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This study presents a fast and simple estimation method for dosimetry assessment of a clothed human body exposed to electromagnetic fields in millimeter-wave bands. A multivariate regression algorithm was given instead of conventional numerical methods for the first time, which utilized the exact value of skin surface temperature rise that was computed by the Monte-Carlo method by changing the oblique incidence wave angle, the cross-polarization power ratio, and the air gap spacing from the cloth to the body. The detailed formula for bioheat transfer analysis considering the abovementioned key parameters is first introduced. Then, a polynomial function is derived by multivariate regression analysis using the simulation result to estimate the skin temperature rises under various exposure conditions and scenarios. In comparison with those results obtained by the traditional methods, the proposed regression algorithm significantly reduced the expense of computational resources. The results estimated using the developed function show a maximum error of less than 0.013 °C for the skin tissues of different body parts and different cloth materials, demonstrating that the proposed method is easy and efficient for the estimation of a high degree of skin temperature rise in a relatively complicated radio-wave exposure environment in millimeter-wave bands. [ABSTRACT FROM AUTHOR]

Brain functions have been proved to be affected by external stimuli. Low-Level-Light Therapy (LLLT) using near-infrared photons is one of the effective ways to modulate the hemodynamic activities in the brain. However, the biphasic hormetic dose-response where bioenergetics are stimulated at a low dose and inhibited at a high dose is well observed in all photon stimulations. The amount of photon energy delivered to the brain are affected by the wavelength as well as the multilayered head structure with variations of optical parameters (OPs). A real 3D volume head model is built for each participant in this study, and the boundary conditions of each OP in each layer is considered. The Monte Carlo simulation with wavelengths ranging from 650 nm to 1064 nm is implemented to investigate the energy delivered to the brain under different radiation profiles. Results show that 1064-nm photons penetrate deeper than 810-nm photons except for scalp absorption at the lower bound due to low melanin content. Collimated-beam radiation is better than diverging-beam due to a more uniform intensity distribution at the scalp surface. Further research to optimize LLLT dosage for each individual is imperative due to the high inter-person variability in structure and OPs. [ABSTRACT FROM AUTHOR]

In order to calibrate radiation protection instruments and determine radiation dose, it is critical to establish an X-ray narrow-spectrum series recommended by the standard ISO 4037-1 which is published by the International Standardisation Organisation (ISO). By experiment, the qualities of filtered X radiation of narrow-spectrum series in the range from 60 to 300 kV were established. The differences of the first half-value layer (HVL1) and the second HVL (HVL2) between experimental results and the values given in the standard ISO 4037-1 were all within 5%. What is more, the Monte Carlo code EGSnrc was used to simulate the spectra of these radiation qualities, which showed a good agreement with these spectra given in the standard ISO 4037-1. The spectral resolution, mean energy and effective energy of these reference qualities can be obtained, which are all have good agreement with the given values.

In recent years, the measurement of X-rays has attracted much attention. Especially in the aspects of environmental monitoring, radiation protection, and radiation therapy, to obtain more accurate measurement values, it is necessary to set up a corresponding reference device to trace the magnitude. This paper aims to establish a 350–450 kV wide spectrum and high air-kerma rate X-ray radiant mass. First, the Monte Carlo simulation was used to simulate the establishment of a model of the light engine, and the energy spectra of different tube voltages and different additional filters were calculated to obtain the average photon energy. Then the average photon energy was compared with the extrapolated average photon energy so that the deviation was within ±5%. Finally, half-value measurements are made on additional filters that meet the requirements.

In order to calibrate radiation protection instruments and determine radiation dose, it is critical to establish an X-ray narrow-spectrum series recommended by the standard ISO 4037-1 which is published by the International Standardisation Organisation (ISO). By experiment, the qualities of filtered X radiation of narrow-spectrum series in the range from 60 to 300 kV were established. The differences of the first half-value layer (HVL1) and the second HVL (HVL2) between experimental results and the values given in the standard ISO 4037-1 were all within 5%. What is more, the Monte Carlo code EGSnrc was used to simulate the spectra of these radiation qualities, which showed a good agreement with these spectra given in the standard ISO 4037-1. The spectral resolution, mean energy and effective energy of these reference qualities can be obtained, which are all have good agreement with the given values. [ABSTRACT FROM AUTHOR]

In recent years, the measurement of X-rays has attracted much attention. Especially in the aspects of environmental monitoring, radiation protection, and radiation therapy, to obtain more accurate measurement values, it is necessary to set up a corresponding reference device to trace the magnitude. This paper aims to establish a 350–450 kV wide spectrum and high air-kerma rate X-ray radiant mass. First, the Monte Carlo simulation was used to simulate the establishment of a model of the light engine, and the energy spectra of different tube voltages and different additional filters were calculated to obtain the average photon energy. Then the average photon energy was compared with the extrapolated average photon energy so that the deviation was within ±5%. Finally, half-value measurements are made on additional filters that meet the requirements. [ABSTRACT FROM AUTHOR]

Purpose A brachytherapy (BT) device has been developed with shielding to provide directional BT for preferentially irradiating malignancies while sparing healthy tissues. The CivaSheet is a flexible low-dose-rate BT device containing CivaDots with 103 Pd shielded by a thin Au disk. This is the first report of a clinical dosimetric characterization of the CivaSheet device. Methods and Materials Radiation dose distributions near a CivaDot were estimated using the MCNP6 radiation transport code. CivaSheet arrays were also modeled to evaluate the dose superposition principle for treatment planning. The resultant data were commissioned in a treatment planning system (TPS) (VariSeed 9.0), and the accuracy of the dose superposition principle was evaluated for summing individual elements comprising a planar CivaSheet. Results The dose-rate constant (0.579 cGy/h/U) was lower than for 103 Pd seeds due to Au L-shell x-rays increasing the air-kerma strength. Radial dose function values at 0.1, 0.5, 2, 5, and 10 cm were 1.884, 1.344, 0.558, 0.088, and 0.0046, respectively. The two-dimensional anisotropy function exhibited dramatic reduction between the forward (0°) and rearward (180°) directions by a factor of 276 at r = 0.1 cm, 24 at r = 1 cm, and 5.3 at r = 10 cm. This effect diminished due to increasingly scattered radiation. The largest gradient in the two-dimensional anisotropy function was in contact with the device at 92° due to the Au disk shielding. TPS commissioning and dose superposition accuracies were typically within 2%. Conclusions Simulations of the CivaDot yielded comprehensive dosimetry parameters that were entered into a TPS and deemed acceptable for clinical use. Dosimetry measurements of the CivaSheet are also of interest to the BT community. [ABSTRACT FROM AUTHOR]

Purpose: The first main aim of this study was to illustrate the absorbed dose rate distribution from 177Lu in sections of xenografted prostate cancer (PCa) tumors using high resolution digital autoradiography (DAR) and compare it with hypothetical identical radioactivity distributions of 90Y or 7 MeV alpha-particles. Three dosimetry models based on either dose point kernels or Monte Carlo simulations were used and evaluated. The second and overlapping aim, was to perform DAR imaging and dosimetric analysis of the distribution of radioactivity, and hence the absorbed dose rate, in tumor sections at an early time point after injection during radioimmunotherapy using 177Lu-h11B6, directed against the human kallikrein 2 antigen. Methods: Male immunodeficient BALB/c nude mice, aged 6-8 w, were inoculated by subcutaneous injection of ~107 LNCaP cells in a 200 µl suspension of a 1:1 mixture of medium and Matrigel. The antibody h11B6 was conjugated with the chelator CHX-A''-DTPA after which conjugated h11B6 was mixed with 177LuCl3. The incubation was performed at room temperature for 2 h, after which the labeling was terminated and the solution was purified on a NAP-5 column. About 20 MBq 177Lu-h11B6 was injected intravenously in the tail vein. At approximately 10 h postinjection (hpi), the mice were sacrificed and one tumor was collected from each of the five animals and cryosectioned into 10 μm thick slices. The tumor slices were measured and imaged using the DAR MicroImager system and the M3Vision software. Then the absorbed dose rate was calculated using a dose point kernel generated with the Monte Carlo code GATE v7.0. Results: The DAR system produced high resolution images of the radioactivity distribution, close to the resolution of single PCa cells. The DAR images revealed a pronounced heterogeneous radioactivity distribution, i.e., count rate per area, in the tumors, indicated by the normalized intensity variations along cross sections as mean ± SD: 0.15±0.15, 0.20±0.18, 0.12±0.17, 0.15±0.16, and 0.23±0.22, for each tumor section, respectively. The absorbed dose rate distribution for 177Lu at the time of dissection 10 hpi showed a maximum value of 2.9±0.4 Gy/h (mean ± SD), compared to 6.0±0.9 and 159±25 Gy/h for the hypothetical 90Y and 7 MeV alpha-particle cases assuming the same count rate densities. Mean absorbed dose rate values were 0.13, 0.53, and 6.43 Gy/h for 177Lu, 90Y, and alpha-particles, respectively. Conclusions: The initial uptake of 177Lu-h11B6 produces a high absorbed dose rate, which is important for a successful therapeutic outcome. The hypothetical 90Y case indicates a less heterogeneous absorbed dose rate distribution and a higher mean absorbed dose rate compared to 177Lu, although with a potentially increased irradiation of surrounding healthy tissue. The hypothetical alpha-particle case indicates the possibility of a higher maximum absorbed dose rate, although with a more heterogeneous absorbed dose rate distribution. [ABSTRACT FROM AUTHOR]

Purpose: Magnetic resonance imaging (MRI)-only radiotherapy treatment planning requires accurate pseudo-CT (pCT) images for precise dose calculation. The current work introduced an atlas-based method combined with MR intensity information. pCT analyses and Monte Carlo dose calculations for intracranial stereotactic treatments were performed. Methods: Twenty-two patients, representing 35 tumor targets, were scanned using a 3D T1-weighted MRI sequence according to the clinical protocol. The MR atlas image was registered to the MR patient image using a deformable algorithm, and the deformation was then applied to the atlas CT. Two methods were applied. The first method (MRdef) was based on deformations only, while the second (MRint) also used the actual MR intensities. pCT analysis was performed using the mean (absolute) error, as well as an in-house tool based on a gamma index. Dose differences between pCT and true CT were analyzed using dose-volume histogram (DVH) parameters, statistical tests, the gamma index, and probability density functions. An unusual case, where the patient underwent an operation (part of the skull bone was removed), was studied in detail. Results: Soft tissues presented a mean error inferior to 50 HUs, while low-density tissues and bones presented discrepancies up to 600 HUs for hard bone. The MRdef method led to significant dose differences compared with the true CT (p-value < 0.05; Wilcoxon-signed-rank test). The MRint method performed better. The DVH parameter differences compared with CT were between -2.9% and 3.1%, except for two cases where the tumors were located within the sphenoid bone. For these cases, the dose errors were up to 6.6% and 5.4% (D98 and D95). Furthermore, for 85% of the tested patients, the mean dose to the planning target volume agreed within 2% with the calculation using the actual CT. Fictitious bone was generated in the unusual case using atlas-based methods. Conclusions: Generally, the atlas-based method led to acceptable dose distributions. The use of common T1 sequences allows the implementation of this method in clinical routine. However, unusual patient anatomy may produce large dose calculation errors. The detection of large anatomic discrepancies using MR image subtraction can be realized, but an alternative way to produce synthetic CT numbers in these regions is still required. [ABSTRACT FROM AUTHOR]

