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Background: Optically stimulated luminescent dosimeters (OSLDs) can be bleached and reused, but questions remain about the effects of repeated bleaching and fractionation schedules on OSLD performance. Purpose: The aim of this study was to investigate how light sources with different wavelengths and different fractionation schemes affect the performance of reused OSLDs. Methods: OSLDs (N = 240) were irradiated on a cobalt‐60 beam in different step sizes until they reached an accumulated dose of 50 Gy. Between irradiations they were bleached using light sources of different wavelengths: the Imaging and Radiation Oncology Core (IROC) bleaching system (our control); monochromatic red, green, yellow, and blue lights; and a polychromatic white light. Sensitivity and linearity‐based correction factors were determined as a function of dose step‐size. The rate of signal removal from different light sources was characterized by sampling these OSLDs at various time points during their bleaching process. Relative doses were calculated according to the American Association of Physicists in Medicine Task Group‐191. Signal repopulation was investigated by irradiating OSLDs (N = 300) to various delivered doses of 2, 10, 20, 30, 40, and 50 Gy in a single fraction, bleached with one of the colors, and read over time. Fractionation effects were evaluated by irradiating OSLDs up to 30 Gy in different size steps. After reading, the OSLDs were bleached following IROC protocol. OSLDs (N = 40) received irradiations in 5, 10, 15, 30 Gy fractions until they had an accumulated dose of 30 Gy; The sensitivity response of these OSLDs was compared with reference OSLDs that had no accumulated dose. Results: Light sources with polychromatic spectrums (IROC and white) bleached OSLDs faster than did sources with monochromatic spectra. Polychromatic light sources (white light and IROC system) provided the greatest dose stability for OSLDs that had larger amounts of accumulated dose. Signal repopulation was related to the choice of bleaching light source, timing of bleaching, and amount of accumulated dose. Changes to relative dosimetry were more pronounced in OSLDs that received larger fractions. At 5‐Gy fractions and above, all OSLDs had heightened sensitivity, with OSLDs exposed to 30‐Gy fractions being 6.4% more sensitive than reference dosimeters. Conclusions: The choice of bleaching light plays a role in how fast an OSLD is bleached and how much accumulated dose an OSLD can be exposed to while maintaining stable signal sensitivity. We have expanded upon investigations into signal repopulation to show that bleaching light plays a role in the migration of deep traps to dosimetric traps after bleaching. Our research concludes that the bleaching light source and fractionation need to be considered when reusing OSLD. [ABSTRACT FROM AUTHOR]

Background: Ultra‐high‐dose‐rate (UHDR) electron beams have been commonly utilized in FLASH studies and the translation of FLASH Radiotherapy (RT) to the clinic. The EDGE diode detector has potential use for UHDR dosimetry albeit with a beam energy dependency observed. Purpose: The purpose is to present the electron beam response for an EDGE detector in dependence on beam energy, to characterize the EDGE detector's response under UHDR conditions, and to validate correction factors derived from the first detailed Monte Carlo model of the EDGE diode against measurements, particularly under UHDR conditions. Methods: Percentage depth doses (PDDs) for the UHDR Mobetron were measured with both EDGE detectors and films. A detailed Monte Carlo (MC) model of the EDGE detector has been configured according to the blueprint provided by the manufacturer under an NDA agreement. Water/silicon dose ratios of EDGE detector for a series of mono‐energetic electron beams have been calculated. The dependence of the water/silicon dose ratio on depth for a FLASH relevant electron beam was also studied. An analytical approach for the correction of PDD measured with EDGE detectors was established. Results: Water/silicon dose ratio decreased with decreasing electron beam energy. For the Mobetron 9 MeV UHDR electron beam, the ratio decreased from 1.09 to 1.03 in the build‐up region, maintained in range of 0.98–1.02 at the fall‐off region and raised to a plateau in value of 1.08 at the tail. By applying the corrections, good agreement between the PDDs measured by the EDGE detector and those measured with film was achieved. Conclusions: Electron beam response of an UHDR capable EDGE detector was derived from first principles utilizing a sophisticated MC model. An analytical approach was validated for the PDDs of UHDR electron beams. The results demonstrated the capability of EDGE detector in measuring PDDs of UHDR electron beams. [ABSTRACT FROM AUTHOR]

Background: The TRS‑483, an IAEA/AAPM International Code of Practice on dosimetry of small static photon fields, underwent testing via an IAEA coordinated research project (CRP). Alongside small field output factors (OFs) measurements using active dosimeters by CRP participants, the IAEA Dosimetry Laboratory received a mandate to formulate a remote small field dosimetry audit method using its passive dosimetry systems. Purpose: This work aimed to develop a small field dosimetry audit methodology employing radiophotoluminescent dosimeters (RPLDs) and radiochromic films. The methodology was subsequently evaluated through a multicenter pilot study with CRP participants. Methods: The developments included designing and manufacturing a dosimeter holder set and the characterization of an RPLD system for measurements in small photon fields using the new holder. The audit included verification of small field OFs and lateral beam profiles for small fields. At first, treatment planning system (TPS) calculated OFs were checked against a reference data set that was available for conventional linacs. Second, calculated OFs were verified through the RPLD measurement of point doses in a machine‐specific reference field, 4 cm × 4 cm, 2 cm × 2 cm, and 1 cm × 1 cm, corresponding size circular fields or nearest achievable field sizes. Lastly, profile checks in in‐plane and cross‐plane directions were done for the two smallest fields by comparing film measurements with TPS calculations at 20%, 50%, and 80% isodose levels. Results: RPLD correction factors for small field measurements were approximately unity. However, they influenced the dose determination's overall uncertainty in small fields, estimated at 2.30% (k = 1 level). Considering the previous experience in auditing reference beam output following the TRS‐398 Code of Practice, the acceptance limit of 5% for the ratio of the dose determined by RPLD to the dose calculated by TPS, DRPLD/DTPS, was considered adequate. The multicenter pilot study included 15 participants from 14 countries (39 beams). Consistent with the previous findings, the results of the OF check against the reference data confirmed that TPSs tend to overestimate OFs for the smallest fields included in this exercise. All except three RPLD measurement results were within the acceptance limit, and the spread of results increased for smaller field sizes. The differences between the film measured and TPS calculated dose profiles were within 3 mm for most of the beams checked; deviated results revealed problems with TPS commissioning and calibration of the treatment unit collimation systems. Conclusion: The newly developed small field dosimetry audit methodology proved effective and successfully complemented the CRP OF measurements by participants with RPLD audit results. [ABSTRACT FROM AUTHOR]

Background: Water phantoms are required to perform reference dosimetry and beam quality measurements but there are no published studies about the size requirements for such phantoms. Purpose: To investigate, using Monte Carlo techniques, the size requirements for water phantoms used in reference dosimetry and/or to measure the beam quality specifiers %dd(10)x$\%dd(10)_{\sf x}$ and TPR1020$TPR^{20}_{10}$. Methods: The EGSnrc application DOSXYZnrc is used to calculate D(10)$D(10)$, the dose per incident fluence at 10 cm depth in a water phantom irradiated by incident 10×10cm2$10\,\times \,10 \, {\rm {cm}}^{2}$ beams of 60Co$^{60}{\rm {Co}}$ or 6 MV photons. The water phantom dimensions are varied from 30×30×40cm3$30 \,\times \, 30 \,\times \, 40 \, {\rm {cm}}^3$ to 15×15×22cm3$15 \,\times \, 15 \,\times \, 22 \, {\rm {cm}}^3$ and occasionally smaller. The %dd(10)x$\%dd(10)_{\sf x}$ and TPR1020$TPR^{20}_{10}$ values are also calculated with care being taken to distinguish TPR1020$TPR^{20}_{10}$ results when using Method A (changing depth of water in phantom) and Method B (moving entire phantom). Typical statistical uncertainties are 0.03%. Results: Phantom dimensions have only minor effects for phantoms larger than 20×20×25cm3$20 \,\times \, 20 \,\times \, 25 \, {\rm {cm}}^3$. A table of corrections to the dose at 10 cm depth in 10×10cm2$10 \,\times \, 10 \, {\rm {cm}}^{2}$ beams of 60Co$^{60}{\rm {Co}}$ or 6 MV photons are provided and range from no correction to 0.75% for a 60Co$^{60}{\rm {Co}}$ beam incident on a 20×20×15cm3$20 \,\times \, 20 \,\times \, 15 \, {\rm {cm}}^3$ phantom. There can be distinct differences in the TPR1020$TPR^{20}_{10}$ values measured using Method A or Method B, especially for smaller phantoms. It is explicitly demonstrated that, within ±$\pm$ 0.15%, TPR1020$TPR^{20}_{10}$ values for a 30×30×30cm3$30 \,\times \, 30 \,\times \, 30 \, {\rm {cm}}^3$ phantom measured using Method A or B are independent of source detector distance between 40 and 200 cm. Conclusions: The phantom sizes recommended in the TG‐51 and IAEA TRS‐398 reference dosimetry protocols are adequate for accurate reference dosimetry and in some cases are even conservative. Correction factors are necessary for accurate measurement of the dose at 10 cm depth in smaller phantoms and these factors are provided. Very accurate beam quality specifiers are not required for reference dosimetry itself, but for specifying beam stability and characteristics it is important to specify phantom sizes and also the method used for TPR1020$TPR^{20}_{10}$ measurements. [ABSTRACT FROM AUTHOR]

Background: Plane‐parallel ionization chambers are the recommended secondary standard systems for clinical reference dosimetry of electrons. Dosimetry in high dose rate and dose‐per‐pulse (DPP) is challenging as ionization chambers are subject to ion recombination, especially when dose rate and/or DPP is increased beyond the range of conventional radiotherapy. The lack of universally accepted models for correction of ion recombination in UDHR is still an issue as it is, especially in FLASH‐RT research, which is crucial in order to be able to accurately measure the dose for a wide range of dose rates and DPPs. Purpose: The objective of this study was to show the feasibility of developing an Artificial Intelligence model to predict the ion‐recombination factor—ksat for a plane‐parallel Advanced Markus ionization chamber for conventional and ultra‐high dose rate electron beams based on machine parameters. In addition, the predicted ksat of the AI model was compared with the current applied analytical models for this correction factor. Methods: A total number of 425 measurements was collected with a balanced variety in machine parameter settings. The specific ksat values were determined by dividing the output of the reference dosimeter (optically stimulated luminescence [OSL]) by the output of the AM chamber. Subsequently, a XGBoost regression model was trained, which used the different machine parameters as input features and the corresponding ksat value as output. The prediction accuracy of this regression model was characterized by R2‐coefficient of determination, mean absolute error and root mean squared error. In addition, the model was compared with the Two‐Voltage (TVA) method and empirical Petersson model for 19 different dose‐per‐pulse values ranging from conventional to UDHR regimes. The Akiake Information criterion (AIC) was calculated for the three different models. Results: The XGBoost regression model reached a R2‐score of 0.94 on the independent test set with a MAE of 0.067 and RMSE of 0.106. For the additional 19 random data points, the ksat values predicted by the XGBoost model showed to be in agreement, within the uncertainties, with the ones determined by the Petersson model and better than the TVA method for doses per pulse >3.5 Gy with a maximum deviation from the ground truth of 14.2%, 16.7%, and −36.0%, respectively, for DPP >4 Gy. Conclusion: The proposed method of using AI for ksat determination displays efficiency. For the investigated DPPs, the ksat values obtained with the XGBoost model were in concurrence with the ones obtained with the current available analytical models within the boundaries of uncertainty, certainly for the DPP characterizing UDHR. But the overall performance of the AI model, taking the number of free parameters into account, lacked efficiency. Future research should optimize the determination of the experimental ksat, and investigate the determination the ksat for DPPs higher than the ones investigated in this study, while also evaluating the prediction of the proposed XGBoost model for UDHR machines of different centers. [ABSTRACT FROM AUTHOR]

Background: Ionization chambers play an essential role in dosimetry measurements for kilovoltage (kV) x‐ray beams. Despite their widespread use, there is limited data on the absolute values for the polarity correction factors across a range of commonly employed ionization chambers. Purpose: This study aimed to investigate the polarity effects for five different ionization chambers in kV x‐ray beams. Methods: Two plane‐parallel chambers being the Advanced Markus and Roos and three cylindrical chambers; 3D PinPoint, Semiflex and Farmer chamber (PTW, Freiburg, Germany), were employed to measure the polarity correction factors. The kV x‐ray beams were produced from an Xstrahl 300 unit (Xstrahl Ltd., UK). All measurements were acquired at 2 cm depth in a PTW‐MP1 water tank for beams between 60 kVp (HVL 1.29 mm Al) and 300 kVp (HVL 3.08 mm Cu), and field sizes of 2–10 cm diameter for 30 cm focus‐source distance (FSD) and 4 × 4 cm2 – 20 × 20 cm2 for 50 cm FSD. The ionization chambers were connected to a PTW‐UNIDOS electrometer, and the polarity effect was determined using the AAPM TG‐61 code of practice methodology. Results: The study revealed significant polarity effects in ionization chambers, especially in those with smaller volumes. For the plane‐parallel chambers, the Advanced Markus chamber exhibited a maximum polarity effect of 2.5%, whereas the Roos chamber showed 0.3% at 150 KVp with the 10 cm circular diameter open‐ended applicator. Among the cylindrical chambers at the same beam energy and applicator, the Pinpoint chamber exhibited a 3% polarity effect, followed by Semiflex with 1.7%, and Farmer with 0.4%. However, as the beam energy increased to 300 kVp, the polarity effect significantly increased reaching 8.5% for the Advanced Markus chamber and 13.5% for the PinPoint chamber at a 20 × 20 cm2 field size. Notably, the magnitude of the polarity effect increased with both the field size and beam energy, and was significantly influenced by the size of the chamber's sensitive volume. Conclusions: The findings demonstrate that ionization chambers can exhibit substantial polarity effects in kV x‐ray beams, particularly for those chambers with smaller volumes. Therefore, it is important to account for polarity corrections when conducting relative dose measurements in kV x‐ray beams to enhance the dosimetry accuracy and improve patient dose calculations. [ABSTRACT FROM AUTHOR]