Purpose: To present and evaluate an ion chamber-based method for the cross-calibration between sites of activity measurements of unsealed radionuclides. Notably, the method allows direct comparison of short lived (i.e., clinically used) radioisotopes and the cross-calibration of radionuclide activity meters (also known as "dose calibrators"). Methods: A portable ion chamber has been designed which is easily shipped between sites, e.g., between a standards laboratory and a nuclear medicine department. The cylindrical chamber accommodates a syringe filled with unsealed radionuclide. Low background and staff shielding are achieved by designing the ion chamber small enough to fit into the well of a dose calibrator. The chamber's sensitivity for the clinically important unsealed radioisotopes 99mTc, 131I, and 18F was measured and compared to Monte Carlo calculations. The influence of syringe fill volume, positioning, and construction (wall diameter, length) was also investigated using Monte Carlo simulations. The chamber's linearity was measured over 5.5 orders of magnitude and its constancy tested over a period of >14.5 months. An overall uncertainty budget is presented. Results: Measured chamber sensitivity was 12.1 pA/100 MBq, 12.5 pA/100 MBq and 29.4 pA/100 MBq for 131I, 99mTc, and 18F, respectively. The uncertainty budget for the ion chamber alone yields an overall uncertainty of less than 1%, with the greatest contribution arising from constancy and linearity (0.5% each). Strategies to further reduce uncertainties are discussed. Conclusions: The investigation presented in this paper confirms the feasibility of the concept introduced here. To optimize its practical implementation, the concept would benefit from computerization for the purpose of data acquisition, evaluation, processing, and storage of measured values. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the feasibility of three-dimensional (3D) dose measurements near thin high-Z materials placed in a water-like medium by using a polymer gel dosimeter (PGD) when the medium was irradiated with high energy photon beams. Methods: PGD is potentially a useful tool for this application because it can record the dose around a small object made of a high-Z material in a continuous 3D medium. In this study, the authors manufactured a methacrylic acid-based normoxic PGD, nMAG. Two 0.5 mm thick lead foils (1×1 cm) were placed in foil supports with 0.7 cm separation in a 1000 ml polystyrene container filled with nMAG. The authors used two foil configurations, i.e., orthogonal and parallel. In the orthogonal configuration, two foils were placed in the direction orthogonal to the beam axis. The parallel configuration had two foils arranged in parallel to the beam axis. The phantom was irradiated with an 18 MV photon beam of 5×5 cm field size. It was imaged with a three-Tesla (3 T) magnetic resonance imaging (MRI) scanned using the Car-Purcell-Meiboom-Gill pulse sequence. The spin-spin relaxation time (R2) to-dose calibration data were obtained by using small vials filled with nMAG and exposing to known doses. The DOSXYZnrc Monte Carlo (MC) codewas used to get the expected dose distributions. More than 35×106 of histories were simulated so that the average errorwas less than 1%. Anin-house MATLAB-based softwarewas used to obtain the dose distributions from the measured R2 data as well as to compare the measurements and the MC predictions. The dose change due to the presence of the foils was studied by comparing the dose distributions with and without foils (or the reference). Results: For the orthogonal configuration, the measured dose along the beam axis showed an increase in the upstream side of the first foil, between the foils, and on the downstream side of the second foil. The range of increased dose area was 1.1 cm in the upstream of the first foil. However, in the downstream of the second foil, it was 0.2 cm, beyond which the dose fell below the reference dose by 10%. The dose profile between the foils showed a well-like shape with the minimum dose still larger than the reference dose by 1.8%. The minimum dose point was closer to the first foil than to the second foil. For the parallel configuration, the dose between foils was the largest at the center. The increased dose area opposite to the gap between foils extended outward to 1 cm. The spatial dose distributions of PGD and MC showed the same geometrical patterns except for the points inside the foils for both orthogonal and parallel foil arrangements. Conclusions: The authors demonstrated that the nMAG PGD with MRI could be used to measure the 3D dosimetric structures at the mm-scale in the vicinity of the foil. The current study provided more accurate 3D spatial dose distribution than the previous studies. Furthermore, the measurements were validated by the MC simulation. [ABSTRACT FROM AUTHOR]

Purpose: CT images are used for patient specific Monte Carlo treatment planning in radionuclide therapy. The authors investigated the impact of tissue classification, CT image segmentation, and CT errors on Monte Carlo calculated absorbed dose estimates in nuclear medicine. Methods: CT errors as a function of patient size, CT reconstruction, and tube current modulation methods were assessed in a phantom experiment on a clinical CT system. The impact of tissue segmentation methods and CT number variations on EGSnrc Monte Carlo calculated absorbed dose distributions was assessed for 99mTc and 131I in the ICRP/ICRU male phantom and in a patient PET/CT-scanned with 124I prior to radioiodine therapy. Results: CT number variations <20 HU were obtained for whole-body CT examinations at effective CT doses ~2 mSv. Monte Carlo calculated absorbed doses depended on both the number of media types and accurate calibration of the CT number-to-density conversion ramp. Tissue segmentation by a 13-tissue CT conversion ramp, calibrated by a stoichiometric method, resulted in low (<4%) dose errors in selected organs for both isotopes. Conclusions: A calibrated CT scanner specific conversion ramp is required for accurate patient specific dosimetry in nuclear medicine. Accurate dosimetry was obtained with a 13-tissue ramp that included five different bone types. [ABSTRACT FROM AUTHOR]

Purpose: To introduce a fast ray-tracing algorithm in pencil proton radiography (PR) with a multilayer ionization chamber (MLIC) for in vivo range error mapping. Methods: Pencil beam PR was obtained by delivering spots uniformly positioned in a square (45×45 mm² field-of-view) of 9×9 spots capable of crossing the phantoms (210 MeV). The exit beam was collected by a MLIC to sample the integral depth dose (IDDMLIC). PRs of an electrondensity and of a head phantom were acquired by moving the couch to obtain multiple 45×45 mm² frames. To map the corresponding range errors, the two-dimensional set of IDDMLIC was compared with (i) the integral depth dose computed by the treatment planning system (TPS) by both analytic (IDDTPS) and Monte Carlo (IDDMC) algorithms in a volume of water simulating the MLIC at the CT, and (ii) the integral depth dose directly computed by a simple ray-tracing algorithm (IDDdirect) through the same CT data. The exact spatial position of the spot pattern was numerically adjusted testing different in-plane positions and selecting the one that minimized the range differences between IDDdirect and IDDMLIC. Results: Range error mapping was feasible by both the TPS and the ray-tracing methods, but very sensitive to even small misalignments. In homogeneous regions, the range errors computed by the direct ray-tracing algorithm matched the results obtained by both the analytic and the Monte Carlo algorithms. In both phantoms, lateral heterogeneities were better modeled by the ray-tracing and the Monte Carlo algorithms than by the analytic TPS computation. Accordingly, when the pencil beam crossed lateral heterogeneities, the range errors mapped by the direct algorithm matched better the Monte Carlo maps than those obtained by the analytic algorithm. Finally, the simplicity of the ray-tracing algorithm allowed to implement a prototype procedure for automated spatial alignment. Conclusions: The ray-tracing algorithm can reliably replace the TPS method in MLIC PR for in vivo range verification and it can be a key component to develop software tools for spatial alignment and correction of CT calibration. [ABSTRACT FROM AUTHOR]

Purpose: In the nuclear medicine department, the activity of radiopharmaceuticals is measured using dose calibrators (DCs) prior to patient injection. TheDCconsists of an ionization chamber that measures current generated by ionizing radiation (emitted from the radiotracer). In order to obtain an activity reading, the current is converted into units of activity by applying an appropriate calibration factor (also referred to as DC dial setting). Accurate determination of DC dial settings is crucial to ensure that patients receive the appropriate dose in diagnostic scans or radionuclide therapies. The goals of this study were (1) to describe a practical method to experimentally determine dose calibrator settings using a thyroid-probe (TP) and (2) to investigate the accuracy, reproducibility, and uncertainties of the method. As an illustration, the TP method was applied to determine 188Re dial settings for two dose calibrator models: Atomlab 100plus and Capintec CRC-55tR. Methods: Using the TP to determine dose calibrator settings involved three measurements. First, the energy-dependent efficiency of the TP was determined from energy spectra measurements of two calibration sources (152Euand 22Na). Second, thegammaemissionsfromthe investigated isotope (188Re) were measured using theTPand its activitywas determined using ?-ray spectroscopy methods. Ambient background, scatter, and source-geometry corrections were applied during the efficiency and activity determination steps. Third, the TP-based 188Re activity was used to determine the dose calibrator settings following the calibration curve method [B. E. Zimmerman et al., J. Nucl. Med. 40, 1508-1516 (1999)]. The interobserver reproducibility of TP measurements was determined by the coefficient of variation (COV) and uncertainties associated to each step of the measuring process were estimated. The accuracy of activity measurements using the proposed methodwas evaluated by comparing the TP activity estimates of 99mTc, 188Re, 131I, and 57Co samples to high purity Ge (HPGe) γ-ray spectroscopy measurements. Results: The experimental 188Re dial settings determined with the TP were 76.5±4.8 and 646±43 for Atomlab 100plus and Capintec CRC-55tR, respectively. In the case of Atomlab 100plus, the TP-based dial settings improved the accuracyof 188Reactivity measurements (confirmedbyHPGemeasurements) as compared to manufacturer-recommended settings. For Capintec CRC-55tR, the TP-based settings were in agreement with previous results [B. E. Zimmerman et al., J. Nucl. Med. 40, 1508-1516 (1999)] which demonstrated that manufacturer-recommended settings overestimate 188Re activity by more than 20%. The largest source of uncertainty in the experimentally determined dial settings was due to the application of a geometry correction factor, followed by the uncertainty of the scatter-corrected photopeak counts and the uncertainty of the TP efficiency calibration experiment. When using the most intense photopeak of the sample's emissions, the TP method yielded accurate (within 5% errors) and reproducible(COV= 2%)measurements of sample's activity. The relative uncertainties associated with such measurements ranged from 6% to 8% (expanded uncertainty at 95% confidence interval, k = 2). Conclusions: Accurate determination/verification of dose calibrator dial settings can be performed using a thyroid-probe in the nuclear medicine department. [ABSTRACT FROM AUTHOR]