Background: The conventional theories and methods of determining the ion recombination correction factor, such as Boag theory and the related two voltage method and Jaffé plot extrapolation, do not seem to yield accurate results in FLASH /high dose per pulse (DPP) beams (>$>$10 mGy DPP). This is due to the presence of a large free electron fraction that distorts the electric field inside the chamber sensitive volume. To understand the influence of these effects on the ion recombination correction factor and to develop new expressions for it, it is necessary to re‐visit the underlying physics. Purpose: To present a mathematical procedure to develop an analytical expression for the ion recombination correction factor. The expression is the basis for an extrapolation method so the correction factor can be determined in a clinical setting. Methods: A semi‐analytical solution method, the homotopy perturbation method (HPM), is used to solve the partial differential equations (PDEs) describing the charge carrier physics, including space charge and free electrons. The electron velocity and attachment rate are modeled as functions of the electric field strength. An expression for the charge collection efficiency and ion recombination correction factor are developed. A fit procedure based on this expression is used to compare it to measured data from previously published articles. Another fit procedure using a general equation is also proposed and compared to the data. Results: The series obtained for the charge collection efficiency and the ion recombination correction factor are determined to be asymptotic series and the optimal truncation established. The ion recombination correction factor exhibits a 1/V2$1/V^2$ dependency due to the free electron presence. The fit using this expression agrees well with measured data as long as (1) the DPP is below 1 Gy for chambers with a 1 mm plate separation and (2) when the DPP is below 3 Gy for chambers with a 0.5 mm plate separation. In these DPP ranges, the deviation between measured and fit value did not exceed 6%. In both chamber cases the voltage range where the fit applies decreases as DPP increases. The general equation yielded comparable results. Conclusions: The HPM was shown to be applicable to a complex system of PDEs and generate meaningful and novel solutions, as they include both space charge and free electrons. The HPM also lends itself to other chamber geometries. The fit procedure was also shown to yield accurate results for the ion recombination correction up to the 1 Gy DPP level. [ABSTRACT FROM AUTHOR]

Background: Dose area product in water (DAPw) in small fields relies on the use of detectors with a sensitive area larger than the irradiation field. This quantity has recently been used to establish primary standards down to 5 mm field size, with an uncertainty smaller than 0.7%. It has the potential to decrease the uncertainty related to field output factors, but is not currently integrated into treatment planning systems. Purpose: This study aimed to explore the feasibility of converting DAPw into a point dose in small fields by determining the volume averaging correction factor. By determining the field output factors, a comparison between the so‐called "DAPw to point dose" approach and the IAEA TRS483 methodology was performed. Method: Diodes, microdiamonds, and a micro ionization chamber were used to measure field output factors following the IAEA TRS483 methodology on two similar linacs equipped with circular cones down to 6 mm diameter. For the "DAPw to point dose" approach, measurements were performed with a dedicated and built‐in‐house 3 cm diameter plane‐parallel ionization chamber calibrated in terms of DAPw in the French Primary Dosimetry Standards Laboratory LNE‐LNHB. Beam profile measurements were performed to generate volume averaging correction factors enabling the conversion of an integral DAPw measurement into a point dose and the determination of the field output factors. Both sets of field output factors were compared. Results: According to the IAEA TRS483 methodology, field output factors were within ±3% for all detectors on both linacs. Large variations were observed for the volume averaging correction factors with a maximum spread between the detectors of 26% for the smallest field size. Consequently, deviations of up to 15% between the "IAEA TRS483" and the "DAPw to point dose" methodologies were found for the field output factor of the smallest field size. This was attributed to the difficulty in accurately determining beam profiles in small fields. Conclusion: Although primary standards associated with small uncertainties can be established in terms of DAPw in a primary laboratory, the "DAPw to point dose" methodology requires volume averaging correction to derive a field output factor from DAPw measurements. None of the point detectors studied provided satisfactory results, and additional work using other detectors, such as film, is still required to allow the transfer of a DAP primary standard to users in terms of absorbed point dose. [ABSTRACT FROM AUTHOR]

Background: Our previous work introduced and evaluated a standard for surface absorbed dose rate per unit radioactivity to water from unsealed alpha‐emitting radionuclides used in targeted radionuclide therapy (TRT). An overall uncertainty over 4.0% at k = 1 was reported for the absorbed dose to air measurements, which was partially attributed to the rotational alignment uncertainty in the geometrical setup. Purpose: A printed circuit board (PCB) with a segmented guard was constructed to align the extrapolation chamber (EC) and the source plates using a differential capacitance technique. The PCB EC aimed to enhance the repeatability of the ionization current measurements. The PCB EC was evaluated using a thin film 210Po source. The measured absorbed dose to air cavity was compared with the Monte Carlo (MC) calculations. Using the extrapolation method, the surface absorbed dose rate to water was calculated. Methods: The PCB EC was constructed with a 4.50 mm diameter collector surrounded by four sectors and a guard electrode. The sectors were isolated for rotational alignment and later connected to the guard for ionization current measurements. A bridge circuit measured differential capacitance between opposing sectors, and a hexapod motion stage rotated the source substrate to minimize the differential capacitance. The EC was evaluated using a 210Po source with a 3.20 mm diameter and 1.253 μ$\mu $Ci radioactivity. MC simulations were performed to calculate the kpoint${k}_{point}$, kbackscatter${k}_{backscatter}$, and kdiv${k}_{div}$ correction factors. Ionization current measurements were performed for air gaps in the 0.3–0.525 mm range and surface absorbed dose rate to water was calculated. Results: Rotational offsets of up to 3.0° were found and the current repeatability was found to increase with the absorbed dose to air uncertainty calculated to be ∼2.0%. Using the capacitance method, the effective EC diameter was measured to be 4.53 mm. The recombination, polarity, and electrometer corrections were reported to be within 1.00% across all measurement trials. The MC‐calculated correction factors were calculated to be much larger than the recombination and polarity correction factors. The average kpoint${k}_{point}$, kbackscatter${k}_{backscatter}$, and kdiv${k}_{div}$ corrections were calculated to be 1.063, 0.9402, and 2.136, respectively. The MC‐calculated absorbed dose to air was found to overestimate the absorbed dose by over 4.00% when compared with the measured absorbed dose to air. The surface absorbed dose rate to water was calculated to be 2.304×10−6$2.304 \times {10}^{ - 6}$ Gy/s/Bq with an overall uncertainty of 4.07%. Conclusions: The constructed PCB EC was deemed suitable as an absorbed dose standard. A repeatable rotational alignment was achieved using the differential capacitance technique. The metal electrodes on the PCB made a difference of < 1.00% on the backscatter correction when compared to the EC comprised of polystyrene‐equivalent collector. A 20% difference in the surface absorbed dose rate to water was found between the two ECs, which is attributed to the cavity diameter differences leading to different magnitudes of dose fall‐off along the lateral direction. [ABSTRACT FROM AUTHOR]

Background: The world's first clinical 0.5 T inline rotating biplanar Linac‐MR system is commissioned for clinical use. For reference dosimetry, unique features to device, including an SAD = 120 cm, bore clearance of 60 cm × 110 cm, as well as 0.5 T inline magnetic field, provide some challenges to applying a standard dosimetry protocol (i.e., TG‐51). Purpose: In this work, we propose a simple and practical clinical reference dosimetry protocol for the 0.5T biplanar Linac‐MR and validated its results. Methods: Our dosimetry protocol for this system is as follows: tissue phantom ratios at 20 and 10 cm are first measured and converted into %dd10x beam quality specifier using equations provided and Kalach and Rogers. The converted %dd10x is used to determine the ion chamber correction factor, using the equations in the TG‐51 addendum for the Exradin A12 farmer chamber used, which is cross‐calibrated with one calibrated at a standards laboratory. For a 0.5 T parallel field, magnetic field effect on chamber response is assumed to have no effect and is not explicitly corrected for. Once the ion chamber correction factor for a non‐standard SAD (kQ,msr) is determined, TG‐51 is performed to obtain dose at a depth of 10 cm at SAD = 120 cm. The dosimetry protocol is repeated with the magnetic field ramped down. To validate our dosimetry protocol, Monte Carlo (EGSnrc) simulations are performed to confirm the determined kQ,msr values. MC Simulations and magnetic Field On versus Field Off measurements are performed to confirm that the magnetic field has no effect. To validate our overall dosimetry protocol, external dose audits, based on optical simulated luminescent dosimeters, thermal luminescent dosimeters, and alanine dosimeters are performed on the 0.5 T Linac‐MR system. Results: Our EGSnrc results confirm our protocol‐determined kQ,msr values, as well as our assumptions about magnetic field effects (kB = 1) within statistical uncertainty for the A‐12 chamber. Our external dosimetry procedures also validated our overall dosimetry protocol for the 0.5 T biplanar Linac‐MR hybrid. Ramping down the magnetic field has resulted in a dosimetric difference of 0.1%, well within experimental uncertainty. Conclusion: With the 0.5 T parallel magnetic field having minimal effect on the ion chamber response, a TPR20,10 approach to determine beam quality provides an accurate method to perform clinical dosimetry for the 0.5 T biplanar Linac‐MR. [ABSTRACT FROM AUTHOR]

Background: The combination of magnetic resonance imaging and proton therapy offers the potential to improve cancer treatment. The magnetic field (MF)‐dependent change in the dosage of ionization chambers in magnetic resonance imaging‐integrated proton therapy (MRiPT) is considered by the correction factor kB⃗,M,Q$k_{\vec{B},M,Q}$, which needs to be determined experimentally or computed via Monte Carlo (MC) simulations. Purpose: In this study, kB⃗,M,Q$k_{\vec{B},M,Q}$ was both measured and simulated with high accuracy for a plane‐parallel ionization chamber at different clinical relevant proton energies and MF strengths. Material and methods: The dose‐response of the Advanced Markus chamber (TM34045, PTW, Freiburg, Germany) irradiated with homogeneous 10 ×$\times$ 10 cm 2$^2$ quasi mono‐energetic fields, using 103.3, 128.4, 153.1, 223.1, and 252.7 MeV proton beams was measured in a water phantom placed in the MF of an electromagnet with MF strengths of 0.32, 0.5, and 1 T. The detector was positioned at a depth of 2 g/cm 2$^2$ in water, with chamber electrodes parallel to the MF lines and perpendicular to the proton beam incidence direction. The measurements were compared with TOPAS MC simulations utilizing COMSOL‐calculated 0.32, 0.5, and 1 T MF maps of the electromagnet. kB⃗,M,Q$k_{\vec{B},M,Q}$ was calculated for the measurements for all energies and MF strengths based on the equation: kB⃗,M,Q=MQMQB⃗$k_{\vec{B},M,Q}=\frac{M_\mathrm{Q}}{M_\mathrm{Q}^{\vec{B}}}$, where MQB⃗$M_\mathrm{Q}^{\vec{B}}$ and MQ$M_\mathrm{Q}$ were the temperature and air‐pressure corrected detector readings with and without the MF, respectively. MC‐based correction factors were determined as kB⃗,M,Q=DdetDdetB⃗$k_{\vec{B},M,Q}=\frac{D_\mathrm{det}}{D_\mathrm{det}^{\vec{B}}}$, where DdetB⃗$D_\mathrm{det}^{\vec{B}}$ and Ddet$D_\mathrm{det}$ were the doses deposited in the air cavity of the ionization chamber model with and without the MF, respectively. Furthermore, MF effects on the chamber dosimetry are studied using MC simulations, examining the impact on the absorbed dose‐to‐water (DW$D_{W}$) and the shift in depth of the Bragg peak. Results: The detector showed a reduced dose‐response for all measured energies and MF strengths, resulting in experimentally determined kB⃗,M,Q$k_{\vec{B},M,Q}$ values larger than unity. For all energies and MF strengths examined, kB⃗,M,Q$k_{\vec{B},M,Q}$ ranged between 1.0065 and 1.0205. The dependence on the energy and the MF strength was found to be non‐linear with a maximum at 1 T and 252.7 MeV. The MC simulated kB⃗,M,Q$k_{\vec{B},M,Q}$ values agreed with the experimentally determined correction factors within their standard deviations with a maximum difference of 0.6%. The MC calculated impact on DW$D_{W}$ was smaller 0.2 %. Conclusion: For the first time, measurements and simulations were compared for proton dosimetry within MFs using an Advanced Markus chamber. Good agreement of kB⃗,M,Q$k_{\vec{B},M,Q}$ was found between experimentally determined and MC calculated values. The performed benchmarking of the MC code allows for calculating kB⃗,M,Q$k_{\vec{B},M,Q}$ for various ionization chamber models, MF strengths and proton energies to generate the data needed for a proton dosimetry protocol within MFs and is, therefore, a step towards MRiPT. [ABSTRACT FROM AUTHOR]

Ion chambers are required for calibration and reference dosimetry applications in radiation therapy (RT). However, exposure of ion chambers in ultra‐high dose rate (UHDR) conditions pertinent to FLASH‐RT leads to severe saturation and ion recombination, which limits their performance and usability. The purpose of this study was to comprehensively evaluate a set of commonly used commercially available ion chambers in RT, all with different design characteristics, and use this information to produce a prototype ion chamber with improved performance in UHDR conditions as a first step toward ion chambers specific for FLASH‐RT. The Advanced Markus and Exradin A10, A26, and A20 ion chambers were evaluated. The chambers were placed in a water tank, at a depth of 2 cm, and exposed to an UHDR electron beam at different pulse repetition frequency (PRF), pulse width (PW), and pulse amplitude settings on an IntraOp Mobetron. Ion chamber responses were investigated for the various beam parameter settings to isolate their dependence on integrated dose, mean dose rate and instantaneous dose rate, dose‐per‐pulse (DPP), and their design features such as chamber type, bias voltage, and collection volume. Furthermore, a thin parallel‐plate (TPP) prototype ion chamber with reduced collector plate separation and volume was constructed and equally evaluated as the other chambers. The charge collection efficiency of the investigated ion chambers decreased with increasing DPP, whereas the mean dose rate did not affect the response of the chambers (± 1%). The dependence of the chamber response on DPP was found to be solely related to the total dose within the pulse, and not on mean dose rate, PW, or instantaneous dose rate within the ranges investigated. The polarity correction factor (Ppol) values of the TPP prototype, A10, and Advanced Markus chambers were found to be independent of DPP and dose rate (± 2%), while the A20 and A26 chambers yielded significantly larger variations and dependencies under the same conditions. Ion chamber performance evaluated under different irradiation conditions of an UHDR electron beam revealed a strong dependence on DPP and a negligible dependence on the mean and instantaneous dose rates. These results suggest that modifications to ion chambers design to improve their usability in UHDR beamlines should focus on minimizing DPP effects, with emphasis on optimizing the electric field strength, through the construction of smaller electrode separation and larger bias voltages. This was confirmed through the production and evaluation of a prototype ion chamber specifically designed with these characteristics. [ABSTRACT FROM AUTHOR]