Purpose: To explore the use of the frequency of the energy deposition (ED) clusters of different sizes (cluster order, CO) as a surrogate (instead of, e.g., LET) classification of the physical characteristics of ionizing radiation at a nanometer scale, to construct a framework for the calculation of relative biological effectiveness (RBE) with cell survival as endpoint. Methods: The frequency of cluster order ƒCO is calculated by sorting the ED sites generated with the Monte Carlo track structure code LIonTrack into clusters based on a single parameter called the cluster distance dC being the maximum allowed distance between two neighboring EDs belonging to a cluster. Published cell survival data parameterized with the linear-quadratic (LQ) model for V79 cells exposed to 15 different radiation qualities (including brachytherapy sources, proton, and carbon ions) were used as input to a fitting procedure, designed to determine a weighting function wCO that describes the capacity of a cluster of a certain CO to damage the cell's sensitive volume. The proposed framework uses both ƒCO and wCO to construct surrogate based functions for the LQ parameters α and β from which RBE values can be derived. Results: The results demonstrate that radiation quality independent weights wCO exist for both the α and β parameters. This enables the calculation of α values that correlate to their experimental counterparts within experimental uncertainties (relative residual of 15% for dC = 2.5 nm). The combination of both α and β surrogate based functions, despite the higher relative residuals for β values, yielded an RBE function that correlated to experimentally derived RBE values (relative residual of 16.5% for dC = 2.5 nm) for all radiation qualities included in this work. Conclusions: The ƒCO cluster characterization of ionizing radiation at a nanometer scale can effectively be used to calculate particle and energy dependent α and β values to predict RBE values with potential applications to, e.g., treatment planning systems in radiotherapy. [ABSTRACT FROM AUTHOR]

Purpose: Primary hepatocellular carcinoma and metastatic liver tumors are highly malignant tumors in Asia. The incidence of fatal liver cancer is also increasing in the United States. The aim of this study was to establish a spherical tumor model and determine its accuracy in predicting the absorbed dose in yttrium-90 (Y-90) microsphere therapy for liver cancer. Methods: Liver morphology can be approximated by a spherical model comprising three concentric regions representing necrotic, tumor, and normal liver tissues. The volumes of these three regions represent those in the actual liver. A spherical tumor model was proposed to calculate the absorbed fractions in the spherical tumor, necrotic, and normal tissue regions. The THORplan treatment planning system and Monte Carlo N-particle extended codes were used for this spherical tumor model. Using the volume-equivalent method, a spherical tumor model was created to calculate the total absorbed fraction [under different tumor-to-healthy-liver ratios (TLRs)]. The patient-specific model (THORplan) results were used to verify the spherical tumor model results. Results: The results for both the Y-90 spectrum and the Y-90 mean energy indicated that the absorbed fraction was a function of the tumor radius and mass. The absorbed fraction increased with tumor radius. The total absorbed fractions calculated using the spherical tumor model for necrotic, liver tumor, and normal liver tissues were in good agreement with the THORplan results, with differences of less than 3% for TLRs of 2-5. The results for the effect of TLR indicate that for the same tumor configuration, the total absorbed fraction decreased with increasing TLR; for the same shell tumor thickness and TLR, the total absorbed fraction was approximately constant; and for tumors with the same radius, the total fraction absorbed by the tumor increased with the shell thickness. Conclusions: The results from spherical tumor models with different tumor-to-healthy-liver ratios were highly consistent with the reference results (THORplan). These findings indicate that a spherical tumor model can provide good estimates of Y-90 doses in microsphere therapy and can be considered a first approximation for dose estimation in Y-90 microsphere therapy. [ABSTRACT FROM AUTHOR]

Purpose: This study provides a proof of concept for real-time 4D dose reconstruction for lung stereotactic body radiation therapy (SBRT) with multileaf collimator (MLC) tracking and assesses the impact of tumor tracking on the size of target margins. Methods: The authors have implemented real-time 4D dose reconstruction by connecting their tracking and delivery software to an Agility MLC at an Elekta Synergy linac and to their in-house treatment planning software (TPS). Actual MLC apertures and (simulated) target positions are reported to the TPS every 40 ms. The dose is calculated in real-time from 4DCT data directly after each reported aperture by utilization of precalculated dose-influence data based on a Monte Carlo algorithm. The dose is accumulated onto the peak-exhale (reference) phase using energy-mass transfer mapping. To investigate the impact of a potentially reducible safety margin, the authors have created and delivered treatment plans designed for a conventional internal target volume (ITV) + 5 mm, a midventilation approach, and three tracking scenarios for four lung SBRT patients. For the tracking plans, a moving target volume (MTV) was established by delineating the gross target volume (GTV) on every 4DCT phase. These were rigidly aligned to the reference phase, resulting in a unified maximum GTV to which a 1, 3, or 5 mm isotropic margin was added. All scenarios were planned for 9-beam step-and-shoot IMRT to meet the criteria of RTOG 1021 (3×18 Gy). The GTV 3D center-of-volume shift varied from 6 to 14 mm. Results: Real-time dose reconstruction at 25 Hz could be realized on a single workstation due to the highly efficient implementation of dose calculation and dose accumulation. Decreased PTV margins resulted in inadequate target coverage during untracked deliveries for patients with substantial tumor motion. MLC tracking could ensure the GTV target dose for these patients. Organ-at-risk (OAR) doses were consistently reduced by decreased PTV margins. The tracked MTV+1 mm deliveries resulted in the following OAR dose reductions: lung V20 up to 3.5%, spinal cord D2 up to 0.9 Gy/Fx, and proximal airways D2 up to 1.4 Gy/Fx. Conclusions: The authors could show that for patient data at clinical resolution and realistic motion conditions, the delivered dose could be reconstructed in 4D for the whole lung volume in real-time. The dose distributions show that reduced margins yield lower doses to healthy tissue, whilst target dose can be maintained using dynamic MLC tracking. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this work is to search for new metrics that could give more reliable acceptance/ rejection criteria on the IMRT verification process and to offer solutions to the discrepancies found among different conventional metrics. Therefore, besides conventional metrics, new ones are proposed and evaluated with new tools to find correlations among them. These new metrics are based on the processing of the dose-volume histogram information, evaluating the absorbed dose differences, the dose constraint fulfillment, or modified biomathematical treatment outcome models such as tumor control probability (TCP) and normal tissue complication probability (NTCP). An additional purpose is to establish whether the new metrics yield the same acceptance/rejection plan distribution as the conventional ones. Methods: Fifty eight treatment plans concerning several patient locations are analyzed. All of them were verified prior to the treatment, using conventional metrics, and retrospectively after the treatment with the new metrics. These new metrics include the definition of three continuous functions, based on dose-volume histograms resulting from measurements evaluated with a reconstructed dose system and also with a Monte Carlo redundant calculation. The 3D gamma function for every volume of interest is also calculated. The information is also processed to obtain ΔTCP or ΔNTCP for the considered volumes of interest. These biomathematical treatment outcome models have been modified to increase their sensitivity to dose changes. A robustness index from a radiobiological point of view is defined to classify plans in robustness against dose changes. Results: Dose difference metrics can be condensed in a single parameter: the dose difference global function, with an optimal cutoff that can be determined from a receiver operating characteristics (ROC) analysis of the metric. It is not always possible to correlate differences in biomathematical treatment outcome models with dose difference metrics. This is due to the fact that the dose constraint is often far from the dose that has an actual impact on the radiobiological model, and therefore, biomathematical treatment outcome models are insensitive to big dose differences between the verification system and the treatment planning system. As an alternative, the use of modified radiobiological models which provides a better correlation is proposed. In any case, it is better to choose robust plans from a radiobiological point of view. The robustness index defined in this work is a good predictor of the plan rejection probability according to metrics derived from modified radiobiological models. The global 3D gamma-based metric calculated for each plan volume shows a good correlation with the dose difference metrics and presents a good performance in the acceptance/rejection process. Some discrepancies have been found in dose reconstruction depending on the algorithm employed. Significant and unavoidable discrepancies were found between the conventional metrics and the new ones. Conclusions: The dose difference global function and the 3D gamma for each plan volume are good classifiers regarding dose difference metrics. ROC analysis is useful to evaluate the predictive power of the new metrics. The correlation between biomathematical treatment outcome models and the dose difference-based metrics is enhanced by using modified TCP and NTCP functions that take into account the dose constraints for each plan. The robustness index is useful to evaluate if a plan is likely to be rejected. Conventional verification should be replaced by the new metrics, which are clinically more relevant. [ABSTRACT FROM AUTHOR]

Purpose: In targeted radionuclide therapy, regional tumors are targeted with radionuclides delivering therapeutic radiation doses. Targeted alpha therapy (TAT) is of particular interest due to its ability to deliver alpha particles of high linear energy transfer within the confines of the tumor. However, there is a lack of data related to alpha particle distribution in TAT. These data are required to more accurately estimate the absorbed dose on a cellular level. As a result, there is a need for a dosimeter that can estimate, or better yet determine the absorbed dose deposited by alpha particles in cells. In this study, as an initial step, the authors present a transmission dosimetry design for alpha particles using A549 lung carcinoma cells, an external alpha particle emitting source (radium 223; Ra-223) and a Timepix pixelated semiconductor detector. Methods: The dose delivery to the A549 lung carcinoma cell line from a Ra-223 source, considered to be an attractive radionuclide for alpha therapy, was investigated in the current work. A549 cells were either unirradiated (control) or irradiated for 1/2, 1, 2, or 3 h with alpha particles emitted from a Ra-223 source positioned below a monolayer of A549 cells. The Timepix detector was used to determine the number of transmitted alpha particles passing through the A549 cells and DNA double strand breaks (DSBs) in the form of γ-H2AX foci were examined by fluorescence microscopy. The number of transmitted alpha particles was correlated with the observed DNA DSBs and the delivered radiation dose was estimated. Additionally, the dose deposited was calculated using Monte Carlo code SRIM. Results: Approximately 20% of alpha particles were transmitted and detected by Timepix. The frequency and number of γ-H2AX foci increased significantly following alpha particle irradiation as compared to unirradiated controls. The equivalent dose delivered to A549 cells was estimated to be approximately 0.66, 1.32, 2.53, and 3.96 Gy after 1/2, 1, 2, and 3 h irradiation, respectively, considering a relative biological effectiveness of alpha particles of 5.5. Conclusions: The study confirmed that the Timepix detector can be used for transmission alpha particle dosimetry. If cross-calibrated using biological dosimetry, this method will give a good indication of the biological effects of alpha particles without the need for repeated biological dosimetry which is costly, time consuming, and not readily available. [ABSTRACT FROM AUTHOR]

Purpose: Radiation dose to patients undergoing examinations with Multislice Computed Tomography (MSCT) as well as Cone Beam Computed Tomography (CBCT) is a matter of concern. Risk management could benefit from efficient replace rational dose calculation tools. The paper aims to verify MSCT dose calculations using a Treatment Planning System (TPS) for radiotherapy and to evaluate four different variations of bow-tie filter characterizations for the beam model used in the dose calculations. Methods: A TPS (RayStation™, RaySearch Laboratories, Stockholm, Sweden) was configured to calculate dose from a MSCT (GE Healthcare, Wauwatosa, WI, USA). The x-ray beam was characterized in a stationary position the by measurements of the Half-Value Layer (HVL) in aluminum and kerma along the principal axes of the isocenter plane perpendicular to the beam. A Monte Carlo source model for the dose calculation was applied with four different variations on the beam-shaping bow-tie filter, taking into account the different degrees of HVL information but reconstructing the measured kerma distribution after the bow-tie filter by adjusting the photon sampling function. The resulting dose calculations were verified by comparison with measurements in solid water as well as in an anthropomorphic phantom. Results: The calculated depth dose in solid water as well as the relative dose profiles was in agreement with the corresponding measured values. Doses calculated in the anthropomorphic phantom in the range 26-55 mGy agreed with the corresponding thermo luminescence dosimeter (TLD) measurements. Deviations between measurements and calculations were of the order of the measurement uncertainties. There was no significant difference between the different variations on the bow-tie filter modeling. Conclusions: Under the assumption that the calculated kerma after the bow-tie filter replicates the measured kerma, the central specification of the HVL of the x-ray beam together with the kerma distribution can be used to characterize the beam. Thus, within the limits of the study, a flat bow-tie filter with an HVL specified by the vendor suffices to calculate the dose distribution. The TPS could be successfully configured to replicate the beam movement and intensity modulation of a spiral scan with dose modulation, on the basis of the specifications available in the metadata of the digital images and the log file of the CT. [ABSTRACT FROM AUTHOR]

Purpose: A practical means of delivering both therapeutic radiation and hyperthermia to a deepseated target has been identified in the literature as highly desirable, provided it is capable of generating sufficient temperatures over the defined target volume. The authors present continued development of a dual-modality thermobrachytherapy (TB) seed, investigating its capabilities in delivering prescribed hyperthermia to realistic deep-seated targets. Methods: The TB seed is based on the ubiquitous low dose-rate (LDR) brachytherapy permanent implant. Heat is generated by incorporating a ferromagnetic core within the seed and placing the patient in an oscillating external magnetic field, producing eddy currents within the core and hence Joule heating. A strategically selected Curie temperature results in thermal self-regulation. The magnetic and thermal properties of the TB seed were studied experimentally by means of seed prototypes placed in a tissue-mimicking phantom and heated with an industrial induction heater, as well as computationally in the finite element analysis solver COMSOL Multiphysics. Patient-specific seed distributions derived from LDR permanent prostate implants previously conducted at their institution were modeled in COMSOL to evaluate their ability to adequately cover a defined target volume and to overcome the loss of heat due to blood perfusion within tissue. The calculated temperature distributions were analyzed by generating temperature-volume histograms, which were used to quantify coverage and temperature homogeneity for varied blood perfusion rates, seed Curie temperatures, and thermal power production rates. Use of additional hyperthermia-only (HT-only) seeds in unused spots within the implantation needles was investigated, as was an increase in these seeds' core size to increase their power. The impact of the interseed attenuation and scatter (ISA) effect on radiation dose distributions of this seed was also quantified by Monte Carlo studies in the software package Monte Carlo N-Particle Version 5. Results: Increasing the power production of the seeds, as well as increasing their Curie point, would increase the maximum blood perfusion rate that a given seed distribution could overcome to obtain an acceptable temperature distribution. However, this would also increase the maximum temperatures generated at the seed surfaces. Auxiliary HT-only seeds serve to improve the temperature uniformity within the target, as well as decrease the seed power generation requirements. Both an increase in their core size and an increase in both seed types' Curie temperatures enhance the resulting temperature coverage. The interseed and scatter effect caused by both the TB and HT-only seeds was found to reduce the dose to 90% of the target volume (D90) by a factor of 1.10±0.02. Conclusions: A systematic approach of combining LDR prostate brachytherapy with hyperthermia is described, and its ability to provide sufficient and uniform temperature distributions in realistic patient-specific implants evaluated. A combination of TB and HT-only seeds may be used to produce a uniform temperature distribution in a defined target. Various modeled changes to their design, such as optimization of their Curie temperature, improve their ability to overcome the thermal effects of blood perfusion. The enhanced ISA of the TB and HT-only seeds must be taken into account for dose calculations, but is manageable. [ABSTRACT FROM AUTHOR]

Purpose: Proton beam dosimetry using bare plastic optical fibers has emerged as a simple approach to proton beam dosimetry. The source of the signal in this method has been attributed to C? erenkov radiation. The aim of this work was a phenomenological study of the nature of the visible light responsible for the signal in bare fiber optic dosimetry of proton therapy beams. Methods: Plastic fiber optic probes embedded in solid water phantoms were irradiated with proton beams of energies 100, 180, and 225 MeV produced by a proton therapy cyclotron. Luminescence spectroscopy was performed by a CCD-coupled spectrometer. The spectra were acquired at various depths in phantom to measure the percentage depth dose (PDD) for each beam energy. For comparison, the PDD curves were acquired using a standard multilayer ion chamber device. In order to further analyze the contribution of the C? erenkov radiation in the spectra, Monte Carlo simulation was performed using ????? Monte Carlo code to stochastically simulate radiation transport, ionizing radiation dose deposition, and optical emission of C? erenkov radiation. Results: The measured depth doses using the bare fiber are in agreement with measurements performed by the multilayer ion chamber device, indicating the feasibility of using bare fiber probes for proton beam dosimetry. The spectroscopic study of proton-irradiated fibers showed a continuous spectrum with a shape different from that of C? erenkov radiation. The Monte Carlo simulations confirmed that the amount of the generated C? erenkov light does not follow the radiation absorbed dose in a medium. Conclusions: The source of the optical signal responsible for the proton dose measurement using bare optical fibers is not C? erenkov radiation. It is fluorescence of the plastic material of the fiber. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study is to develop a Monte Carlo simulation model for in vivo associated particle neutron elemental imaging (APNEI) and to study the feasibility of using APNEI to determine the iron distribution in a human liver with the defined model. Methods: The model presented in this study was defined in ???? by the basic geometry of the human body, the use of D+D source neutrons, iron as the element of interest, an iron-containing voxel in the liver as the target region, and 2 large germanium detectors anterior and posterior to the trunk of the body. The f 8 pulse height tally was employed in ???? to determine the signal acquired from iron inelastic scatter gamma rays at various iron concentrations in the target liver voxel. Correspondingly, the f 4 average flux tally in ???? was modified by a dose function such that the equivalent dose to the whole liver and the effective dose to the whole body could be estimated and used as the basis for a limiting number of neutron histories which could feasibly allow for the collection of a sufficient volume of data to construct a 2D image of iron distribution in the liver voxel. Results: Assuming an allowable equivalent dose to the liver of 5 mSv, 143 inelastic scatter iron gamma ray counts (at ~847 keV) would ideally be registered at the germanium detectors for a 1 cm³ cube-shaped liver voxel with an iron concentration of 1000 ppm. According to the simulation model, an image of iron distribution in the liver can be constructed with a 1 cm resolution at the level of 1000 ppm iron. Collecting such an image would yield an estimated whole body dose of 0.82 mSv. The mathematical introduction of image uncertainty resulting from source spot diameter and detector timing resolution more closely approximates the result of real world application. Conclusions: APNEI of certain elements in vivo appears feasible given several timing, sensitivity, and resolution caveats. However, further study is required to determine what the detection limit of iron would be and what image resolution would be in an experimental setup as the present model contains idealized assumptions which overestimate the signal attributable to iron inelastic scatter gamma rays. [ABSTRACT FROM AUTHOR]

Purpose: The primary objective of this work is to measure the secondary neutron field produced by an uncollimated proton pencil beam impinging on different tissue-equivalent phantom materials using organic scintillation detectors. Additionally, the Monte Carlo code ?????-PoliMi was used to simulate the detector response for comparison to the measured data. Comparison of the measured and simulated data will validate this approach for monitoring secondary neutron dose during proton therapy. Methods: Proton beams of 155- and 200-MeV were used to irradiate a variety of phantom materials and secondary particles were detected using organic liquid scintillators. These detectors are sensitive to fast neutrons and gamma rays: pulse shape discrimination was used to classify each detected pulse as either a neutron or a gamma ray. The ?????-PoliMi code was used to simulate the secondary neutron field produced during proton irradiation of the same tissue-equivalent phantom materials. Results: An experiment was performed at the Loma Linda University Medical Center proton therapy research beam line and corresponding models were created using the ?????-PoliMi code. The authors' analysis showed agreement between the simulations and the measurements. The simulated detector response can be used to validate the simulations of neutron and gamma doses on a particular beam line with or without a phantom. Conclusions: The authors have demonstrated a method of monitoring the neutron component of the secondary radiation field produced by therapeutic protons. The method relies on direct detection of secondary neutrons and gamma rays using organic scintillation detectors. These detectors are sensitive over the full range of biologically relevant neutron energies above 0.5 MeV and allow effective discrimination between neutron and photon dose. Because the detector system is portable, the described system could be used in the future to evaluate secondary neutron and gamma doses on various clinical beam lines for commissioning and prospective data collection in pediatric patients treated with proton therapy. [ABSTRACT FROM AUTHOR]

Purpose: In multidetector CT, the volume CT dose index (CTDIvol) is reported for each scan series and dose conversion factors are used for the size-specific dose estimate (SSDE) and scannerindependent organ dose evaluation. This study aimed at examining the dependencies of conversion factors on scan length, tube voltage, and subject size. The results may be insightful for evaluating the dose from CT examinations with large variations in patient size and scan length. Methods: A previously developed Monte Carlo simulation program was used to simulate single rotation axial scans of two standard CTDI phantoms [material polymethyl methacrylate (PMMA), diameters 16 and 32 cm] and multiple water cylinders at five tube voltages (70, 80, 100, 120, and 140 kV). The resultant longitudinal dose profiles were used to calculate CTDIL(water)/CTDIw (PMMA), where L was dose integration length. The ratio was equal to the midpoint dose DL(0) (water) to CTDIvol ratio in a CT scan series with a scan length equal to L. Results: For water phantom diameters from 11 to 50 cm and scan lengths from 15 to 30 cm, the changes of DL(0)/CTDIvol from that of 120 kV and 20-cm scan length were between -18.4% and 11.7%. This was consistent with the CTDIvol to SSDE conversion factors of AAPM Report No. 204. For scan lengths less than 15 cm, DL(0)/CTDIvol decreased considerably as L decreased. DL(0)/CTDIvol was shown to be 17.3%-31.4% lower for L = 5 cm than for L = 15 cm, when the tube voltage was 120 kV and phantom diameter ranged from 11 to 50 cm. As tube voltage increased from 80 to 140 kV, DL(0)/CTDIvol decreased at small diameters while it increased at large diameters. The change was 9.4% with a diameter of 18 cm and a scan length of 20 cm and 17.6% with a diameter of 40 cm and a scan length of 30 cm. Conclusions: The midpoint dose to CTDIvol ratio varies widely across the clinical scan lengths from a few millimeters to about 1 m and varies moderately across tube voltages from 70 to 140 kV. The comprehensive data provided in the Appendix can be used for assessing the dose from short-length scans and improving the dose evaluation in CT. [ABSTRACT FROM AUTHOR]