Background: Independent dosimetry audits are an important intervention in radiotherapy for quality assurance. Electron beams, used for superficial radiotherapy treatments, must also be tested in dosimetry audits as part of a good quality assurance program to help prevent clinical errors. Purpose: To establish a new service for IAEA/WHO postal dosimetry audits in electron beams using RPL dosimeters. Methods: A novel postal audit methodology employing a PMMA holder system for RPLDs was developed. The associated correction factors including holder dependence, energy dependence, dose response non‐linearity, and fading were obtained and tested in a multi‐center (n = 12) pilot study. A measurement uncertainty budget was estimated and employed in analyzing the irradiated dosimeters. Results: Holder and energy correction factors ranged from 1.004 to 1.010 and 1.019 to 1.059 respectively across the energy range. The non‐linearity and fading correction models used for photon beams were tested in electron beams and did not significantly increase measurement uncertainty. The mean dose ratio ± SD of the multi‐center study was 1.001 ± 0.011. The overall uncertainty budget was estimated as ± 1.42% (k = 1). Conclusions: A methodology for IAEA/WHO postal dosimetry audits in electron beams was developed and validated in a multi‐center study and is now made available to radiotherapy centers as a routine service. [ABSTRACT FROM AUTHOR]

Background: Although flattening filter free (FFF) beams are commonly used in clinical treatment, the accuracy of dose measurements in FFF beams has been questioned. Higher dose per pulse (DPP) such as FFF beams from a linear accelerator may cause problems in dose profile measurements using an ionization chamber due to the change of the charge collection efficiency. Ionization chambers are commonly used for percent depth dose (PDD) measurements. Changes of DPP due to chamber movement during PDD measurement can vary the ion collection efficiency of ionization chambers. In the case of FF beams, the DPP fluctuation is negligible, but in the case of the FFF beams, the DPP is 2.5 ∼ 4 times larger than that of the FF beam, and the change in ion collection efficiency is larger than that of the FF beam. PDD profile normalized by maximum dose depth, 10 cm depth for example, may therefore be affected by the ion collection efficiency. Purpose: In this study, we investigate the characteristics of the ion collection efficiency change depending on the DPP of each ionization chamber in the FFF beam. We furthermore propose a method to obtain the chamber‐ independent PDD by applying a DPP‐dependent ion recombination correction. Methods: Prior to investigating the relationship between DPP and charge collection efficiency, Jaffe‐plots were generated with different DPP settings to investigate the linearity between the applied voltage and collected charge. The absolute dose measurement using eight ionization chambers under the irradiation settings of 0.148, 0.087, and 0.008 cGy/pulse were performed. Applied voltages for the Jaffe‐plots were 100, 125, 150, 200, 250, and 300 V. The ion recombination correction factor Pion was calculated by the two‐voltage analysis (TVA) method at the applied voltages of 300 and 100 V. The DPP dependency of the charge collection efficiency for each ionization chamber were evaluated from the DPP‐ Pion plot. PDD profiles for the 10 MV FFF beam were measured using Farmer type chambers (TN30013, FC65‐P, and FC65‐G) and mini‐type chambers (TN31010, TN31021, CC13, CC04, and FC23‐C). The PDD profiles were corrected with ion recombination correction at negative and positive polar applied voltages of 100 and 300 V. Results: From the DPP‐Pion relation for each ionization chamber with DPP ranging from 0.008 cGy/pulse to 0.148 cGy/pulse, all Farmer and mini‐type chambers satisfied the requirements described in AAPM TG‐51 addendum. However, Pion for the CC13 was most affected by DPP among tested chambers. The maximum deviation among PDDs using eight ionization chambers for 10 MV FFF was about 1%, but the deviation was suppressed to about 0.5% by applying ion recombination correction at each depth. Conclusions: In this study, the deviation of PDD profile among the ionization chambers was reduced by the ion recombination coefficient including the DPP dependency, especially for high DPP beams such as FFF beams. The present method is particularly effective for CC13, where the ion collection efficiency is highly DPP dependent. [ABSTRACT FROM AUTHOR]

Background: The response of various detectors in the radiotherapy energy range has been investigated, especially for 6 and 10 MV energies for small fields, and is summarized in TRS‐483. However, data for accelerator energies above 10 MV are sparse or unavailable for many detectors, especially for the energy of 18 MV. Small variations in field output factors for the commissioning of a treatment planning system can have a high impact on calculation of dose distributions. Purpose: Many studies describe an energy dependence of the response for a large number of detectors. We wanted to close the gap for the 18 MV energy regime and determined field output correction factors for different detectors at 18 MV. Methods: An ELEKTA Versa HD accelerator at 18 MV was used together with a PTW MP3 water phantom at an SSD of 90 cm. The following detectors were examined: PTW Semiflex 31021, PinPoint 3D 31022, diode 60012, diode 60008 and microDiamond 60019, Sun Nuclear EDGE detector, IBA PFD, SFD, Razor Chamber, Razor Nano Chamber and Razor Diode, Standard Imaging Scintillator Exradin W2 1x3, W2 1x1 and Gafchromic EBT3 film. The dose response was determined at a depth of 10 cm for square fields between 0.5 and 10 cm side length. As reference data a composure of radiochromic film data for small fields (s≤3$s\le 3$ cm) and data of all compatible chambers for larger fields (s≥3$s\ge 3$ cm) was used. The effective field sizes of small fields were determined from profiles obtained on radiochromic film. The obtained field output correction factors obey the rules of the TRS‐483 protocol. Results: The W2 1x1 scintillator and the Razor Chamber showed the smallest deviations from the reference curve. The shielded diodes (diode 60008, EDGE detector) showed the highest over‐response at small fields, followed by PFD, microDiamond and the unshielded diodes (diode 60012, SFD). The ionization chambers exhibited the well‐known volume effect, that is, strong under‐response at small fields of up to 9% for the PinPoint 3D, 7% for the Razor Chamber and up to 30% for the Semiflex detector for the smallest studied field size. The small chambers showed a polarity effect in axial orientation, especially the Razor Nano Chamber. Corrections at 18 MV are generally larger than those provided by TRS‐483, continuing the trend of increasing corrections between 6 and 10 MV also at a higher accelerator energy. Only the PinPoint 3D Chamber showed a slightly smaller correction. Conclusions: Field output correction factors were determined for square field sizes between 0.5 and 10 cm at 18 MV. Most detectors needed a larger correction than at 6 and 10 MV. Thus, the use of correction factors will improve beam data for 18 MV. [ABSTRACT FROM AUTHOR]

Background: Gafchromic film's unique properties of tissue‐equivalence, dose‐rate independence, and high spatial resolution make it an attractive choice for many dosimetric applications. However, complicated calibration processes and film handling limits its routine use. Purpose: We evaluated the performance of Gafchromic EBT3 film after irradiation under a variety of measurement conditions to identify aspects of film handling and analysis for simplified but robust film dosimetry. Methods: The short‐ (from 5 min to 100 h) and long‐term (months) film response was evaluated for clinically relevant doses of up to 50 Gy for accuracy in dose determination and relative dose distributions. The dependence of film response on film‐read delay, film batch, scanner type, and beam energy was determined. Results: Scanning the film within a 4‐h window and using a standard 24‐h calibration curve introduced a maximum error of 2% over a dose range of 1–40 Gy, with lower doses showing higher uncertainty in dose determination. Relative dose measurements demonstrated <1 mm difference in electron beam parameters such as depth of 50% of the maximum dose value (R50), independent of when the film was scanned after irradiation or the type of calibration curve used (batch‐specific or time‐specific calibration curve) if the same default scanner was used. Analysis of films exposed over a 5‐year period showed that using the red channel led to the lowest variation in the measured net optical density values for different film batches, with doses >10 Gy having the lowest coefficient of variation (<1.7%). Using scanners of similar design produced netOD values within 3% after exposure to doses of 1–40 Gy. Conclusions: This is the first comprehensive evaluation of the temporal and batch dependence of Gafchromic EBT3 film evaluated on consolidated data over 8 years. The relative dosimetric measurements were insensitive to the type of calibration applied (batch‐ or time‐specific) and in‐depth time‐dependent dosimetric signal behaviors can be established for film scanned outside of the recommended 16–24 h post‐irradiation window. We generated guidelines based on our findings to simplify film handling and analysis and provide tabulated dose‐ and time‐dependent correction factors to achieve this without reducing the accuracy of dose determination. [ABSTRACT FROM AUTHOR]

Background and purpose: As a part of the commissioning and quality assurance in proton beam therapy, lateral dose profiles and output factors have to be acquired. Such measurements can be performed with point detectors and are especially challenging in small fields or steep lateral penumbra regions as the detector's volume effect may lead to perturbations. To address this issue, this work aims to quantify and correct for such perturbations of six point detectors in small proton fields created via three different delivery techniques. Methods: Lateral dose profile and output measurements of three proton beam delivery techniques (pencil beam scanning, pencil beam scanning combined with collimators, passive scattering with collimators) were performed using high‐resolution EBT3 films, a PinPoint 3D 31022 ionization chamber, a microSilicon diode 60023 and a microDiamond detector 60019 (all PTW Freiburg, Germany). Detector specific lateral dose response functions K(x,y) acting as the convolution kernel transforming the undisturbed dose distribution D(x,y) into the measured signal profiles M(x,y) were applied to quantify perturbations of the six investigated detectors in the proton fields and correct the measurements. A signal theoretical analysis in Fourier space of the dose distributions and detector's K(x,y) was performed to aid the understanding of the measurement process with regard to the combination of detector choice and delivery technique. Results: Quantification of the lateral penumbra broadening and signal reduction at the fields center revealed that measurements in the pencil beam scanning fields are only compromised slightly even by large volume ionization chambers with maximum differences in the lateral penumbra of 0.25 mm and 4% signal reduction at the field center. In contrast, radiation techniques with collimation are not accurately represented by the investigated detectors as indicated by a penumbra broadening up to 1.6 mm for passive scattering with collimators and 2.2 mm for pencil beam scanning with collimators. For a 3 mm diameter collimator field, a signal reduction at field center between 7.6% and 60.7% was asserted. Lateral dose profile measurements have been corrected via deconvolution with the corresponding K(x,y) to obtain the undisturbed D(x,y). Corrected output ratios of the passively scattered collimated fields obtained for the microDiamond, microSilicon and PinPoint 3D show agreement better than 0.9% (one standard deviation) for the smallest field size of 3 mm. Conclusion: Point detector perturbations in small proton fields created with three delivery techniques were quantified and found to be especially pronounced for collimated small proton fields with steep dose gradients. Among all investigated detectors, the microSilicon diode showed the smallest perturbations. The correction strategies based on detector's K(x,y) were found suitable for obtaining unperturbed lateral dose profiles and output factors. Approximation of K(x,y) by considering only the geometrical averaging effect has been shown to provide reasonable prediction of the detector's volume effect. The findings of this work may be used to guide the choice of point detectors in various proton fields and to contribute toward the development of a code of practice for small field proton dosimetry. [ABSTRACT FROM AUTHOR]

Background: The integration of magnetic resonance tomography into clinical linear accelerators provides high‐contrast, real‐time imaging during treatment and facilitates online‐adaptive workflows in radiation therapy treatments. The associated magnetic field also bends the trajectories of charged particles via the Lorentz force, which may alter the dose distribution in a patient or a phantom and affects the dose response of dosimetry detectors. Purpose: To perform an experimental and Monte Carlo‐based determination of correction factors kB,Q$k_{B,Q}$, which correct the response of ion chambers in the presence of external magnetic fields in high‐energy photon fields. Methods: The response variation of two different types of ion chambers (Sun Nuclear SNC125c and SNC600c) in strong external magnetic fields was investigated experimentally and by Monte Carlo simulations. The experimental data were acquired at the German National Metrology Institute, PTB, using a clinical linear accelerator with a nominal photon energy of 6 MV and an external electromagnet capable of generating magnetic flux densities of up to 1.5 T in opposite directions. The Monte Carlo simulation geometries corresponded to the experimental setup and additionally to the reference conditions of IAEA TRS‐398. For the latter, the Monte Carlo simulations were performed with two different photon spectra: the 6 MV spectrum of the linear accelerator used for the experimental data acquisition and a 7 MV spectrum of a commercial MRI‐linear accelerator. In each simulation geometry, three different orientations of the external magnetic field, the beam direction and the chamber orientation were investigated. Results: Good agreement was achieved between Monte Carlo simulations and measurements with the SNC125c and SNC600c ionization chambers, with a mean deviation of 0.3% and 0.6%, respectively. The magnitude of the correction factor kB,Q$k_{B,Q}$ strongly depends on the chamber volume and on the orientation of the chamber axis relative to the external magnetic field and the beam directions. It is greater for the SNC600c chamber with a volume of 0.6 cm3 than for the SNC125c chamber with a volume of 0.1 cm3. When the magnetic field direction and the chamber axis coincide, and they are perpendicular to the beam direction, the ion chambers exhibit a calculated overresponse of less than 0.7(6)% (SNC600c) and 0.3(4)% (SNC125c) at 1.5 T and less than 0.3(0)% (SNC600c) and 0.1(3)% (SNC125c) for 0.35 T for nominal beam energies of 6 MV and 7 MV. This chamber orientation should be preferred, as kB,Q$k_{B,Q}$ may increase significantly in other chamber orientations. Due to the special geometry of the guard ring, no dead‐volume effects have been observed in any orientation studied. The results show an intra‐type variation of 0.17% and 0.07% standard uncertainty (k=1) for the SNC125c and SNC600c, respectively. Conclusion: Magnetic field correction factors kB,Q$k_{B,Q}$ for two different ion chambers and for typical clinical photon beam qualities were presented and compared with the few data existing in the literature. The correction factors may be applied in clinical reference dosimetry for existing MRI‐linear accelerators. [ABSTRACT FROM AUTHOR]

Background: The machine‐specific reference (msr) correction factors (kQmsr,Q0fmsr,fref$k_{{Q_{{\rm{msr}}}},\;{Q_0}}^{{f_{{\rm{msr}}}},{f_{{\rm{ref}}}}}$) were introduced in International Atomic Energy Agency (IAEA) Technical Report Series 483 (TRS‐483) for reference dosimetry of small fields. Several correction factor sets exist for a Leksell Gamma Knife (GK) Perfexion or Icon. Nevertheless, experiments have not rigorously validated the correction factors from different studies. Purpose: This study aimed to assess the role and accuracy of kQmsr,Q0fmsr,fref$k_{{Q_{{\rm{msr}}}},\;{Q_0}}^{{f_{{\rm{msr}}}},{f_{{\rm{ref}}}}}$ values in determining the absorbed dose rates to water in the reference dosimetry of Gamma Knife. Methods: The dose rates in the 16 mm collimator field of a GK were determined following the international code of practices with three ionization chambers: PTW T31010, PTW T31016 (PTW Freiberg GmbH, New York, NY), and Exradin A16 (Standard Imaging, Inc., Middleton, WI). A chamber was placed at the center of a solid water phantom (Elekta AB, Stockholm, Sweden) using a detector‐specific insert. The reference point of the ionization chamber was confirmed using cone‐beam CT images. Consistency checks were repeated five times at a GK site and performed once at seven GK sites. Correction factors from six simulations reported in previous studies were employed. Variations in the dose rates and relative dose rates before and after applying the kQmsr,Q0fmsr,fref$k_{{Q_{msr}},\;{Q_0}}^{{f_{msr}},{f_{ref}}}$ were statistically compared. Results: The standard deviation of the dose rates measured by the three chambers decreased significantly after any correction method was applied (p = 0.000). When the correction factors of all studies were averaged, the standard deviation was reduced significantly more than when any single correction method was applied (p ≤ 0.030), except for the IAEA TRS‐483 correction factors (p = 0.148). Before any correction was applied, there were statistically significant differences among the relative dose rates measured by the three chambers (p = 0.000). None of the single correction methods could remove the differences among the ionization chambers (p ≤ 0.038). After TRS‐483 correction, the dose rate of Exradin A16 differed from those of the other two chambers (p ≤ 0.025). After the averaged factors were applied, there were no statistically significant differences between any pairs of chambers according to Scheffe's post hoc analyses (p ≥ 0.051); however, PTW T31010 differed from PTW 31016 according to Tukey's HSD analyses (p = 0.040). Conclusion: The kQmsr,Q0fmsr,fref$k_{{Q_{{\rm{msr}}}},\;{Q_0}}^{{f_{{\rm{msr}}}},{f_{{\rm{ref}}}}}$ significantly reduced variations in the dose rates measured by the three ionization chambers. The mean correction factors of the six simulations produced the most consistent results, but this finding was not explicitly proven in the statistical analyses. [ABSTRACT FROM AUTHOR]

Background: Radiation fields encountered in proton therapy (PT) and ionbeam therapy (IBT) are characterized by a variable linear energy transfer (LET), which lead to a variation of relative biological effectiveness and also affect the response of certain dosimeters. Therefore, reliable tools to measure LET are advantageous to predict and correct LET effects. Fluorescent nuclear track detectors (FNTDs) are suitable to measure LET spectra within the range of interest for PT and IBT, but so far the accuracy and precision have been challenged by sensitivity variations between individual crystals. Purpose: To develop a novel methodology to correct changes in the fluorescent intensity due to sensitivity variations among FNTDs. This methodology is based on exposing FNTDs to alpha particles in order to derive a detector-specific correction factor. This will allow us to improve the accuracy and precision of LET spectra measurements with FNTDs. Methods: FNTDs were exposed to alpha particles. Afterward, the detectors were irradiated to monoenergetic protons, 4He-, 12C-, and 16O-ions. At each step, the detectors were imaged with a confocal laser scanning microscope. The tracks were reconstructed and analyzed using in-house developed tools. Alpha-particle tracks were used to derive a detector-specific sensitivity correction factor (ks,i). Proton, 4He-, 12C-, and 16O-ion tracks were used to establish a traceable calibration curve that relates the fluorescence intensity with the LET in water (LETH2O). FNTDs from a second batch were exposed and analyzed following the same procedures, to test if ks,i can be used to extend the applicability of the calibration curve to detectors from different batches. Finally, a set of blind tests was performed to assess the accuracy of the proposed methodology without user bias. Throughout all stages, the main sources of uncertainty were evaluated. Results: Based on a sample of 100 FNTDs, our findings show a high sensitivity heterogeneity between FNTDs, with ks,i having values between 0.57 and 2.55. The fitting quality of the calibration curve, characterized by the mean absolute percentage residuals and correlation coefficient, was improved when ks,i was considered. Results for detectors from the second batch show that, if the fluorescence signal is corrected by ks,i,the differences in the predicted LETH2O with respect to the reference set are reduced from 55%, 141%, 41%, and 186% to 4.2%, 6.5%, 5.0%, and 11.0%,for protons, 4He-, 12C-, and 16O-ions, respectively. The blind tests showed that it is possible to measure the track- and doseaverage LETH2O with an accuracy of 0.3%, 16%, and 9.6% and 1.7%, 28%, and 30% for protons, 12C-ions and mixed beams, respectively. On average, the combined uncertainty of the measured LETH2O was 11%, 13%, 21%, and 26% for protons, 4He-, 12C-, and 16O-ions, respectively. These values were increased by a mean factor of 2.0 when ks,i was not applied. Conclusions: We have demonstrated for the first time that alpha particles can be used to derive a detector-specific sensitivity correction factor. The proposed methodology allows us to measure LET spectra using FNTD-technology, with a degree of accuracy and precision unreachable before with sole experimental approaches. [ABSTRACT FROM AUTHOR]

Background: Small-field dosimetry is challenging for radiotherapy dosimetry because of the loss of lateral charged equilibrium, partial occlusion of the primary photon source by the collimating devices, perturbation effects caused by the detector materials and their design, and the detector size relative to the radiation field size, which leads to a volume averaging effect. Therefore, a suitable tool for small-field dosimetry requires high spatial resolution, tissue equivalence, angular independence, and energy and dose rate independence to achieve sufficient accuracy. Recently, with the increasing use of combinations of coplanar and non-coplanar beams for small-field dosimetry, there is a need to clarify angular dependence for dosimetry where the detector is oriented at various angles to the incident beam. However, the effect of angular dependence on small-field dosimetry with coplanar and non-coplanar beams has not been fully clarified. Purpose: This study clarified the effect of angular dependence on small-field dosimetry with coplanar and non-coplanar beams using various detectors. Methods: Seven different detectors were used: CC01, RAZOR, RAZOR Nano, Pinpoint 3D, stereotactic field diode (SFD), microSilicon, and microDiamond. All measurements were taken using a TrueBeam STx with 6 MV and 10 MV flattening filter-free (FFF) energies using a water-equivalent spherical phantom with a source-to-axis distance of 100 cm. The detector was inserted in a perpendicular orientation, and the gantry was rotated at 15° increments from the incidence beam angle. A multi-leaf collimator (MLC) with four field sizes of 0.5 × 0.5, 1 × 1, 2 × 2, and 3 × 3 cm², and four couch angles from 0°, 30°, 60°, and 90° (coplanar and non-coplanar) were adopted. The angular dependence response (AR) was defined as the ratio of the detector response at a given irradiation gantry angle normalized to the detector response at 0°. The maximum AR differences were calculated between the maximum and minimum AR values for each detector, field size, energy, and couch angle. Results: The maximum AR difference for the coplanar beam was within 3.3% for all conditions, excluding the maximum AR differences in 0.5 × 0.5 cm² field for CC01 and RAZOR. The maximum AR difference for non-coplanar beams was within 2.5% for fields larger than 1 × 1 cm², excluding the maximum AR differences for RAZOR Nano, SFD, and microSilicon. The Pinpoint 3D demonstrated stable AR tendencies compared to other detectors. The maximum difference was within 2.0%, except for the 0.5 × 0.5 cm² field and couch angle at 90°. The tendencies of AR values for each detector were similar when using different energies. Conclusion: This study clarified the inherent angular dependence of seven detectors that were suitable for small-field dosimetry. The Pinpoint 3D chamber had the smallest angular dependence of all detectors for the coplanar and non-coplanar beams. The findings of this study can contribute to the calculation of the AR correction factor, and it may be possible to adapt detectors with a large angular dependence on coplanar and non-coplanar beams. However, note that the gantry sag and detector-specific uncertainties increase as the field size decreases. [ABSTRACT FROM AUTHOR]

Background: In a recent study, we reported beam quality correction factors, fQ, in carbon ion beams using Monte Carlo (MC) methods for a cylindrical and a parallel-plate ionization chamber (IC). A non-negligible perturbation effect was observed;however, the magnitude of the perturbation correction due to the specific IC subcomponents was not included. Furthermore, the stopping power data presented in the International Commission on Radiation Units and Measurements (ICRU) report 73 were used, whereas the latest stopping power data have been reported in the ICRU report 90. Purpose: The aim of this study was to extend our previous work by computing fQ correction factors using the ICRU 90 stopping power data and by reporting ICspecific perturbation correction factors. Possible energy or linear energy transfer (LET) dependence of the fQ correction factor was investigated by simulating both pristine beams and spread-out Bragg peaks (SOBPs). Methods: The TOol for PArticle Simulation (TOPAS)/GEANT4 MC code was used in this study. A 30 × 30 × 50 cm3 water phantom was simulated with a uniform 10 × 10 cm2 parallel beam incident on the surface. A Farmer-type cylindrical IC (Exradin A12) and two parallel-plate ICs (Exradin P11 and A11) were simulated in TOPAS using the manufacturer-provided geometrical drawings. The fQ correction factor was calculated in pristine carbon ion beams in the 150-450 MeV/u energy range at 2 cm depth and in the middle of the flat region of four SOBPs. The kQ correction factor was calculated by simulating the fQo correction factor in a 60Co beam at 5 cm depth. The perturbation correction factors due to the presence of the individual IC subcomponents, such as the displacement effect in the air cavity, collecting electrode, chamber wall, and chamber stem, were calculated at 2 cm depth for monoenergetic beams only. Additionally, the mean dose-averaged and track-averaged LET was calculated at the depths at which the fQ was calculated. Results: The ICRU 90 fQ correction factors were reported. The pdis correction factor was found to be significant for the cylindrical IC with magnitudes up to 1.70%. The individual perturbation corrections for the parallel-plate ICs were <1.0% except for the A11 pcel correction at the lowest energy. The fQ correction for the P11 IC exhibited an energy dependence of >1.00% and displayed differences up to 0.87% between pristine beams and SOBPs. Conversely, the fQ for A11 and A12 displayed a minimal energy dependence of <0.50%. The energy dependence was found to manifest in the LET dependence for the P11 IC. A statistically significant LET dependence was found only for the P11 IC in pristine beams only with a magnitude of <1.10%. Conclusions: The perturbation and kQ correction factor should be calculated for the specific IC to be used in carbon ion beam reference dosimetry as a function of beam quality. [ABSTRACT FROM AUTHOR]

Background: In Magnetic Resonance-Linac (MR-Linac) dosimetry formalisms, a new correction factor, kB, Q, has been introduced to account for corresponding changes to detector readings under the beam quality, Q, and the presence of magnetic field, B. Purpose: This study aims to develop and implement a Monte Carlo (MC)-based framework for the determination of kB, Q correction factors for a series of ionization chambers utilized for dosimetry protocols and dosimetric quality assurance checks in clinical 1.5 T MR-Linacs. Their dependencies on irradiation setup conditions are also investigated. Moreover, to evaluate the suitability of solid phantoms for dosimetry checks and end-to-end tests, changes to the detector readings due to the presence of small asymmetrical air gaps around the detector's tip are quantified. Methods: Phase space files for three irradiation fields of the ELEKTA Unity 1.5 T/7 MV flattening-filter-free MR-Linac were provided by the manufacturer and used as source models throughout this study. Twelve ionization chambers (three farmer-type and nine small-cavity detectors, from three manufacturers) were modeled (including their dead volume) using the EGSnrc MC code package. kB, Q values were calculated for the 10 × 10 cm2 irradiation field and for four cardinal orientations of the detectors' axes with respect to the 1.5 T magnetic field. Potential dependencies of kB,Q values with respect to field size, depth, and phantom material were investigated by performing additional simulations. Changes to the detectors' readings due to the presence of small asymmetrical air gaps (0.1 up to 1 mm) around the chambers'sensitive volume in an RW3 solid phantom were quantified for three small-cavity chambers and two orientations. Results: For both parallel (to the magnetic field) orientations, kB,Q values were found close to unity. The maximum correction needed was 1.1%.For each detector studied, the kB,Q values calculated for the two parallel orientations agreed within uncertainties. Larger corrections (up to 5%) were calculated when the detectors were oriented perpendicularly to themagnetic field. Results were compared with corresponding ones found in the literature, wherever available. No considerable dependence of kB,Q with respect to field size (down to 3 × 3 cm2), depth, or phantom material was noticed, for the detectors investigated. As compared to the perpendicular one, in the parallel to the magnetic field orientation, the air gap effect is minimized but is still considerable even for the smallest air gap considered (0.1 mm). Conclusion: For the 10 × 10 cm2 field, magnetic field correction factors for 12 ionization chambers and four orientations were determined. For each detector, the kB,Q value may be also applied for dosimetry procedures under different irradiation parameters provided that the orientation is taken into account. Moreover, if solid phantoms are used, even the smallest asymmetrical air gap may still bias small-cavity chamber response. This work substantially expands the availability and applicability of kB,Q correction factors that are detector- and orientationspecific, enabling more options in MR-Linac dosimetry checks, end-to-end tests, and quality assurance protocols. [ABSTRACT FROM AUTHOR]