Purpose: Magnetic resonance imaging (MRI)-guided Co-60 provides daily and intrafractional MRI soft tissue imaging for improved target and critical organ tracking. To increase delivery efficiency, the system uses three Co-60 sources at 120? apart, allowing up to 600 cGy combined dose rate at isocenter. Despite the potential tripling in output, creating a delivery plan that uses all three sources is considerably unintuitive. Here, the authors computerize the triplet orientation optimization using column generation, an approach that was demonstrated effective in integrated beam orientation and fluence optimization for noncoplanar therapies. To achieve a better plan quality without increasing the treatment time, the authors then solve a fluence map optimization (FMO) problem while regularizing the fluence maps to reduce the number of deliverable MLC segments. Methods: Three patients--one prostate, one lung, and one head and neck boost plan (H&NBoost)-- were evaluated in this study. For each patient, the beamlet doses were calculated using Monte Carlo, under a 0.35 T magnetic field, for 180 equally spaced coplanar beams grouped into 60 triplets. The beamlet size is 1.05×0.5 cm determined by the MLC leaf thickness and step size. The triplets were selected using the column generation algorithm. The FMO problem was formulated using an L2-norm dose fidelity term and an L1-norm anisotropic total variation regularization term, which allows controlling the number of MLC segments, and hence the treatment time, with minimal degradation to the dose. The authors' Fluence Regularization and Optimized Selection of Triplets (FROST) plans were compared against the clinical treatment plans (CLNs) produced by an experienced dosimetrist. PTV homogeneity, max dose, mean dose, D95, D98, and D99 were evaluated. OAR max and mean doses, as well as R50, defined as the ratio of the 50% isodose volume over the planning target volume were investigated. Results: The mean PTV D95, D98, and D99 differ by +0.04%, +0.07%, and +0.25% of the prescription dose between planning methods. The mean PTV homogeneity was virtually same with values at 0.8788 (FROST) and 0.8812 (CLN). R50 decreased by 0.67 comparing FROST to CLN. On average, FROST reduced Dmax and Dmean of OARs by 7.30% and 6.08% of the prescription dose, respectively. The manual CLN planning processes required numerous trial and error runs. The FROST plans on the other hand required minimal human intervention. Conclusions: Efficient delivery of MRI-guided Co-60 therapy needs the output of multiple sources yet suffers from unintuitive and laborious manual beam selection processes. Computerized triplet orientation optimization improves both planning efficiency and plan dosimetry. The novel fluence map regularization provides additional controls over the number of MLC segments and treatment time. [ABSTRACT FROM AUTHOR]

Purpose: Small field measurements are challenging, due to the physical characteristics coming from the lack of charged particle equilibrium, the partial occlusion of the finite radiation source, and to the detector response. These characteristics can be modeled in the dose calculations in the treatment planning systems. Aim of the present work is to evaluate the MU calculation accuracy for small fields, defined by jaw or MLC, for anisotropic analytical algorithm (AAA) and Acuros XB algorithms, relative to output measurements on the beam central axis. Methods: Single point output factor measurement was acquired with a PTW microDiamond detector for 6 MV, 6 and 10 MV unflattened beams generated by a Varian TrueBeam STx equipped with high definition-MLC. Fields defined by jaw or MLC apertures were set; jaw-defined: 0.6×0.6, 0.8×0.8, 1×1, 2×2, 3×3, 4×4, 5×5, and 10×10 cm²; MLC-defined: 0.5×0.5 cm² to the maximum field defined by the jaw, with 0.5 cm stepping, and jaws set to: 2×2, 3×3, 4×4, 5×5, and 10×10 cm². MU calculation was obtained with 1 mm grid in a virtual water phantom for the same fields, for AAA and Acuros algorithms implemented in the Varian ECLIPSE treatment planning system (version 13.6). Configuration parameters as the effective spot size (ESS) and the dosimetric leaf gap (DLG) were varied to find the best parameter setting. Differences between calculated and measured doses were analyzed. Results: Agreement better than 0.5% was found for field sizes equal to or larger than 2×2 cm² for both algorithms. A dose overestimation was present for smaller jaw-defined fields, with the best agreement, averaged over all the energies, of 1.6% and 4.6% for a 1×1 cm² field calculated by AAA and Acuros, respectively, for a configuration with ESS = 1 mm for both X and Y directions for AAA, and ESS = 1.5 and 0 mm for X and Y directions for Acuros. Conversely, a calculated dose underestimation was found for small MLC-defined fields, with the best agreement averaged over all the energies, of -3.9% and 0.2% for a 1×1 cm² field calculated by AAA and Acuros, respectively, for a configuration with ESS = 0 mm for both directions and both algorithms. Conclusions: For optimal setting applied in the algorithm configuration phase, the agreement of Acuros calculations with measurements could achieve the 3% for MLC-defined fields as small as 0.5×0.5 cm². Similar agreement was found for AAA for fields as small as 1×1 cm². [ABSTRACT FROM AUTHOR]

Purpose: Inaccurate density information may introduce dose calculation errors when inhomogeneity correction is applied. The aim of the present study was to examine the effect of density variation on photon dose calculation accuracy using the convolution/superposition (CS) algorithm with the focus on newer treatment technologies including intensity modulated radiotherapy, volumetric modulated arc radiotherapy, and stereotactic body radiotherapy (SBRT). Methods: Calculations were first performed using simple inhomogeneity phantoms in order to determine clinically relevant tolerance levels for different tissue types. The clinical validity of these tolerance levels was then demonstrated by evaluating their dosimetric impact on clinical treatment plans. The dose difference was examined by comparing the dose-volume histogram statistics and the spatial distribution of dose errors calculated on a voxel-by-voxel basis. In order to gain some insight into this issue for the Monte Carlo (MC) algorithm, the authors also performed additional validation using a MC dose calculation system. Results: For soft tissue and bone, the tolerance levels determined from this study appear to be consistent with the values previously calculated using simpler inhomogeneity correction methods. However, the tolerance level for low density lung tissue has been found to be much smaller than what previous studies had reported. The results from this study also suggest that if density variation is restricted within ±0.02, ±0.03, and ±0.10 g/cm³ for lung, soft tissue, and bone, respectively, the resulting dose error in target volumes can be limited to <2% for most clinical cases and <3% for more challenging lung SBRT cases. When the same amount of density variation is introduced, MC algorithm yields ~0.3%-0.9% and ~0.0%-1.2% smaller dose errors for the target and organs-at-risk as compared to CS. Conclusions: It is important to include lung substitute material into the periodic quality assurance of CT simulators and treatment planning systems. This study suggests that the tolerance value of CT number for lung material is ~±20 HU in order to keep the associated dose uncertainty at 2%-3% or less. Further studies with larger number of lung cases are warranted to validate this new tolerance value before it can be applied to clinical practice. [ABSTRACT FROM AUTHOR]

Purpose: CyberKnife system is initially equipped with fixed circular cones for stereotactic radiosurgery. Two dose calculation algorithms, Ray-Tracing and Monte Carlo, are available in the supplied treatment planning system. A multileaf collimator system was recently introduced in the latest generation of system, capable of arbitrarily shaped treatment field. The purpose of this study is to develop a model based dose calculation algorithm to better handle the lateral scatter in an irregularly shaped small field for the CyberKnife system. Methods: A pencil beam dose calculation algorithm widely used in linac based treatment planning system was modified. The kernel parameters and intensity profile were systematically determined by fitting to the commissioning data. The model was tuned using only a subset of measured data (4 out of 12 cones) and applied to all fixed circular cones for evaluation. The root mean square (RMS) of the difference between the measured and calculated tissue-phantom-ratios (TPRs) and off-center-ratio (OCR) was compared. Three cone size correction techniques were developed to better fit the OCRs at the penumbra region, which are further evaluated by the output factors (OFs). The pencil beam model was further validated against measurement data on the variable dodecagon-shaped Iris collimators and a half-beam blocked field. Comparison with Ray-Tracing and Monte Carlo methods was also performed on a lung SBRT case. Results: The RMS between the measured and calculated TPRs is 0.7% averaged for all cones, with the descending region at 0.5%. The RMSs of OCR at infield and outfield regions are both at 0.5%. The distance to agreement (DTA) at the OCR penumbra region is 0.2 mm. All three cone size correction models achieve the same improvement in OCR agreement, with the effective source shift model (SSM) preferred, due to their ability to predict more accurately the OF variations with the source to axis distance (SAD). In noncircular field validation, the pencil beam calculated results agreed well with the film measurement of both Iris collimators and the half-beam blocked field, fared much better than the Ray-Tracing calculation. Conclusions: The authors have developed a pencil beam dose calculation model for the CyberKnife system. The dose calculation accuracy is better than the standard linac based system because the model parameters were specifically tuned to the CyberKnife system and geometry correction factors. The model handles better the lateral scatter and has the potential to be used for the irregularly shaped fields. Comprehensive validations on MLC equipped system are necessary for its clinical implementation. It is reasonably fast enough to be used during plan optimization. [ABSTRACT FROM AUTHOR]

Purpose: Accurate reconstruction of the dose delivered by 90Y microspheres using a postembolization PET scan would permit the establishment of more accurate dose-response relationships for treatment of hepatocellular carcinoma with 90Y. However, the quality of the PET data obtained is compromised by several factors, including poor count statistics and a very high random fraction. This work uses Monte Carlo simulations to investigate what impact factors other than low count statistics have on the quantification of 90Y PET. Methods: PET acquisitions of two phantoms--a NEMA PET phantom and the NEMA IEC PET body phantom-containing either 90Y or 18F were simulated using GATE. Simulated projections were created with subsets of the simulation data allowing the contributions of random, scatter, and LSO background to be independently evaluated. The simulated projections were reconstructed using the commercial software for the simulated scanner, and the quantitative accuracy of the reconstruction and the contrast recovery of the reconstructed images were evaluated. Results: The quantitative accuracy of the 90Y reconstructions were not strongly influenced by the high random fraction present in the projection data, and the activity concentration was recovered to within 5% of the known value. The contrast recovery measured for simulated 90Y data was slightly poorer than that for simulated 18F data with similar count statistics. However, the degradation was not strongly linked to any particular factor. Using a more restricted energy range to reduce the random fraction in the projections had no significant effect. Conclusions: Simulations of 90Y PET confirm that quantitative 90Y is achievable with the same approach as that used for 18F, and that there is likely very little margin for improvement by attempting to model aspects unique to 90Y, such as the much higher random fraction or the presence of bremsstrahlung in the singles data. [ABSTRACT FROM AUTHOR]

Purpose: Small field output correction factors have been studied by several research groups for the PTW 60019 microDiamond (MD) dosimeter, by comparing the response of such a device with both reference dosimeters and Monte Carlo simulations. A general good agreement is observed for field sizes down to about 1 cm. However, evident inconsistencies can be noticed when comparing some experimental results and Monte Carlo simulations obtained for smaller irradiation fields. This issue was tentatively attributed by some authors to unintentional large variations of the MD active surface area. The aim of the present study is a nondestructive experimental determination of the MD active surface area and active volume. Methods: Ten MD dosimeters, one MD prototype, and three synthetic diamond samples were investigated in the present work. 2D maps of the MD response were recorded under scanned soft x-ray microbeam irradiation, leading to an experimental determination of the device active surface area. Profiles of the device responses were measured as well. In order to evaluate the MD active volume, the thickness of the diamond sensing layer was independently evaluated by capacitance measurements and alpha particle detection experiments. The MD sensitivity, measured at the PTW calibration laboratory, was also used to calculate the device active volume thickness. Results: An average active surface area diameter of (2.19±0.02) mm was evaluated by 2D maps and response profiles of all the MDs. Average active volume thicknesses of (1.01±0.13) µm and (0.97±0.14) µm were derived by capacitance and sensitivity measurements, respectively. The obtained results are well in agreement with the nominal values reported in the manufacturer dosimeter specifications. A homogeneous response was observed over the whole device active area. Besides the one from the device active volume, no contributions from other components of the housing nor from encapsulation materials were observed in the 2D response maps. Conclusions: The obtained results demonstrate the high reproducibility of the MD fabrication process. The observed discrepancies among the output correction factors reported by several authors for MD response in very small fields are very unlikely to be ascribed to unintentional variations of the device active surface area and volume. It is the opinion of the authors that the role of the volume averaging as well as of other perturbation effects should be separately investigated instead, both experimentally and by Monte Carlo simulations, in order to better clarify the behaviour of the MD response in very small fields. [ABSTRACT FROM AUTHOR]