Background: 7T MRI offers significant benefits to spatial and contrast resolution compared to lower field strengths. This superior image quality can help better delineate targets in stereotactic neurosurgical procedures; however, the potential for increased geometric distortions at 7T has impaired its widespread use for these applications. Image geometric distortions can be due to distortions of B0 arising from tissue magnetic susceptibility effects or inherent field inhomogeneities, and nonlinearity of the magnetic field gradients. Purpose: The purpose of this study was to investigate the use of 7T MRI for neurosurgical frameless stereotactic navigation procedures. Image geometric distortions at the skin surface in 7T images were minimized and compared to results from clinical 3T frameless imaging protocols. Methods: A 3D-printed grid phantom filled with oil was designed to perform a fine calibration of the 7T imaging gradients, and an oil-filled head phantom with internal targets was used to determine ground truth (from computed tomography [CT]) positioning errors. Three volunteers and the head phantom were imaged consecutively at 3T and 7T. Ten skin-adhesive fiducial markers were placed on each subject's exposed skin surface at standard clinical placement locations for frameless procedures. Imaging sequences included MPRAGE (three bandwidths at 7T: 400, 690, and 1020 Hz/pixel, and one at 3T: 400 Hz/pixel), T2 SPACE, and T2 SPACE FLAIR acquisitions. An additional GRE field map was acquired on both scanners using a multi-echo GRE sequence. Custom Matlab code was used to perform additional distortion correction of the images using the unwrapped field maps. Fiducial localization was performed with 3D Slicer, with absolute fiducial positioning errors determined in phantom experiments following rigid registration to the CT images. For human experiments, 3T and 7T images were registered and relative differences in fiducial locations were compared using two-tailed paired t-tests. Results: Phantom measurements at 7T yielded gradient distance scaling errors of 1.1%, 2.2%, and 1.0% along the x-, y-, and z-axes, respectively. These system miscalibrations were traced back to phantom manufacturing deviations in the sphericity of the vendor's gradient calibration phantom. Correction factors along each gradient axis were applied, and afterward, geometric distortions of less than 1 mm were obtained in the 7T MR head phantom images for the 1020 Hz/pixel bandwidth MPRAGE sequence. For the human subjects, four fiducial locations were excluded from the analysis due to patient positioning differences. Differences between 3T and 7T MPRAGE with low/medium/high bandwidth were 2.2/2.6/2.3 mm, respectively, before the correction, reducing to 1.6/1.3/1.0 mm after the correction (p < 0.001). T2 SPACE and T2 SPACE FLAIR yielded a similar pattern when the correction was applied, decreasing from 2.1 to 0.8 mm, and 2.6 to 1.0 mm, respectively. Conclusions: 7T MRI can be used to perform frameless presurgical planning with skin-adhesive fiducials. Geometric distortions can be reduced to a clinically relevant level (errors < ~1 mm) with no significant susceptibility-related distortions, by using high receiver bandwidth, ensuring gradients are properly calibrated, and placing skin fiducials in areas where distortions from patient positioning are minimal. [ABSTRACT FROM AUTHOR]

Purpose: The dosimetric effect of edema on prostate implants have long been realized, but large uncertainties still exist in the estimation of dose actually received by the prostate. This study attempted to develop a new method to accurately estimate dose delivered to the prostate accounting for the variation of prostate volume and seed distribution, edema half‐lives, and times for postimplant evaluation. Methods and materials: A series of prostate seed implants for Cs‐131, Pd‐103, and I‐125 with various prostate volumes were simulated in a water phantom using the TG‐43 algorithm on the Varian Eclipse treatment planning system. Dose analysis was performed to derive a quantitative relationship between the prostate peripheral dose and the prostate radius with the variation of prostate volume and seed distribution. Using this relationship to calculate dynamically, the total dose accumulated in the prostate (DT) accounting for the variation of prostate volume and seed distribution and edema half‐lives. Moreover, the total dose can be estimated statically based on the prostate volume that can be determined in a computerized tomography (CT) image taken at a time after implantation. The statically estimated total dose (DCT) was compared with DT to determine optimal imaging times as well as dose correction factors for other imaging times. Results: An inverse power law was established between the prostate peripheral dose and prostate radius. The value of the power was 1.3 for Cs‐131 and I‐125, and 1.5 for Pd‐103, respectively. DT was derived dynamically using the inverse power law. Given the edema half‐lives, TE, of 4, 9.3, and 25 days and the volume expansion of 1.1 and 2.0 times of the prostate without edema, the optimal times for postimplant imaging were: 7, 9, and 16 days for TE = 4 days; 10, 14, and 28 days for TE = 9.3 days; and 12, 19, 45 days TE = 25 days, for Cs‐131, Pd‐103, and I‐125, respectively. DCT calculated using the prostate volume determined on the optimal days agreed with DT to 0.0%–1.8% and within 0.3% for most cases. For various prostate volumes, edema half‐lives, and nonoptimal times, DCT was able to achieve a 1% accuracy. Conclusion: The postimplant dose calculation based on the proposed inverse power law for prostate seed implants with edema has improved the accuracy of postimplant dosimetry with accurate and patient‐specific dose corrections accounting for prostate size, edema half‐life, and postimplant imaging times. Optimal times for postimplant imaging have been accurately determined, and the high accuracy of postimplant dose calculation can be achieved for both optimal imaging times and nonoptimal imaging times. [ABSTRACT FROM AUTHOR]

Background: In reference dosimetry, radiation quality correction factors are used in order to account for changes in the detector's response among different radiation qualities, improving dosimetric accuracy. Purpose: Reference dosimetry radiation quality corrections factors for the PTW microDiamond were calculated for preclinical X‐ray and proton minibeams, and their impact in dosimetric accuracy was evaluated. Methods: A formalism for the calculation of radiation quality correction factors for absolute dosimetry in minibeam fields was developed. Following our formalism, radiation quality correction factors were calculated for the PTW microDiamond detector, using the Monte Carlo method. Models of the detector, and X‐ray and proton irradiation platform, were imported into the TOPAS Monte Carlo simulation toolkit. The radiation quality correction factors were calculated in the following scenarios: (i) reference dosimetry open field to minibeam center of the central peak, (ii) different positions at the minibeam profile (along the peaks and valleys direction) to the center of the central minibeam, and (iii) some representative depth positions. In addition, the radiation quality correction factors needed for the calculation of the peak‐to‐valley dose ratio at different depths were calculated. Results: An important overestimation of the dose (about 10%) was found in the case of the open to minibeam field for both X‐rays and proton beams, when the correction factors were used. Smaller differences were observed in the other cases. Conclusions: The usage of the PTW microDiamond detector requires radiation quality correction factors in order to be used in minibeam reference dosimetry. [ABSTRACT FROM AUTHOR]

Background: With the emergence of more complex and novel proton delivery techniques, there is a need for quality assurance tools with high spatiotemporal resolution to conveniently measure the spatial and temporal properties of the beam. In this context, scintillation‐based dosimeters, if synchronized with the radiation beam and corrected for ionization quenching, are appealing. Purpose: To develop a synchronized high‐speed scintillation imaging system for characterization and verification of the proton therapy beams on a pulse‐by‐pulse basis. Materials and methods: A 30 cm × 30 cm × 5 cm block of BC‐408 plastic scintillator placed in a light‐tight housing was irradiated by proton beams generated by a Mevion S250 proton therapy synchrocyclotron. A high‐speed camera system, placed perpendicular to the beam direction and facing the scintillator, was synchronized to the accelerator's pulses to capture images. Opening and closing of the camera's shutter was controlled by setting a proper time delay and exposure time, respectively. The scintillation signal was recorded as a set of two‐dimensional (2D) images. Empirical correction factors were applied to the images to correct for the nonuniformity of the pixel sensitivity and quenching of the scintillator. Proton range and modulation were obtained from the corrected images. Results: The camera system was able to capture all data on a pulse‐by‐pulse basis at a rate of ∼504 frames per second. The applied empirical correction method for ionization quenching was effective and the corrected composite image provided a 2D map of dose distribution. The measured range (depth of distal 90%) through scintillation imaging agreed within 1.2 mm with that obtained from ionization chamber measurement. Conclusion: A high‐speed camera system capable of capturing scintillation signals from individual proton pulses was developed. The scintillation imaging system is promising for rapid proton beam characterization and verification. [ABSTRACT FROM AUTHOR]

The clinical translation of FLASH radiotherapy (RT) requires challenges related to dosimetry and beam monitoring of ultra‐high dose rate (UHDR) beams to be addressed. Detectors currently in use suffer from saturation effects under UHDR regimes, requiring the introduction of correction factors. There is significant interest from the scientific community to identify the most reliable solutions and suitable experimental approaches for UHDR dosimetry. This interest is manifested through the increasing number of national and international projects recently proposed concerning UHDR dosimetry. Attaining the desired solutions and approaches requires further optimization of already established technologies as well as the investigation of novel radiation detection and dosimetry methods. New knowledge will also emerge to fill the gap in terms of validated protocols, assessing new dosimetric procedures and standardized methods. In this paper, we discuss the main challenges coming from the peculiar beam parameters characterizing UHDR beams for FLASH RT. These challenges vary considerably depending on the accelerator type and technique used to produce the relevant UHDR radiation environment. We also introduce some general considerations on how the different time structure in the production of the radiation beams, as well as the dose and dose‐rate per pulse, can affect the detector response. Finally, we discuss the requirements that must characterize any proposed dosimeters for use in UDHR radiation environments. A detailed status of the current technology is provided, with the aim of discussing the detector features and their performance characteristics and/or limitations in UHDR regimes. We report on further developments for established detectors and novel approaches currently under investigation with a view to predict future directions in terms of dosimetry approaches, practical procedures, and protocols. Due to several on‐going detector and dosimetry developments associated with UHDR radiation environment for FLASH RT it is not possible to provide a simple list of recommendations for the most suitable detectors for FLASH RT dosimetry. However, this article does provide the reader with a detailed description of the most up‐to‐date dosimetric approaches, and describes the behavior of the detectors operated under UHDR irradiation conditions and offers expert discussion on the current challenges which we believe are important and still need to be addressed in the clinical translation of FLASH RT. [ABSTRACT FROM AUTHOR]

Purpose: Silicon diodes are often the detector of choice for relative dose measurements, particularly in the context of radiotherapy involving small photon beams. However, a major drawback lies in their dose‐rate dependency. Although ionization chambers are often too large for small field output factor (OF) measurements, they are valuable instruments to provide reliable percent‐depth dose (PDD) curves in reference beams. The aim of this work is to propose a practical and accurate method for the characterization of silicon diode dose‐rate dependence correction factors using ionization chamber measurements as a reference. Methods: The robustness of ionization chambers for PDD measurements is used to quantify the dose‐rate dependency of a diode detector. A mathematical formalism, which exploits the error induced in percent‐depth ionization (PDI) curves for diodes by their dose‐rate dependency, is developed to derive a dose‐rate correction factor applicable to diode relative measurements. The method is based on the definition of the recombination correction factor given in the addendum to TG 51 and is applied to experimental measurements performed on a CyberKnife M6 radiotherapy unit using a PTW 60012 diode detector. A measurement‐based validation is provided by comparing corrected PDI curves to measurements performed with a PTW 60019 diamond detector, which does not exhibit dose‐rate dependence. Results: Results of dose‐rate correction factors for PDI curves, off‐axis ratios (OARs), tissue‐phantom ratios, and small field OFs are coherent with the expected behavior of silicon diode detectors. For all considered setups and field sizes, the maximum correction and the maximum impact of the uncertainties induced by the correction are obtained for OARs for the 60 mm collimator, with a correction of 2.5% and an uncertainty of 0.34%. For OFs, corrections range from 0.33% to 0.82% for all field sizes considered, and increase with the reduction of the field size. Comparison of PDI curves corrected for dose‐rate and for in‐depth beam quality variations illustrates excellent agreement with measurements performed using the diamond detector. Conclusion: The proposed method allows the efficient and precise correction of the dose‐rate dependence of silicon diode detectors in the context of clinical relative dosimetry. [ABSTRACT FROM AUTHOR]

Purpose: Scintillation detectors were 3D printed based on a gamma knife (GK) dose distribution to calculate the volume averaging effect. The collimator output factors were measured using isodose‐shaped scintillators (ISSs) and compared with those of a micro‐diamond detector and previous reports. Methods: An absorbed dose distribution in a spherical dosimetry phantom with a radius of 8 cm was obtained from GK treatment planning software (Leksell GammaPlan [LGP], Elekta AB, Stockholm, Sweden). Two types of ISSs were fabricated to fit the 97.2% (ISS‐1) and 95.6% (ISS‐2) isodose surfaces. The volume averaging correction factors were obtained by dividing the absorbed dose to water in the central voxel (CV) by that in the ISS. The correction effect due to the difference between the ISS and water was calculated by Monte Carlo simulations. Ten ISS detectors, five of each type, were used to measure the output factors of the 4‐ and 8‐mm collimators of a GK Icon to assess system consistency. The output factors of seven GKs were measured using two ISS detectors, one of each type, and a PTW T60019 (PTW, Freiburg, Germany) micro‐diamond detector. Results: The detector output ratios (DORs) measured using the five ISSs of each type were consistent, with standard uncertainties less than 0.2%. In the 4‐mm field, the volume averaging correction factor ratios were 1.018 and 1.026, and the output factors after all corrections were 0.827 (0.006) and 0.825 (0.006) for ISS‐1 and ISS‐2, respectively. In the 8‐mm field, the volume averaging correction factor ratios were 1.000 for both ISS types, and the output factors were 0.898 (0.003) and 0.900 (0.003) for ISS‐1 and ISS‐2, respectively. The ISS detectors could measure the output factors of a GK with uncertainties comparable to that of the PTW 60019 detector. The output factors of all detectors decreased with the dose rate. Conclusion: The volume averaging effect of an ISS developed in‐house could be calculated using known dose distributions. The collimator output factors of the GK Perfexion/Icon models measured using ISS detectors were consistent with those of a commercial synthetic micro‐diamond detector and recent studies. [ABSTRACT FROM AUTHOR]

Purpose: The RefleXion™ X1 is a novel radiotherapy system that is designed for image‐guided radiotherapy, and eventually, biology‐guided radiotherapy (BgRT). BgRT is a treatment paradigm that tracks tumor motion using real‐time positron emission signals. This study reports the small‐field measurement results and the validation of a Monte Carlo (MC) model of the first clinical RefleXion unit. Methods: The RefleXion linear accelerator (linac) produces a 6 MV flattening filter free (FFF) photon beam and consists of a binary multileaf collimator (MLC) system with 64 leaves and two pairs of y‐jaws. The maximum clinical field size achievable is 400 × 20 mm2. The y‐jaws provide either a 10 or 20 mm opening at source‐to‐axis distance (SAD) of 850 mm. The width of each MLC leaf at SAD is 6.25 mm. Percentage depth doses (PDDs) and relative beam profiles were acquired using an Edge diode detector in a water tank for field sizes from 12.5 × 10 to 100 × 20 mm2. Beam profiles were also measured using films. Output factors of fields ranging from 6.25 × 10 to 100 × 20 mm2 were measured using W2 scintillator detector, Edge detector, and films. Output correction factors k of the Edge detector for RefleXion were calculated. An MC model of the linac including pre‐MLC beam sources and detailed structures of MLC and lower y‐jaws was validated against the measurements. Simulation codes BEAMnrc and GATE were utilized. Results: The diode measured PDD at 10 cm depth (PDD10) increases from 53.6% to 56.9% as the field opens from 12.5 × 10 to 100 × 20 mm2. The W2‐measured output factor increases from 0.706 to 1 as the field opens from 6.25 × 10 to 100 × 20 mm2 (reference field size). The output factors acquired by diode and film differ from the W2 results by 1.65% (std = 1.49%) and 2.09% (std = 1.41%) on average, respectively. The profile penumbra and full‐width half‐maximum (FWHM) measured by diode agree well with the film results with a deviation of 0.60 mm and 0.73% on average, respectively. The averaged beam profile consistency calculated between the diode‐ and film‐measured profiles among different depths is within 1.72%. By taking the W2 measurements as the ground truth, the output correction factors k for Edge detector ranging from 0.958 to 1 were reported. For the MC model validation, the simulated PDD10 agreed within 0.6% to the diode measurement. The MC‐simulated output factor differed from the W2 results by 2.3% on average (std = 3.7%), while the MC simulated beam penumbra differed from the diode results by 0.67 mm on average (std = 0.42 mm). The MC FWHM agreed with the diode results to within 1.40% on average. The averaged beam profile consistency calculated between the diode and MC profiles among different depths is less than 1.29%. Conclusions: This study represents the first small‐field dosimetry of a clinical RefleXion system. A complete and accurate MC model of the RefleXion linac has been validated. [ABSTRACT FROM AUTHOR]

Purpose: Determination of absorbed dose is well established in many dosimetry protocols and considered to be highly reliable using ionization chambers under reference conditions. If dosimetry is performed under other conditions or using other detectors, however, open questions still remain. Such questions frequently refer to appropriate correction factors. A converted energy per mass (cema)‐based approach to formulate such correction factors offers a good understanding of the specific response of a detector for dosimetry under various measuring conditions and thus an estimate of pros and cons of its application. Methods: Determination of absorbed dose requires the knowledge of the beam quality correction factor kQ,Qo, where Q denotes the quality of a user beam and Qo is the quality of the radiation used for calibration. In modern Monte Carlo (MC)‐based methods, kQ,Qo is directly derived from the MC‐calculated dose conversion factor, which is the ratio between the absorbed dose at a point of interest in water and the mean absorbed dose in the sensitive volume of an ion chamber. In this work, absorbed dose is approximated by the fundamental quantity cema. This approximation allows the dose conversion factor to be substituted by the cema conversion factor. Subsequently, this factor is decomposed into a product of cema ratios. They are identified as the stopping power ratio water to the material in the sensitive detector volume, and as the correction factor for the fluence perturbation of the secondary charged particles in the detector cavity caused by the presence of the detector. This correction factor is further decomposed with respect to the perturbation caused by the detector cavity and that caused by external detector properties. The cema‐based formalism was subsequently tested by MC calculations of the spectral fluence of the secondary charged particles (electrons and positrons) under various conditions. Results: MC calculations demonstrate that considerable fluence perturbation may occur particularly under non‐reference conditions. Cema‐based correction factors to be applied in a 6‐MV beam were obtained for a number of ionization chambers and for three solid‐state detectors. Feasibility was shown at field sizes of 4 × 4 and 0.5 cm × 0.5 cm. Values of the cema ratios resulting from the decomposition of the dose conversion factor can be well correlated with detector response. Under the small field conditions, the internal fluence correction factor of ionization chambers is considerably dependent on volume averaging and thus on the shape and size of the cavity volume. Conclusions: The cema approach is particularly useful at non‐reference conditions including when solid‐state detectors are used. Perturbation correction factors can be expressed and evaluated by cema ratios in a comprehensive manner. The cema approach can serve to understand the specific response of a detector for dosimetry to be dependent on (a) radiation quality, (b) detector properties, and (c) electron fluence changes caused by the detector. This understanding may also help to decide which detector is best suited for a specific measurement situation. [ABSTRACT FROM AUTHOR]

Purpose: The safety and clinical efficacy of 125I seed‐loaded stent for the treatment of portal vein tumor thrombosis (PVTT) have been shown. Accurate and fast dose calculation of the 125I seeds with the presence of the stent is necessary for the plan optimization and evaluation. However, the dosimetric characteristics of the seed‐loaded stents remain unclear and there is no fast dose calculation technique available. This paper aims to explore a fast and accurate analytical dose calculation method based on Monte Carlo (MC) dose calculation, which takes into account the effect of stent and tissue inhomogeneity. Methods: A detailed model of the seed‐loaded stent was developed using 3D modeling software and subsequently used in MC simulations to calculate the dose distribution around the stent. The dose perturbation caused by the presence of the stent was analyzed, and dose perturbation kernels (DPKs) were derived and stored for future use. Then, the dose calculation method from AAPM TG‐43 was adapted by integrating the DPK and appropriate inhomogeneity correction factors (ICF) to calculate dose distributions analytically. To validate the proposed method, several comparisons were performed with other methods in water phantom and voxelized CT phantoms for three patients. Results: The stent has a considerable dosimetric effect reducing the dose up to 47.2% for single‐seed stent and 11.9%–16.1% for 16‐seed stent. In a water phantom, dose distributions from MC simulations and TG‐43‐DP‐ICF showed a good agreement with the relative error less than 3.3%. In voxelized CT phantoms, taking MC results as the reference, the relative errors of TG‐43 method can be up to 33%, while those of TG‐43‐DP‐ICF method were less than 5%. For a dose matrix with 256 × 256 × 46 grid (corresponding to a phantom of 17.2 × 17.2 × 11.5 cm3) for 16‐seed‐loaded stent, it only takes 17 s for TG‐43‐DP‐ICF to compute, compared to 25 h for the full MC calculation. Conclusions: The combination of DPK and inhomogeneity corrections is an effective approach to handle both the presence of stent and tissue heterogeneity. Exhibiting good agreement with MC calculation and computational efficiency, the proposed TG‐43‐DP‐ICF method is adequate for dose evaluation and optimization in seed‐loaded stent implantation treatment planning. [ABSTRACT FROM AUTHOR]

Purpose: Previous publications have described how the standard temperature and pressure correction will overcorrect measurements with a low‐energy photon low‐dose rate brachytherapy source at low ambient air pressures. To account for this effect, an additional correction factor is applied after the standard temperature and pressure correction. This additional correction is dependent on the source being measured and the chamber it is measured in. Well chamber corrections for two sources and findings regarding aspects that may affect the altitude response of the sources are presented. Methods: A purpose‐built pressure vessel was constructed previously, which could achieve pressures ranging from 74.661 to 106.66 kPa (560–800 mmHg). Three Cesium Blu sources (131Cs) from Isoray Inc. and three CivaDots (103Pd) from CivaTech Oncology Inc. were tested over this pressure range in increments of 2.7 kPa (20 mmHg) in three HDR 1000 Plus chambers, and the Cesium Blu sources were also tested in two IVB 1000 chambers. Both chamber models are air communicating well‐type ionization chambers produced by Standard Imaging Inc. Multiple runs of each source/chamber combination were completed, corrected with the standard temperature and pressure correction, normalized to the result at 101.325 kPa, and averaged with runs of the same combination. The chamber response was also simulated using MCNP6 to validate the experimental results. Results: Measurements of both sources in all chambers followed the expected power dependence on ambient pressure as seen in previous studies. The Cesium Blu source, however, demonstrated a significant difference in response in the HDR 1000 Plus chamber versus the IVB 1000 chamber. For an altitude correction factor of the form, PA = k1(P)k2, new coefficients are proposed for both sources for pressure units of kPa and mmHg. The Monte Carlo calculated chamber response agreed with the experimental results within 2% for all sources and chambers at all pressures. Conclusions: Altitude correction coefficients for two new low‐energy photon low‐dose rate brachytherapy sources are provided. The directional dependence of the CivaDot has no bearing on its dependence on pressure; however, the difference in construction materials from other 103Pd sources leads to unique correction coefficients. The higher energy of the Cesium Blu source with respect to 103Pd and 125I sources yields a difference in correction factors depending on which model chamber is used for air‐kerma strength calculations. Clinics must be careful to select the correct pair of coefficients for the chamber model they used. [ABSTRACT FROM AUTHOR]

Purpose: To obtain individualized internal doses with a Monte Carlo (MC) method in patients undergoing diagnostic [18F]FCH‐PET studies and to compare such doses with the MIRD method calculations. Methods: A patient cohort of 17 males were imaged after intravenous administration of a mean [18F]FCH activity of 244.3 MBq. The resulting PET/CT images were processed in order to generate individualized input source and geometry files for dose computation with the MC tool GATE. The resulting dose estimates were studied and compared to the MIRD method with two different computational phantoms. Mass correction of the S‐factors was applied when possible. Potential sources of uncertainty were closely examined: the effect of partial body images, urinary bladder emptying, and biokinetic modeling. Results: Large differences in doses between our methodology and the MIRD method were found, generally in the range ±25%, and up to ±120% for some cases. The mass scaling showed improvements, especially for non‐walled and high‐uptake tissues. Simulations of the urinary bladder emptying showed negligible effects on doses to other organs, with the exception of the prostate. Dosimetry based on partial PET/CT images (excluding the legs) resulted in an overestimation of mean doses to bone, skin, and remaining tissues, and minor differences in other organs/tissues. Estimated uncertainties associated with the biokinetics of FCH introduce variations of cumulated activities in the range of ±10% in the high‐uptake organs. Conclusions: The MC methodology allows for a higher degree of dosimetry individualization than the MIRD methodology, which in some cases leads to important differences in dose values. Dosimetry of FCH‐PET based on a single partial PET study seems viable due to the particular biokinetics of FCH, even though some correction factors may need to be applied to estimate mean skin/bone doses. [ABSTRACT FROM AUTHOR]

Purpose: In previous work, we demonstrated that mixed electron‐photon radiation therapy (MBRT) produces treatment plans with improved normal tissue sparing and similar target coverage, when compared to photon‐only plans. The purpose of this work was to validate the MBRT delivery process on a Varian TrueBeam accelerator and laying the groundwork for a patient‐specific quality assurance (QA) protocol based on ion chamber point measurements and 2D film measurements. Methods: MC beam models used to calculate the MBRT dose distributions of each modality (photons/electrons) were validated with a single‐angle beam MBRT treatment plan delivered on a slab of Solid Water phantom with a film positioned at a depth of 2 cm. The measured film absorbed dose was compared to the calculated dose. To validate clinical deliveries, a polymethyl methacrylate (PMMA) cylinder was machined and holes were made to fit an ionization chamber. A complex MBRT plan involving a photon arc and three electron delivery angles was created with the aim of reproducing a clinically realistic dose distribution in typical soft tissue sarcoma tumours of the extremities. The treatment plan was delivered on the PMMA cylinder. Point measurements were taken with an Exradin A1SL chamber at two nominal depths: 1.4 cm and 2.1 cm. The plan was also delivered on a second identical phantom with an insert at 2 cm depth, where a film was placed. An existing EGSnrc user‐code, SPRRZnrc, was modified to calculate the stopping power ratios between any materials in the same voxelized geometry used for dose calculation purposes. This modified code, called SPRXYZnrc, was used to calculate a correction factor, kMBRT, accounting for the differences in electron fluence spectrum at the measurement point compared to that at reference conditions. The uncertainty associated with neglecting potential ionization chamber fluence perturbation correction factors using this approach was estimated. Results: The film measurement from the Solid Water phantom treatment plan was in good agreement with the simulated dose distribution, with a gamma pass rate of 96.1% for a 3%/2 mm criteria. For the PMMA phantom delivery, for the same gamma criteria, the pass rate was 97.3%. The ion chamber measurements of the total delivered dose agreed with the MC‐simulated dose within 2.1%. The beam quality correction factors amounted to, at most, a 4% correction on the ion chamber measurement. However, individual contribution of low electron energies proved difficult to precisely measure due to their steep dose gradients, with disagreements of up to 28% ± 15% at 2.1 cm depth (6 MeV). Ion chamber measurement procedure of electron beams was achieved in less than 5 min, and the entire validation process including phantom setup was performed in less than 30 min. Conclusion: The agreement between measured and simulated MBRT doses indicates that the dose distributions obtained from the MBRT treatment planning algorithm are realistically achievable. The SPRXYZnrc MC code allowed for convenient calculations of kMBRT simultaneously with the dose distributions, laying the groundwork for patient‐specific QA protocol practical for clinical use. Further investigation is needed to establish the accuracy of our ionization chamber correction factors kMBRT calculations at low electron energies. [ABSTRACT FROM AUTHOR]