Purpose: Magnetic resonance imaging-guided radiotherapy (MRIgRT) provides superior soft-tissue contrast and real-time imaging compared with standard image-guided RT, which uses x-ray based imaging. Several groups are developing integrated MRIgRT machines. Reference dosimetry with these new machines requires accounting for the effects of the magnetic field on the response of the ionization chambers used for dose calibration. Here, the authors propose a formalism for reference dosimetry with integrated MRIgRT devices. The authors also examined the suitability of the TPR1020 and %dd(10)x beam quality specifiers in the presence of magnetic fields and calculated detector correction factors to account for the effects of the magnetic field for a range of detectors. Methods: The authors used full-head and point-source Monte Carlo models of an MR-linac along with detailed detector models of an Exradin A19, an NE2571, and several PTW Farmer chambers to calculate magnetic field correction factors for six commercial ionization chambers in three chamber configurations. Calculations of ionization chamber response (performed with GEANT4) were validated with specialized Fano cavity tests. %dd(10)x values, TPR1020 values, and Spencer-Attix water-to-air restricted stopping power ratios were also calculated. The results were further validated against measurements made with a preclinical functioning MR-linac. Results: The TPR1020 was found to be insensitive to the presence of the magnetic field, whereas the relative change in %dd(10)x was 2.4% when a transverse 1.5 T field was applied. The parameters chosen for the ionization chamber calculations passed the Fano cavity test to within ~0.1%. Magnetic field correction factors varied in magnitude with detector orientation with the smallest corrections found when the chamber was parallel to the magnetic field. Conclusions: Reference dosimetry can be performed with integrated MRIgRT devices by using magnetic field correction factors, but care must be taken with the choice of beam quality specifier and chamber orientation. The uncertainties achievable under this formalism should be similar to those of conventional formalisms, although this must be further quantified. [ABSTRACT FROM AUTHOR]

Purpose: A dose calculation tool, which combines the accuracy of the dose planning method (DPM) Monte Carlo code and the versatility of a practical analytical multisource model, which was previously reported has been improved and validated for the Varian 6 and 10 MV linear accelerators (linacs). The calculation tool can be used to calculate doses in advanced clinical application studies. One shortcoming of current clinical trials that report dose from patient plans is the lack of a standardized dose calculation methodology. Because commercial treatment planning systems (TPSs) have their own dose calculation algorithms and the clinical trial participant who uses these systems is responsible for commissioning the beam model, variation exists in the reported calculated dose distributions. Today's modern linac is manufactured to tight specifications so that variability within a linac model is quite low. The expectation is that a single dose calculation tool for a specific linac model can be used to accurately recalculate dose from patient plans that have been submitted to the clinical trial community from any institution. The calculation tool would provide for a more meaningful outcome analysis. Methods: The analytical source model was described by a primary point source, a secondary extra-focal source, and a contaminant electron source. Off-axis energy softening and fluence effects were also included. The additions of hyperbolic functions have been incorporated into the model to correct for the changes in output and in electron contamination with field size. A multileaf collimator (MLC) model is included to facilitate phantom and patient dose calculations. An offset to the MLC leaf positions was used to correct for the rudimentary assumed primary point source. Results: Dose calculations of the depth dose and profiles for field sizes 4?4 to 40?40 cm agree with measurement within 2% of the maximum dose or 2 mm distance to agreement (DTA) for 95% of the data points tested. The model was capable of predicting the depth of the maximum dose within 1 mm. Anthropomorphic phantom benchmark testing of modulated and patterned MLCs treatment plans showed agreement to measurement within 3% in target regions using thermoluminescent dosimeters (TLD). Using radiochromic film normalized to TLD, a gamma criteria of 3% of maximum dose and 2 mm DTA was applied with a pass rate of least 85% in the high dose, high gradient, and low dose regions. Finally, recalculations of patient plans using DPM showed good agreement relative to a commercial TPS when comparing dose volume histograms and 2D dose distributions. Conclusions: A unique analytical source model coupled to the dose planning method Monte Carlo dose calculation code has been modified and validated using basic beam data and anthropomorphic phantom measurement. While this tool can be applied in general use for a particular linac model, specifically it was developed to provide a singular methodology to independently assess treatment plan dose distributions from those clinical institutions participating in National Cancer Institute trials. [ABSTRACT FROM AUTHOR]

Purpose: To quantify and explain the backscatter dose effects for clinically relevant high atomic number materials being irradiated in the presence of a 1.5 T transverse magnetic field. Methods: Interface effects were investigated using Monte Carlo simulation techniques. We used GPUMCD (v5.1) and geant4 (v10.1) for this purpose. GPUMCD is a commercial software written for the Elekta AB, MRI linac. Dose was scored using GPUMCD in cubic voxels of side 1 and 0.5 mm, in two different virtual phantoms of dimensions 20 ? 20 ? 20 cm and 5 ? 5 ? 13.3 cm, respectively. A photon beam was generated from a point 143.5 cm away from the isocenter with energy distribution sampled from a histogram representing the true Elekta, MRI linac photon spectrum. A slab of variable thickness and position containing either bone, aluminum, titanium, stainless steel, or one of the two different dental filling materials was inserted as an inhomogeneity in the 20 ? 20 ? 20 cm phantom. The 5 ? 5 ? 13.3 cm phantom was used as a clinical test case in order to explain the dose perturbation effects for a head and neck cancer patient. The back scatter dose factor (BSDF) was defined as the ratio of the doses at a given depth with and without the presence of the inhomogeneity. Backscattered electron fluence was calculated at the inhomogeneity interface using geant4. A 1.5 T magnetic field was applied perpendicular to the direction of the beam in both phantoms, identical to the geometry in the Elekta MRI linac. Results: With the application of a 1.5 T magnetic field, all the BSDF's were reduced by 12%-47%, compared to the no magnetic field case. The corresponding backscattered electron fluence at the interface was also reduced by 45%-64%. The reduction in the BSDF at the interface, due to the application of the magnetic field, is manifested in a different manner for each material. In the case of bone, the dose drops at the interface contrary to the expected increase when no magnetic field is applied. In the case of aluminum, the dose at the interface is the same with and without the presence of the aluminum. For all of the other materials the dose increases at the interface. Conclusions: The reduction in dose at the interface, in the presence of the magnetic field, is directly related to the reduction in backscattered electron fluence. This reduction occurs due to two different reasons. First, the electron spectrum hitting the interface is changed when the magnetic field is turned on, which results in changes in the electron scattering probability. Second, some electrons that have curved trajectories due to the presence of the magnetic field are absorbed by the higher density side of the interface and no longer contribute to the backscattered electron fluence. [ABSTRACT FROM AUTHOR]

Purpose: Plastic scintillation detectors are promising candidates for the dosimetry of low- to medium-energy photons but quantitative knowledge of their energy response is a prerequisite for their correct use. The purpose of this study was to characterize the energy dependent response of small scintillation detectors (active volume <1 mm³) made from the commonly used plastic scintillator BC400. Methods: Different detectors made from BC400 were calibrated at a number of radiation qualities ranging from 10 to 280 kV and at a 60Co beam. All calibrations were performed at the Physikalisch- Technische Bundesanstalt, the National Metrology Institute of Germany. The energy response in terms of air kerma, dose to water, and dose to the scintillator was determined. Conversion factors from air kerma to dose to water and to dose to the scintillator were derived from Monte Carlo simulations. In order to quantitatively describe the energy dependence, a semiempirical model known as unimolecular quenching or Birks' formula was fitted to the data and from this the response to secondary electrons generated within the scintillator material BC400 was derived. Results: The detector energy response in terms of air kerma differs for different scintillator sizes and different detector casings. It is therefore necessary to take attenuation within the scintillator and in the casing into account when deriving the response in terms of dose to water from a calibration in terms of air kerma. The measured energy response in terms of dose to water for BC400 cannot be reproduced by the ratio of mean mass energy-absorption coefficients for polyvinyl toluene to water but shows evidence of quenching. The quenching parameter kB in Birks' formula was determined to be kB = (12.3±0.9) mg MeV-1 cm-2. Conclusions: The energy response was quantified relative to the response to 60Co which is the common radiation quality for the calibration of therapy dosemeters. The observed energy dependence could be well explained with the assumption of ionization quenching as described by Birks' formula. Plastic scintillation detectors should be calibrated at the same radiation quality that they will be used at and changes of the spectrum within the application need to be considered. The authors results can be used to evaluate the range of validity of a given calibration. [ABSTRACT FROM AUTHOR]

Purpose: Magnetic resonance imaging (MRI) guidance in radiation therapy brings real-time imaging and adaptive planning into the treatment vault where it can account for interfraction and intrafraction movement of soft tissue. The only commercially available MRI-guided radiation therapy device is a three-head 60Co and MRI system with an integrated treatment planning system (TPS). Couch attenuation of the beam of up to 20% is well modeled in the TPS. Variations in the patient's day-to-day position introduce discrepancies in the actual couch attenuation as modeled in the treatment plan. For this reason, the authors' institution avoids plans with beams that pass through or near the couch edges. This study investigates the effects of differential beam attenuation by the couch due to couch shifts in order to determine whether couch edge avoidance restrictions can be lifted. Couch shifts were simulated using a Monte Carlo treatment planning system and ion chamber measurements performed for validation. Methods: A total of 27 plans from 23 patients were investigated. Couch shifts of 1 and 2 cm were introduced in combinations of lateral and vertical directions to simulate patient position variations giving 16 shifted plans per reference plan. The 1 and 2 cm shifts were based on shifts recorded in 320 treatment fractions. Results: Following TG176 recommendations for measurement methods, couch attenuation measurements agreed with TPS modeled attenuation to within 2.1%. Planning target volume D95 changed less than 1% for 1 and 2 cm couch shifts in only the x-direction and less than 3% for all directions. Conclusions: Dosimetry of all plans tested was robust to couch shifts up to ±2 cm. In general, couch shifts resulted in clinically insignificant dosimetric deviations. It is conceivable that in certain cases with large systematic couch shifts and plans that are particularly sensitive to shifts, dosimetric changes might rise to a clinically significant level. [ABSTRACT FROM AUTHOR]

Purpose: To accurately and efficiently implement charged particle transport in a magnetic field in EGSnrc and validate the code for the use in phantom and ion chamber simulations. Methods: The effect of the magnetic field on the particle motion and position is determined using one- and three-point numerical integrations of the Lorentz force on the charged particle and is added to the condensed history calculation performed by the EGSnrc PRESTA-II algorithm. The code is tested with a Fano test adapted for the presence of magnetic fields. The code is compatible with all EGSnrc based applications, including egs++. Ion chamber calculations are compared to experimental measurements and the effect of the code on the efficiency and timing is determined. Results: Agreement with the Fano test's theoretical value is obtained at the 0.1% level for large step-sizes and in magnetic fields as strong as 5 T. The NE2571 dose calculations achieve agreement with the experiment within 0.5% up to 1 T beyond which deviations up to 1.2% are observed. Uniform air gaps of 0.5 and 1 mm and a misalignment of the incoming photon beam with the magnetic field are found to produce variations in the normalized dose on the order of 1%. These findings necessitate a clear definition of all experimental conditions to allow for accurate Monte Carlo simulations. It is found that ion chamber simulation times are increased by only 38%, and a 10 × 10 × 6 cm³ water phantom with (3 mm)³ voxels experiences a 48% increase in simulation time as compared to the default EGSnrc with no magnetic field. Conclusions: The incorporation of the effect of the magnetic fields in EGSnrc provides the capability to calculate high accuracy ion chamber and phantom doses for the use in MRI-radiation systems. Further, the effect of apparently insignificant experimental details is found to be accentuated by the presence of the magnetic field. [ABSTRACT FROM AUTHOR]