Purpose: Radiochromic film has been established as a detector that can be used without the need for perturbation correction factors for small field dosimetry in water. However, perturbation factors in low density media such as lung have yet to be published. This study calculated the factors required to account for the perturbation of radiochromic film when used for small field dosimetry in lung equivalent material. Method: Monte Carlo simulations were used to calculate dose to Gafchromic EBT3 film when placed inside a lung phantom. The beam simulated had a nominal energy of 6 MV and the field sizes simulated ranged from 10 × 10 mm2 to 30 × 30 mm2. The lung density simulated was varied between 0.2 and 0.3 g/cm3. Each simulation was repeated with the film replaced by lung material (the same as the surrounding medium), and the required correction factors for film dosimetry in lung (DMed,QDDet,Q) were calculated by dividing the dose in lung by the dose in film. Results: For field sizes 30 × 30 mm2 and larger, no correction factors were required. At a 20 × 20 mm2 field size, small corrections were required, but were within the approximate accuracy of film dosimetry (~2%). For a 10 × 10 mm2 field size, significant correction factors need to be applied (0.935 for lung density of 0.20 g/cm3 to 0.963 for lung density of 0.30 g/cm3). The values lower than one mean that the film is over‐responding. At the "upstream" lung‐water interface the correction factors were close to unity; while at the downstream interface the corrections required were marginally smaller to those at the center of lung. One centimeter or more away from the interfaces, the correction factor did not vary as a function distance from the interface (in the beam direction). Away from the central axis (perpendicular to the beam direction), the correction factors increased slightly (away from unity) as a function of off‐axis distance, before abruptly changing direction at the penumbra, with the film actually under‐responding by ~10% outside the field edges. Conclusion: Accurate dosimetry of very small fields (15 × 15 mm2 or smaller) using radiochromic film requires correction factors for the perturbation of the film on the surrounding lung material. This correction factor was as high as 6.5% for a 10 × 10 mm2 field size and a density of 0.2 g/cm3. This will increase if either the density or the field size decrease further. This correction factor does not vary as a function of depth in lung once charged particle equilibrium is established. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study is to provide data for the calibration of the recent RefleXionTM biology‐guided radiotherapy (BgRT) machine (Hayward, CA, USA) following the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS‐483 code of practice (COP) (Palmans et al. International Atomic Energy Agency, Vienna, 2017) and (Mirzakhanian et al. Med Phys, 2020). Methods: In RefleXion BgRT machine, reference dosimetry was performed using two methodologies described in TRS‐483 and (Mirzakhanian et al. Med Phys, 2020) In the first approach (Approach 1), the generic beam quality correction factor kQA,Q0fA,fref was calculated using an accurate Monte Carlo (MC) model of the beam and of six ionization chamber types. The kQA,Q0fA,fref is a beam quality factor that corrects ND,w,Q0fref (absorbed dose to water calibration coefficient in a calibration beam quality Q0) for the differences between the response of the chamber in the conventional reference calibration field fref with beam quality Q0 at the standards laboratory and the response of the chamber in the user's A field fA with beam quality QA. Field A represents the reference calibration field that does not fulfill msr conditions. In the second approach (Approach 2), a square equivalent field size was determined for field A of 10×2cm2 and 10×3cm2. Knowing the equivalent field size, the beam quality specifier for the hypothetical 10×10cm2 field size was derived. This was used to calculate the beam quality correction factor analytically for the six chamber types using the TRS‐398. (Andreo et al. Int Atom Energy Agency 420, 2001) Here, TRS‐398 was used instead of TRS‐483 since the beam quality correction values for the chambers used in this study are not tabulated in TRS‐483. The accuracy of Approach 2 is studied in comparison to Approach 1. Results: Among the chambers, the PTW 31010 had the largest kQA,Q0fA,fref correction due to the volume averaging effect. The smallest‐volume chamber (IBA CC01) had the smallest correction followed by the other microchambers Exradin‐A14 and ‐A14SL. The equivalent square fields sizes were found to be 3.6 cm and 4.8 cm for the 10×2cm2 and 10×3cm2 field sizes, respectively. The beam quality correction factors calculated using the two approaches were within 0.27% for all chambers except IBA CC01. The latter chamber has an electrode made of steel and the differences between the correction calculated using the two approaches was the largest, that is, 0.5%. Conclusions: In this study, we provided the kQA,Q0fA,fref values as a function of the beam quality specifier at the RefleXion BgRT setup (TPR20,10(S) and %dd(10,S)x) for six chamber types. We suggest using the first approach for calibration of the RefleXion BgRT machine. However, if the MC correction is not available for a user's detector, the user can use the second approach for estimating the beam quality correction factor to sufficient accuracy (0.3%) provided the chamber electrode is not made of high Z material. [ABSTRACT FROM AUTHOR]

Purpose: Radiosurgery using the Leksell Gamma Knife® (LGK) Icon™ is an established technique used for treating intracranial lesions. The largest beam field size the LGK Icon can produce is a 16 mm diameter sphere. Despite this, reference dosimetry on the LGK Icon is typically performed using ionization chambers calibrated in 10 × 10 cm2 fields. Furthermore, plastic phantoms are widely used instead of liquid water phantoms. In an effort to resolve these issues, the International Atomic Energy Agency (IAEA) in collaboration with American Association of Physicists in medicine (AAPM) recently published Technical Report Series No. 483 (TRS‐483) as a Code of Practice for small‐field dosimetry. TRS‐483 includes small‐field correction factors, kQmsr,Q0fmsr,fref, intended to account for the differences between setups when using small‐field modalities such as the LGK Icon, and conventional setups. Since the publication of TRS‐483, at least three new sets of values of kQmsr,Q0fmsr,fref for the LGK Icon have been published. The purpose of this study was to experimentally investigate the published values of kQmsr,Q0fmsr,fref for commonly used phantom and ionization chamber (IC) models for the LGK Icon. Methods: Dose‐rates from two LGK units were determined using acrylonitrile butadiene styrene (ABS) and Certified Medical Grade Solid Water® (SW) phantoms, and PTW 31010 and PTW 31016 ICs. Correction factors were applied, and the resulting dose‐rates compared. Relative validity of the correction factors was investigated by taking the ratios of dose‐rate correction factor products. Additionally, dose‐rates from the individual sectors were determined in order to calculate the beam attenuation caused by the ABS phantom adapter. Results and Conclusions: It was seen that the dose‐rate is underestimated by at least 1% when using the ABS phantom, which was attributed to fluence perturbation caused by the IC and phantom adapter. Published correction factors kQmsr,Q0fmsr,fref account for these effects to varying degree and should be used. The SW phantom is unlikely to underestimate the dose‐rate by more than 1%, and applying kQmsr,Q0fmsr,fref could not be shown to be necessary. Out of the two phantom models, the ABS phantom is not recommended for use in LGK reference dosimetry. The use of newly published values of kQmsr,Q0fmsr,fref should be considered. [ABSTRACT FROM AUTHOR]

Purpose: The local depth dose gradient and the displacement correction factor for Farmer‐type ionization chambers are quantified for reference dosimetry at shallow depth in single‐layer scanned proton fields. Method: Integrated radial profiles as a function of depth (IRPDs) measured at three proton therapy centers were smoothed by polynomial fits. The local relative depth dose gradient at measurement depths from 1 to 5 cm were derived from the derivatives of those fits. To calculate displacement correction factors, the best estimate of the effective point of measurement was derived from reviewing experimental and theoretical determinations reported in the literature. Displacement correction factors for the use of Farmer‐type ionization chambers with their reference point (at the center of the cavity volume) positioned at the measurement depth were derived as a ratio of IRPD values at the measurement depth and at the effective point of measurement. Results: Depth dose gradients are as low as 0.1–0.4% per mm at measurement depths from 1 to 5 cm in the highest clinical proton energies (with residual ranges higher than 15 cm) and increase to 1% per mm at a residual range of 4 cm and become larger than 3% per mm for residual ranges lower than 2 cm. The literature review shows that the effective point of measurement of Farmer‐type ionization chambers is, similarly as for carbon ion beams, located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. If a maximum displacement correction of 2% is deemed acceptable to be included in calculated beam quality correction factors, Farmer‐type ICs can be used at measurements depths from 1 to 5 cm for which the residual range is 4 cm or larger. If one wants to use the same beam quality correction factors as applicable to the conventional measurement point for scattered beams, located at the center of the SOBP, the relative standard uncertainty on the assumption that the displacement correction factor is unity can be kept below 0.5% for measurement depths of at least 2 cm and for residual ranges of 15 cm or higher. Conclusion: The literature review confirmed that for proton beams the effective point of measurement of Farmer‐type ionization chambers is located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. Based on the findings in this work, three options can be recommended for reference dosimetry of scanned proton beams using Farmer‐type ionization chambers: (a) positioning the effective point of measurement at the measurement depth, (b) positioning the reference point at the measurement depth and applying a displacement correction factor, and (c) positioning the reference point at the measurement depth without applying a displacement correction factor. Based on limiting the acceptable uncertainty on the gradient correction factor to 0.5% and the maximum deviation of the displacement perturbation correction factor from unity to 2%, the first two options can be allowed for residual ranges of at least 4 cm while the third option only for residual ranges of at least 15 cm. [ABSTRACT FROM AUTHOR]

Purpose: This study aims to investigate the dose response of diode‐type detectors in the presence of strong magnetic field and to understand the underlying mechanisms leading to the observed magnetic field dependence by close examinations on the role of the detector's design. Materials and methods: Three clinical diode‐type detectors (PTW microSilicon type 60023, PTW microDiamond type 60019, and IBA Razor diode) have been studied. Measurements were performed at the linear accelerator experimental facility of the German National Metrology Institute (PTB, Braunschweig) with electromagnets up to 1.4 T to obtain the magnetic field correction factors kB,Q. The experimental results were compared to Monte Carlo simulations. Stepwise modifications of the detectors' models were performed to characterize the contributions of the structural components toward the magnetic field‐dependent dose response. Additionally, systematic Monte Carlo study was conducted to elucidate the influence of the structural layers with varying density located above and beneath the detector's sensitive volume. Results: The dose response of all investigated detectors decreases with magnetic field. As a result, the associated kB,Q factors increase by approximately 10% for the PTW detectors, and by 5% for the IBA Razor diode at 1.5 T. The sensitive volume itself was shown to cause negligible effect but the diode substrate with enhanced density situated directly below the sensitive volume contributes strongest to the observed magnetic field dependence. Systematic simulations revealed that kB,Q increases with magnetic field if the density of the structural layer located beneath the sensitive volume is higher than that of normal water (>1 g/cm3). In the case where the layer consists of low‐density water (1.2 mg/cm3), kB,Q decreases with the magnetic field strength. On the contrary , if the structural layer with varying density is situated above the sensitive volume, the reversed effect could be observed. Discussion and conclusion: The experimental and Monte Carlo results demonstrated that the dose response of the investigated diode‐type detectors decreases in magnetic field. This observation can be generally attributed to the common construction of diode‐type detectors, where structural components with enhanced density, for example the diode substrate, are situated below the sensitive volume. The results provide deeper insights into the behavior of clinical diode detectors when used in strong magnetic field. [ABSTRACT FROM AUTHOR]

Purpose: Beam quality correction factors provided in current codes of practice for proton beams are approximated using the water‐to‐air mass stopping power ratio and by assuming the proton beam quality related perturbation correction factors to be unity. The aim of this work is to use Monte Carlo simulations to calculate energy dependent beam quality and perturbation correction factors for a set of nine ionization chambers in proton beams. Methods: The Monte Carlo code EGSnrc was used to determine the ratio of the absorbed dose to water and the absorbed dose to the sensitive air volume of ionization chambers fQ0 related to the reference photon beam quality (60Co). For proton beams, the quantity fQ was simulated with GATE/Geant4 for five monoenergetic beam energies between 70 MeV and 250 MeV. The perturbation correction factors for the air cavity, chamber wall, chamber stem, central electrode, and displacement effect in proton radiation were investigated separately. Additionally, the correction factors of cylindrical chambers were investigated with and without consideration of the effective point of measurement. Results: The perturbation factors pQ were shown to deviate from unity for the investigated chambers, contradicting the assumptions made in dosimetry protocols. The beam quality correction factors for both plane‐parallel and cylindrical chambers positioned with the effective point of measurement at the measurement depth were constant within 0.8%. An increase of the beam quality correction factors determined for cylindrical ionization chambers placed with their reference point at the measurement depth with decreasing energy is attributed to the displacement perturbation correction factors pdis, which were up to 1.045 ± 0.1% for the lowest energy and 1.005 ± 0.1% for the highest energy investigated. Besides pdis, the largest perturbation was found for the chamber wall where the smallest pwall determined was 0.981 ± 0.3%. Conclusions: Beam quality correction factors applied in dosimetry with cylindrical chambers in monoenergetic proton beams strongly depend on the positioning method used. We found perturbation correction factors different from unity. Consequently, the approximation of ionization chamber perturbations in proton beams by the respective water‐to‐air mass stopping power ratio shall be revised. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study was to provide a method and reference data for the evaluation of the weight dependence of the conversion coefficients from air collision kerma integrated over beam area to the effective dose for patients undergoing diagnostic x‐ray examinations. Method: A simple physics‐based analytical model was developed in order to calculate the weight dependence of the effective dose conversion coefficients based on existing values of the conversion coefficients between patient effective dose and air collision kerma integrated over beam area calculated by the Monte Carlo method on the base of computational MIRD‐5‐type stylized models. Results: The analytical expressions that make possible taking into account a patient's weight when assessing the effective dose were presented. The following categories of diagnostic x‐ray examinations are considered: CT; plain radiography; cardiac angiography; fluoroscopy. The results indicate that the weight dependence of the effective dose conversion coefficients for different categories of diagnostic x‐ray examinations has the same form. Correction factor for evaluation of the new value of the effective dose conversion coefficient for a patient with weight Wpa. when we know its value for a standard patient with weight Wst. depends on the ratios of their weights (Wst./Wpat.) and fractions of the incident energy which is imparted to a patient. Conclusions: The paper presents an approach and necessary data for the estimation of the weight dependence of the effective dose conversion coefficients for patients undergoing different diagnostic x‐ray examinations. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study is to provide a calibration methodology for radiation therapy machines where the closest field to the conventional reference field may not meet the lateral charged particle equilibrium (LCPE) condition of the machine‐specific reference (msr) field. We provided two methodologies by extending the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS‐483 code of practice (COP) (Palmans et al. TRS‐483: Dosimetry of small static fields used in external beam radiotherapy: an international code of practice for reference and relative dose determination; 2017) methodology for the calibration of radiation therapy machines with 6 MV flattening filter free (FFF) beam and with field sizes down to 10 × 2 cm2. Methods: Two methods of calibration were provided following the TRS‐483. In calibration Method I, the generic correction factors kQA,Q0fA,fref were calculated using Monte Carlo (MC) for seven detectors and rectangular physical field sizes ranging from 10 × 2 cm2 to 10 × 10 cm2. In calibration Method II, we extended the methodology in TRS‐483 for deriving the equivalent square msr field sizes for rectangular field sizes down to 10 × 2 cm2. The beam quality specifier for a hypothetical 10 × 10 cm2 field was derived by extending the methodology provided in the TRS‐483. Since the beam quality correction values for the conventional reference field (kQ,Q0fref) tabulated in TRS‐483 are provided only for large reference chambers, we calculated the kQ,Q0fref values analytically for our beam quality specifier and chambers used, using interaction data in TRS‐398 (Andreo, et al. TRS‐398: Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water; 2001). Results: The kQA,Q0fA,fref correction values calculated using the first method for chambers with an electrode made of C552 almost did not vary across the different field sizes studied (within 0.1%) while it varied by 1.6% for IBA CC01 with electrode made of steel. Extending the equivalent field and beam quality specifier determination methodology of TRS‐483 resulted in a maximum error of 1.3% on the beam quality specifier for the 2 × 2 cm2 field size. However, this had a negligible impact on the kQA,Q0fA,fref values (less than 0.1%). For chambers with C552 and Al electrode material, the correction factors determined using the two methods of calibration were in agreement to within 0.5%. However, for the chambers with electrode made of higher atomic number (Z), the difference between the two methodologies could be as large as 1.5%. It was shown that this difference can be reduced to less than 0.5% if central electrode perturbation effects and kQAFFF,QFFFfA,fref values introduced in TRS‐483 were taken into account. Conclusions: In this study, applying the kQA,Q0fA,fref correction values calculated using the calibration Method I to the chamber reading improved the consistency on an absorbed dose determination from 0.5% to 0.1% standard deviation (except for the Exradin A16). For this reason we recommend using calibration Method I. If the kQA,Q0fA,fref values are not available for the user's detector, calibration Method II can be used to predict the correction factors. However, the second methodology should not be used for chambers with electrode made of high‐Z material unless the electrode perturbation effects and kQAFFF,QFFFfA,fref values are taken into account. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study is to provide a calibration methodology for radiation therapy machines where the closest field to the conventional reference field may not meet the lateral charged particle equilibrium (LCPE) condition of the machine‐specific reference (msr) field. We provided two methodologies by extending the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS‐483 code of practice (COP) (Palmans et al. TRS‐483: Dosimetry of small static fields used in external beam radiotherapy: an international code of practice for reference and relative dose determination; 2017) methodology for the calibration of radiation therapy machines with 6 MV flattening filter free (FFF) beam and with field sizes down to 10 × 2 cm2. Methods: Two methods of calibration were provided following the TRS‐483. In calibration Method I, the generic correction factors kQA,Q0fA,fref were calculated using Monte Carlo (MC) for seven detectors and rectangular physical field sizes ranging from 10 × 2 cm2 to 10 × 10 cm2. In calibration Method II, we extended the methodology in TRS‐483 for deriving the equivalent square msr field sizes for rectangular field sizes down to 10 × 2 cm2. The beam quality specifier for a hypothetical 10 × 10 cm2 field was derived by extending the methodology provided in the TRS‐483. Since the beam quality correction values for the conventional reference field (kQ,Q0fref) tabulated in TRS‐483 are provided only for large reference chambers, we calculated the kQ,Q0fref values analytically for our beam quality specifier and chambers used, using interaction data in TRS‐398 (Andreo, et al. TRS‐398: Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water; 2001). Results: The kQA,Q0fA,fref correction values calculated using the first method for chambers with an electrode made of C552 almost did not vary across the different field sizes studied (within 0.1%) while it varied by 1.6% for IBA CC01 with electrode made of steel. Extending the equivalent field and beam quality specifier determination methodology of TRS‐483 resulted in a maximum error of 1.3% on the beam quality specifier for the 2 × 2 cm2 field size. However, this had a negligible impact on the kQA,Q0fA,fref values (less than 0.1%). For chambers with C552 and Al electrode material, the correction factors determined using the two methods of calibration were in agreement to within 0.5%. However, for the chambers with electrode made of higher atomic number (Z), the difference between the two methodologies could be as large as 1.5%. It was shown that this difference can be reduced to less than 0.5% if central electrode perturbation effects and kQAFFF,QFFFfA,fref values introduced in TRS‐483 were taken into account. Conclusions: In this study, applying the kQA,Q0fA,fref correction values calculated using the calibration Method I to the chamber reading improved the consistency on an absorbed dose determination from 0.5% to 0.1% standard deviation (except for the Exradin A16). For this reason we recommend using calibration Method I. If the kQA,Q0fA,fref values are not available for the user's detector, calibration Method II can be used to predict the correction factors. However, the second methodology should not be used for chambers with electrode made of high‐Z material unless the electrode perturbation effects and kQAFFF,QFFFfA,fref values are taken into account. [ABSTRACT FROM AUTHOR]