Purpose: To develop a multilayer ionization chamber (MLIC) and a correction technique that suppresses differences between the MLIC and water phantom measurements in order to achieve fast and accurate depth dose measurements in pencil beam scanning proton therapy. Methods: The authors distinguish between a calibration procedure and an additional correction: 1--the calibration for variations in the air gap thickness and the electrometer gains is addressed without involving measurements in water; 2--the correction is addressed to suppress the difference between depth dose profiles in water and in the MLIC materials due to the nuclear interaction cross sections by a semiempirical model tuned by using measurements in water. In the correction technique, raw MLIC data are obtained for each energy layer and integrated after multiplying them by the correction factor because the correction factor depends on incident energy. The MLIC described here has been designed especially for pencil beam scanning proton therapy. This MLIC is called a dual ring multilayer ionization chamber (DRMLIC). The shape of the electrodes allows the DRMLIC to measure both the percentage depth dose (PDD) and integrated depth dose (IDD) because ionization electrons are collected from inner and outer air gaps independently. Results: IDDs for which the beam energies were 71.6, 120.6, 159, 180.6, and 221.4 MeV were measured and compared with water phantom results. Furthermore, the measured PDDs along the central axis of the proton field with a nominal field size of 10×10 cm² were compared. The spread out Bragg peak was 20 cm for fields with a range of 30.6 and 3 cm for fields with a range of 6.9 cm. The IDDs measured with the DRMLIC using the correction technique were consistent with those that of the water phantom; except for the beam energy of 71.6 MeV, all of the points satisfied the 1% dose/1 mm distance to agreement criterion of the gamma index. The 71.6 MeV depth dose profile showed slight differences in the shallow region, but 94.5% of the points satisfied the 1%/1 mm criterion. The 90% ranges, defined at the 90% dose position in distal fall off, were in good agreement with those in the water phantom, and the range differences from the water phantom were less than ±0.3 mm. The PDDs measured with the DRMLIC were also consistent with those that of the water phantom; 97% of the points passed the 1%/1 mm criterion. Conclusions: It was demonstrated that the new correction technique suppresses the difference between the depth dose profiles obtained with the MLIC and those obtained from a water phantom, and a DRMLIC enabling fast measurements of both IDD and PDD was developed. The IDDs and PDDs measured with the DRMLIC and using the correction technique were in good agreement with those that of the water phantom, and it was concluded that the correction technique and DRMLIC are useful for depth dose profile measurements in pencil beam scanning proton therapy. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the sensitivity of the diagnostic radiological index of protection (DRIP), used to quantify the protective value of radioprotective garments, to procedural factors in fluoroscopy in an effort to determine an appropriate set of scatter-mimicking primary beams to be used in measuring the DRIP. Methods: Monte Carlo simulations were performed to determine the shape of the scattered x-ray spectra incident on the operator in different clinical fluoroscopy scenarios, including interventional radiology and interventional cardiology (IC). Two clinical simulations studied the sensitivity of the scattered spectrum to gantry angle and patient size, while technical factors were varied according to measured automatic dose rate control (ADRC) data. Factorial simulations studied the sensitivity of the scattered spectrum to gantry angle, field of view, patient size, and beam quality for constant technical factors. Average energy (Eavg) was the figure of merit used to condense fluence in each energy bin to a single numerical index. Results: Beam quality had the strongest influence on the scattered spectrum in fluoroscopy. Many procedural factors affect the scattered spectrum indirectly through their effect on primary beam quality through ADRC, e.g., gantry angle and patient size. Lateral C-arm rotation, common in IC, increased the energy of the scattered spectrum, regardless of the direction of rotation. The effect of patient size on scattered radiation depended on ADRC characteristics, patient size, and procedure type. Conclusions: The scattered spectrum striking the operator in fluoroscopy is most strongly influenced by primary beam quality, particularly kV. Use cases for protective garments should be classified by typical procedural primary beam qualities, which are governed by the ADRC according to the impacts of patient size, anatomical location, and gantry angle. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study was to determine fluence corrections necessary to convert absorbed dose to graphite, measured by graphite calorimetry, to absorbed dose to water. Fluence corrections were obtained from experiments and Monte Carlo simulations in low- and high-energy proton beams. Methods: Fluence corrections were calculated to account for the difference in fluence between water and graphite at equivalent depths. Measurements were performed with narrow proton beams. Plane-parallel-plate ionization chambers with a large collecting area compared to the beam diameter were used to intercept the whole beam. High- and low-energy proton beams were provided by a scanning and double scattering delivery system, respectively. A mathematical formalism was established to relate fluence corrections derived from Monte Carlo simulations, using the fluka code [A. Ferrari et al., "fluka: A multi-particle transport code," in CERN 2005-10, INFN/TC 05/11, SLAC-R-773 (2005) and T. T. Böhlen et al., "The fluka Code: Developments and challenges for high energy and medical applications," Nucl. Data Sheets 120, 211-214 (2014)], to partial fluence corrections measured experimentally. Results: A good agreement was found between the partial fluence corrections derived by Monte Carlo simulations and those determined experimentally. For a high-energy beam of 180 MeV, the fluence corrections from Monte Carlo simulations were found to increase from 0.99 to 1.04 with depth. In the case of a low-energy beam of 60 MeV, the magnitude of fluence corrections was approximately 0.99 at all depths when calculated in the sensitive area of the chamber used in the experiments. Fluence correction calculations were also performed for a larger area and found to increase from 0.99 at the surface to 1.01 at greater depths. Conclusions: Fluence corrections obtained experimentally are partial fluence corrections because they account for differences in the primary and part of the secondary particle fluence. A correction factor, F(d), has been established to relate fluence corrections defined theoretically to partial fluence corrections derived experimentally. The findings presented here are also relevant to water and tissue-equivalent-plastic materials given their carbon content. [ABSTRACT FROM AUTHOR]

Purpose: The clinical commissioning of IMRT subject to a magnetic field is challenging. The purpose of this work is to develop a GPU-accelerated Monte Carlo dose calculation platform based on penelope and then use the platform to validate a vendor-provided MRIdian head model toward quality assurance of clinical IMRT treatment plans subject to a 0.35 T magnetic field. Methods: penelope was first translated from fortran to c++ and the result was confirmed to produce equivalent results to the original code. The c++ code was then adapted to cuda in a workflow optimized for GPU architecture. The original code was expanded to include voxelized transport with Woodcock tracking, faster electron/positron propagation in a magnetic field, and several features that make gpenelope highly user-friendly. Moreover, the vendor-provided MRIdian head model was incorporated into the code in an effort to apply gpenelope as both an accurate and rapid dose validation system. A set of experimental measurements were performed on the MRIdian system to examine the accuracy of both the head model and gpenelope. Ultimately, gpenelope was applied toward independent validation of patient doses calculated by MRIdian's kmc. Results: An acceleration factor of 152 was achieved in comparison to the original single-thread fortran implementation with the original accuracy being preserved. For 16 treatment plans including stomach (4), lung (2), liver (3), adrenal gland (2), pancreas (2), spleen(1), mediastinum (1), and breast (1), the MRIdian dose calculation engine agrees with gpenelope with a mean gamma passing rate of 99.1% ± 0.6% (2%/2 mm). Conclusions: A Monte Carlo simulation platform was developed based on a GPU- accelerated version of penelope. This platform was used to validate that both the vendor-provided head model and fast Monte Carlo engine used by the MRIdian system are accurate in modeling radiation transport in a patient using 2%/2 mm gamma criteria. Future applications of this platform will include dose validation and accumulation, IMRT optimization, and dosimetry system modeling for next generation MR-IGRT systems. [ABSTRACT FROM AUTHOR]

Purpose: To provide data for estimation of fetal radiation dose (DF) from prophylactic hypogastric artery balloon occlusion (HABO) procedures. Methods: The Monte-Carlo-N-particle (MCNP)transport code and mathematical phantoms representing a pregnant patient at the ninth month of gestation were employed.PA,RAO20° andLAO20° fluoroscopy projections of left and right internal iliac arteries were simulated. Projection-specific normalized fetal dose (NFD) data were produced for various beam qualities. The effects of projection angle, x-ray field location relative to the fetus, field size, maternal body size, and fetal size onNFDwere investigated. Presented NFD values were compared to corresponding values derived using a physical anthropomorphic phantom simulating pregnancy at the third trimester and thermoluminescence dosimeters. Results: NFDdid not considerably vary when projection anglewas altered by±5°, whereas itwas found to markedly depend on tube voltage, filtration, x-ray field location and size, and maternal body size. Differences in NFD < 7.5% were observed for naturally expected variations in fetal size. A difference of less than 13.5% was observed between NFD values estimated by MCNP and direct measurements. Conclusions: Data and methods provided allow for reliable estimation of radiation burden to the fetus from HABO. [ABSTRACT FROM AUTHOR]

Purpose: The future of radiation therapy will require advanced inverse planning solutions to support single-arc, multiple-arc, and "4π" delivery modes, which present unique challenges in finding an optimal treatment plan over a vast search space, while still preserving dosimetric accuracy. The successful clinical implementation of such methods would benefit from Monte Carlo (MC) based dose calculation methods, which can offer improvements in dosimetric accuracy when compared to deterministic methods. The standard method for MC based treatment planning optimization leverages the accuracy of the MC dose calculation and efficiency of well-developed optimization methods, by precalculating the fluence to dose relationship within a patient with MC methods and subsequently optimizing the fluence weights. However, the sequential nature of this implementation is computationally time consuming and memory intensive. Methods to reduce the overhead of the MC precalculation have been explored in the past, demonstrating promising reductions of computational time overhead, but with limited impact on the memory overhead due to the sequential nature of the dose calculation and fluence optimization. The authors propose an entirely new form of "concurrent" Monte Carlo treat plan optimization: a platform which optimizes the fluence during the dose calculation, reduces wasted computation time being spent on beamlets that weakly contribute to the final dose distribution, and requires only a low memory footprint to function. In this initial investigation, the authors explore the key theoretical and practical considerations of optimizing fluence in such a manner. Methods: The authors present a novel derivation and implementation of a gradient descent algorithm that allows for optimization during MC particle transport, based on highly stochastic information generated through particle transport of very few histories. A gradient rescaling and renormalization algorithm, and the concept of momentum from stochastic gradient descent were used to address obstacles unique to performing gradient descent fluence optimization during MC particle transport. The authors have applied their method to two simple geometrical phantoms, and one clinical patient geometry to examine the capability of this platform to generate conformal plans as well as assess its computational scaling and efficiency, respectively. Results: The authors obtain a reduction of at least 50% in total histories transported in their investigation compared to a theoretical unweighted beamlet calculation and subsequent fluence optimization method, and observe a roughly fixed optimization time overhead consisting of ~10% of the total computation time in all cases. Finally, the authors demonstrate a negligible increase in memory overhead of ~7–8 MB to allow for optimization of a clinical patient geometry surrounded by 36 beams using their platform. Conclusions: This study demonstrates a fluence optimization approach, which could significantly improve the development of next generation radiation therapy solutions while incurring minimal additional computational overhead. [ABSTRACT FROM AUTHOR]