Purpose: To calculate in‐ and out‐of‐field neutron spectra and dose equivalent, using Monte Carlo (MC) simulation, for a Mevion gantry‐mounted passively scattered proton system in craniospinal irradiation. An analytical model based on the MC calculations that estimates in‐ and out‐of‐field neutron dose equivalent from proton Craniospinal irradiation (CSI) was also developed. Methods: The MCNPX MC code was used to simulate a Mevion S250 proton therapy system. The simulated proton depth doses and profiles for pristine and spread‐out Bragg peaks were benchmarked against the measured data. Previous measurements using extended‐range Bonner spheres were used to verify the calculated neutron spectra and dose equivalent. Using the benchmarked results as a reference condition, a correction‐based analytical model was reconstructed by fitting the data to derive model parameters at 95% confidence interval. Sensitivity analysis of brass aperture opening, thickness of the Lucite (PMMA) range compensator, and modulation width was performed to obtain correction parameters for nonreference conditions. Results: For the neutron dose equivalent per therapeutic proton dose, the MCNPX calculated dose equivalent matched the measured values to within 8%. The benchmarked neutron dose equivalent at the isocenter was 41.2 and 20.8 mSv/Gy, for cranial and spinal fields, respectively. For in‐ and out‐of‐field neutron dose calculations, the correction‐based analytical model showed up to 17% discrepancy compared to the MC calculations. The correction factors may provide a conservative estimation of neutron dose, especially for depth ≤ 5 cm and regions underneath the brass aperture. Conclusion: The proposed analytical model can be used to estimate the contribution of the neutron dose to the overall CSI treatment dose. Moreover, the model can be employed to estimate the neutron dose to the implantable cardiac electronic devices. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate three different formulae for calculating the biologically effective dose (BED) by use of a multipopulation reaction–diffusion simulation to determine whether these formulae produce equivalent effects for different treatment regimes. Methods: The standard BED formula, BEDs, was updated to account both for spacial nonuniformity in dose and for cellular regrowth between fractions, by creating two new formulae: BEDϕ and BEDϕT. These BED formulae were used to calculate dose per fraction values for two, three, and five fraction treatments and to compare the tumor volumes of those treatments to those of a single fraction. A spherical tumor model based on the reaction–diffusion equation was used to calculate the final volume of each tumor 185 days after the delivery of the first fraction. The percent difference in volume between single‐fraction and multiple‐fraction treatments was used as a measure to test the accuracy of each BED formula. Results: Percent differences in volume between single‐ and multiple‐fraction treatment regimes varied up to approximately 18.5% if the dose per fraction was calculated using BEDs but the delivered dose was nonuniform. Proper application of spacial nonuniformity in dose and tumor regrowth correction factors modified the dose per fraction values by no more than 5%, but resulted in the improvement of simulated tumor volumes down to around 2% or lower difference in volume. Conclusions: Treatment regimes with the same BED value should have the same effect. However, small changes in the dose per fraction delivered in multiple‐fraction treatments can have a large effect on the tumor volume of a treatment when the dose is delivered nonuniformly or when tumor regrowth between fractions is ignored. Inclusion of these correction factors is important for the underlying assumption that treatments with equal BED will have equal effects on the clinically observed tumor volume. [ABSTRACT FROM AUTHOR]

Purpose: The plan‐class specific reference field concept could theoretically improve the calibration of radiation detectors in a beam environment much closer to clinical deliveries than existing broad beam dosimetry protocols. Due to a lack of quantitative guidelines and representative data, however, the pcsr field concept has not yet been widely implemented. This work utilizes quantitative plan complexity metrics from modulated clinical treatments in order to investigate the establishment of potential plan classes using two different clustering methodologies. The utility of these potential plan clusters is then further explored by analyzing their relevance to actual dosimetric correction factors. Methods: Two clinical databases containing several hundred modulated plans originally delivered on two Varian linear accelerators were analyzed using 21 plan complexity metrics. In the first approach, each database's plans were further subdivided into groups based on the anatomic site of treatment and then compared to one another using a series of nonparametric statistical tests. In the second approach, objective clustering algorithms were used to seek potential plan clusters in the multidimensional complexity‐metric space. Concurrently, beam‐ and detector‐specific dosimetric corrections for a subset of the modulated clinical plans were determined using Monte Carlo for three different ionization chambers. The distributions of the dosimetric correction factors were compared to the derived plan clusters to see which plan clusters, if any, could help predict the correction factor magnitudes. Ultimately, a simplified volume averaging metric (SVAM) is shown to be much more relevant to the total dosimetric correction factor than the established plan clusters. Results: Plan groups based on the site of treatment did not show noticeable distinction from one another in the context of the metrics investigated. An objective clustering algorithm was able to discriminate volumetric modulated arc therapy (VMAT) plans from step‐and‐shoot intensity‐modulated radiation therapy plans with an accuracy of 90.8%, but no clusters were found to exist at any level more specific than delivery modality. Monte Carlo determined correction factors for the modulated plans ranged from 0.970 to 1.104, 0.983 to 1.027, and 0.986 to 1.009 for the A12, A1SL, and A26 chambers, respectively, and were highly variable even within the treatment modality plan clusters. The magnitudes of these correction factors were explained almost entirely by volume averaging with SVAM demonstrating positive correlation with all Monte Carlo established total correction factors. Conclusions: Plan complexity metrics do provide some quantitative basis for the investigation of plan clusters, but an objective clustering algorithm demonstrated that quantifiable differences could only be found between VMAT and step‐and‐shoot beams delivered on the same treatment machine. The inherent variability of the Monte Carlo determined correction factors could not be explained solely by the modality of the treatment but were instead almost entirely dependent upon the volume averaging correction, which itself depends on the detector position within the dose distribution, dose gradients, and other factors. Considering the continued difficulty of determining a relevant plan metric to base plan clusters on, case‐by‐case corrections may instead obviate the need for the pcsr field concept in the future. [ABSTRACT FROM AUTHOR]

Purpose: Specialized treatment machines such as the CyberKnife, TomoTherapy, or the GammaKnife, utilize flattening filter free (FFF) photon beams and may not be able to generate a 10 cm x 10 cm reference field. A new Code of Practice has recently been published (IAEA TRS483) to give recommendations for these machines. Additionally, some standard laboratories provide measured beam quality correction factors for the user's reference chamber, which can be used instead of the published tabulated beam quality correction factors. The purpose of this study was first to assess how different recommendations, as outlined below, affect the reference dosimetry at the CyberKnife, and second, to assess the impact of using measured rather than tabulated beam quality correction factors on measured dose. Methods: Following recommendations in TRS398, three field chambers (IBA CC04, Exradin A19, and Exradin A12S) were cross‐calibrated with a user's reference chamber (IBA FC65‐G), which was calibrated in a Cobalt‐60 (Co‐60) beam by a primary standards laboratory. The chamber response was corrected for influence quantities such as temperature, pressure, ion recombination, polarity, and beam quality. Additionally, correction factors for volume averaging and differences due the FFF beam spectrum were determined for the CyberKnife beam. Three different methods were utilized ‐ TRS398; Intermediate (i.e. TRS398 with additional published recommendations); and TRS483. The measurements were undertaken in a 10 cm × 10 cm field defined by jaws for a uniform flattened (WFF) 6 MV photon beam of a Varian TrueBeam linear accelerator (linac) with a source to detector distance (SDD) of 100 cm, and in a 60 mm diameter circular field for a 6 MV flattening filter free (FFF) Accuray CyberKnife beam with SDD of 80 cm. All measurement was performed at 10 cm deep in a full scatter phantom as defined in TRS398. Results: Differences between the three methods in volume averaging correction factors ranged from 0.01% to 0.45% depending on the chamber assessed. As expected, an increased chamber length leads to a larger correction factor. The differences in beam spectrum correction factors range from 0.09% to 0.3%. Negligible differences in beam quality correction factors were observed; however, differences up to 1% were found between measured and tabulated values. Differences in cross‐calibrated chamber calibration coefficients range from 0.05% to 0.51% depending on the chamber assessed. Differences in measured dose are up to 0.87% between Method TRS398 and Intermediate, again chamber dependent, and 0.28% between Method Intermediate and TRS483. Conclusion: Using chambers cross‐calibrated in the linac beam can lead to differences in measured dose per Monitor Unit (MU) in the CyberKnife beam of approximately 0.5% between chambers. Using Method Intermediate vs using recommendations given in TRS483 led to a difference of 0.28% in measured dose per MU, which is due to differences in volume averaging and beam spectrum correction factors. Using TRS483 is recommended as the cross‐calibration is done in the CyberKnife beam and accounts for its specific reference conditions. It will also ensure consistency between different centers. The measured beam quality correction factors agree within the uncertainties with the tabulated values. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study is the experimental and Monte Carlo‐based determination of small field correction factors for the unshielded silicon detector microSilicon for a standard linear accelerator as well as the Cyberknife System. In addition, a detailed Monte Carlo analysis has been performed by modifying the detector models stepwise to study the influences of the detector's components. Methods: Small field output correction factors have been determined for the new unshielded silicon diode detector, microSilicon (type 60023, PTW Freiburg, Germany) as well as for the predecessors Diode E (type 60017, PTW Freiburg, Germany) and Diode SRS (type 60018, PTW Freiburg, Germany) for a Varian TrueBeam linear accelerator at 6 MV and a Cyberknife system. For the experimental determination, an Exradin W1 scintillation detector (Standard Imaging, Middleton, USA) has been used as reference. The Monte Carlo simulations have been performed with EGSnrc and phase space files from IAEA as well as detector models according to manufacturer blueprints. To investigate the influence of the detector's components, the detector models have been modified stepwise. Results: The correction factors for the smallest field size investigated at the TrueBeam linear accelerator (equivalent dosimetric square field side length Sclin = 6.3 mm) are 0.983 and 0.939 for the microSilicon and Diode E, respectively. At the Cyberknife system, the correction factors of the microSilicon are 0.967 at the smallest 5‐mm collimator compared to 0.928 for the Diode SRS. Monte Carlo simulations show comparable results from the measurements and literature. Conclusion: The microSilicon (type 60023) detector requires less correction than its predecessors, Diode E (type 60017) and Diode SRS (type 60018). The detector housing has been demonstrated to cause the largest perturbation, mainly due to the enhanced density of the epoxy encapsulation surrounding the silicon chip. This density has been rendered more water equivalent in case of the microSilicon detector to minimize the associated perturbation. The sensitive volume itself has been shown not to cause observable field size‐dependent perturbation except for the volume‐averaging effect, where the slightly larger diameter of the sensitive volume of the microSilicon (1.5 mm) is still small at the smallest field size investigated with corrections <2%. The new microSilicon fulfils the 5% correction limit recommended by the TRS 483 for output factor measurements at all conditions investigated in this work. [ABSTRACT FROM AUTHOR]