Purpose: This work evaluates Varian's electron phase–space sources for Monte Carlo simulation of the TrueBeam for modulated electron radiation therapy (MERT) and combined, modulated photon and electron radiation therapy (MPERT) where fields are shaped by the photon multileaf collimator (MLC) and delivered at 70 cm SSD. Methods: Monte Carlo simulations performed with EGSnrc-based BEAMnrc/DOSXYZnrc and PENELOPE-based PRIMO are compared against diode measurements for 5×5, 10×10, and 20×20 cm2 MLC-shaped fields delivered with 6, 12, and 20 MeV electrons at 70 cm SSD (jaws set to 40×40 cm2). Depth dose curves and profiles are examined. In addition, EGSnrc-based simulations of relative output as a function of MLC-field size and jaw-position are compared against ion chamber measurements for MLC-shaped fields between 3×3 and 25×25 cm2 and jaw positions that range from the MLC-field size to 40×40 cm2. Results: Percent depth dose curves generated by BEAMnrc/DOSXYZnrc and PRIMO agree with measurement within 2%, 2 mm except for PRIMO's 12 MeV, 20×20 cm2 field where 90% of dose points agree within 2%, 2 mm. Without the distance to agreement, differences between measurement and simulation are as large as 7.3%. Characterization of simulated dose parameters such as FWHM, penumbra width and depths of 90%, 80%, 50%, and 20% dose agree within 2 mm of measurement for all fields except for the FWHM of the 6 MeV, 20×20 cm2 field which falls within 2 mm distance to agreement. Differences between simulation and measurement exist in the profile shoulders and penumbra tails, in particular for 10×10 and 20×20 cm2 fields of 20 MeV electrons, where both sets of simulated data fall short of measurement by as much as 3.5%. BEAMnrc/DOSXYZnrc simulated outputs agree with measurement within 2.3% except for 6 MeV MLC-shaped fields. Discrepancies here are as great as 5.5%. Conclusions: TrueBeam electron phase–spaces available from Varian have been implemented in two distinct Monte Carlo simulation packages to produce dose distributions and outputs that largely reflect measurement. Differences exist in the profile shoulders and penumbra tails for the 20 MeV phase–space off-axis and in the outputs for the 6 MeV phase–space. [ABSTRACT FROM AUTHOR]

Purpose: An improvement in tissue assignment for low-dose rate brachytherapy (LDRB) patients using more accurate Monte Carlo (MC) dose calculation was accomplished with a metallic artifact reduction (MAR) method specific to dual-energy computed tomography (DECT). Methods: The proposed MAR algorithm followed a four-step procedure. The first step involved applying a weighted blend of both DECT scans (IH/L) to generate a new image (IMix). This action minimized Hounsfield unit (HU) variations surrounding the brachytherapy seeds. In the second step, the mean HU of the prostate in IMix was calculated and shifted toward the mean HU of the two original DECT images (IH/L). The third step involved smoothing the newly shifted IMix and the two original IH/L, followed by a subtraction of both, generating an image that represented the metallic artifact (IA,(H/L)) of reduced noise levels. The final step consisted of subtracting the original IH/L from the newly generated IA,(H/L) and obtaining a final image corrected for metallic artifacts. Following the completion of the algorithm, a DECT stoichiometric method was used to extract the relative electronic density (ρe) and effective atomic number (Zeff) at each voxel of the corrected scans. Tissue assignment could then be determined with these two newly acquired physical parameters. Each voxel was assigned the tissue bearing the closest resemblance in terms of ρe and Zeff, comparing with values from the ICRU 42 database. A MC study was then performed to compare the dosimetric impacts of alternative MAR algorithms. Results: An improvement in tissue assignment was observed with the DECT MAR algorithm, compared to the single-energy computed tomography (SECT) approach. In a phantom study, tissue misassignment was found to reach 0.05% of voxels using the DECT approach, compared with 0.40% using the SECT method. Comparison of the DECT and SECT D90 dose parameter (volume receiving 90% of the dose) indicated that D90 could be underestimated by up to 2.3% using the SECT method. Conclusions: The DECT MAR approach is a simple alternative to reduce metallic artifacts found in LDRB patient scans. Images can be processed quickly and do not require the determination of x-ray spectra. Substantial information on density and atomic number can also be obtained. Furthermore, calcifications within the prostate are detected by the tissue assignment algorithm. This enables more accurate, patient-specific MC dose calculations. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study was to evaluate various approaches for assessing patient organ doses resulting from radiotherapy cone-beam CT (CBCT), by the use of thermoluminescent dosimeter (TLD) measurements in anthropomorphic phantoms, a Monte Carlo based dose calculation software, and different dose indicators as presently defined. Methods: Dose evaluations were performed on a CBCT Elekta XVI (Elekta, Crawley, UK) for different protocols and anatomical regions. The first part of the study focuses on using PCXMC software (PCXMC 2.0, STUK, Helsinki, Finland) for calculating organ doses, adapting the input parameters to simulate the exposure geometry, and beam dose distribution in an appropriate way. The calculated doses were compared to readouts of TLDs placed in an anthropomorphic Rando phantom. After this validation, the software was used for analyzing organ dose variability associated with patients'differences in size and gender. At the same time, various dose indicators were evaluated: kerma area product (KAP), cumulative air-kerma at the isocenter (Kair), cone-beam dose index, and central cumulative dose. The latter was evaluated in a single phantom and in a stack of three adjacent computed tomography dose index phantoms. Based on the different dose indicators, a set of coefficients was calculated to estimate organ doses for a range of patient morphologies, using their equivalent diameters. Results: Maximum organ doses were about 1 mGy for head and neck and 25 mGy for chest and pelvis protocols. The differences between PCXMC and TLDs doses were generally below 10% for organs within the field of view and approximately 15% for organs at the boundaries of the radiation beam. When considering patient size and gender variability, differences in organ doses up to 40% were observed especially in the pelvic region; for the organs in the thorax, the maximum differences ranged between 20% and 30%. Phantom dose indexes provided better correlation with organ doses than Kair and KAP, with average ratios ranging between 0.9 and 1.1 and variations for different organs and protocols below 20%. The triple phantom setup allowed us to take into account scatter dose contributions, but nonetheless, the correlation with the evaluated organ doses was not improved with this method. Conclusions: The simulation of rotational geometry and of asymmetric beam distribution by means of PCXMC 2.0 enabled us to determine patient organ doses depending on weight, height and gender. Alternatively, the measurement of an in phantom dose indicator combined with proper correction coefficients can be a useful tool for a first dose estimation of in-field organs. The data and coefficients provided in this study can be applied to any patient undergoing a scan by an Elekta XVI equipment. [ABSTRACT FROM AUTHOR]

Purpose: To develop and evaluate a fast Monte Carlo (MC) dose calculation model of electronic portal imaging device (EPID) based on its effective atomic number modeling in the XVMC code. Methods: A previously developed EPID model, based on the XVMC code by density scaling of EPID structures, was modified by additionally considering effective atomic number (Zeff) of each structure and adopting a phase space file from the EGSnrc code. The model was tested under various homogeneous and heterogeneous phantoms and field sizes by comparing the calculations in the model with measurements in EPID. In order to better evaluate the model, the performance of the XVMC code was separately tested by comparing calculated dose to water with ion chamber (IC) array measurement in the plane of EPID. Results: In the EPID plane, calculated dose to water by the code showed agreement with IC measurements within 1.8%. The difference was averaged across the in-field regions of the acquired profiles for all field sizes and phantoms. The maximum point difference was 2.8%, affected by proximity of the maximum points to penumbra and MC noise. The EPID model showed agreement with measured EPID images within 1.3%. The maximum point difference was 1.9%. The difference dropped from the higher value of the code by employing the calibration that is dependent on field sizes and thicknesses for the conversion of calculated images to measured images. Thanks to the Zeff correction, the EPID model showed a linear trend of the calibration factors unlike those of the density-only-scaled model. The phase space file from the EGSnrc code sharpened penumbra profiles significantly, improving agreement of calculated profiles with measured profiles. Conclusions: Demonstrating high accuracy, the EPID model with the associated calibration system may be used for in vivo dosimetry of radiation therapy. Through this study, a MC model of EPID has been developed, and their performance has been rigorously investigated for transit dosimetry. [ABSTRACT FROM AUTHOR]

Purpose: To present and characterize a Monte Carlo (MC) tool for the simulation of the relative biological effectiveness for the induction of direct DNA double strand breaks (RBEdirectDSB ) for protons and light ions. Methods: The MC tool uses a pregenerated event-by-event tracks library of protons and light ions that are overlaid on a cell nucleus model. The cell nucleus model is a cylindrical arrangement of nucleosome structures consisting of 198 DNA base pairs. An algorithm relying on k-dimensional trees and cylindrical symmetries is used to search coincidences of energy deposition sites with volumes corresponding to the sugar-phosphate backbone of the DNA molecule. Strand breaks (SBs) are scored when energy higher than a threshold is reached in these volumes. Based on the number of affected strands, they are categorized into either single strand break (SSB) or double strand break (DSB) lesions. The number of SBs composing each lesion (i.e., its size) is also recorded. RBEdirectDSB is obtained by taking the ratio of DSB yields of a given radiation field to a 60Co field. The MC tool was used to obtain SSB yields, DSB yields, and RBEdirectDSB as a function of linear energy transfer (LET) for protons (1H+), 4He2+, 7Li3+, and 12C6+ ions. Results: For protons, the SSB yields decreased and the DSB yields increased with LET. At ≈24.5 keV μm-1, protons generated 15% more DSBs than 12C6+ ions. The RBEdirectDSB varied between 1.24 and 1.77 for proton fields between 8.5 and 30.2 keV μm- 1, and it was higher for iso-LET ions with lowest atomic number. The SSB and DSB lesion sizes showed significant differences for all radiation fields. Generally, the yields of SSB lesions of sizes ⩾2 and the yields of DSB lesions of sizes ⩾3 increased with LET and increased for iso-LET ions of lower atomic number. On the other hand, the ratios of SSB to DSB lesions of sizes 2-4 did not show variability with LET nor projectile atomic number, suggesting that these metrics are independent of the radiation quality. Finally, a variance of up to 8% in the DSB yields was observed as a function of the particle incidence angle on the cell nucleus. This simulation effect is due to the preferential alignment of ion tracks with the DNA nucleosomes at specific angles. Conclusions: TheMCtool can predict SSB and DSB yields for light ions of various LET and estimate RBEdirectDSB . In addition, it can calculate the frequencies of different DNA lesion sizes, which is of interest in the context of biologically relevant absolute dosimetry of particle beams. [ABSTRACT FROM AUTHOR]