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Background: There is increased interest in in vivo dosimetry for 192Ir brachytherapy (BT) treatments using high atomic number (Z) inorganic scintillators. Their high light output enables construction of small detectors with negligible stem effect and simple readout electronics. Experimental determination of absorbed‐dose energy dependence of detectors relative to water is prevalent, but it can be prone to high detector positioning uncertainties and does not allow for decoupling of absorbed‐dose energy dependence from other factors affecting detector response. Purpose: To investigate which measurement conditions and detector properties could affect their absorbed‐dose energy dependence in BT in vivo dosimetry. Methods: We used a general‐purpose Monte Carlo (MC) code PENELOPE for the characterization of high‐Z inorganic scintillators with the focus on ZnSe (Z¯=32$\bar{Z}=32$) Z. Two other promising media CsI (Z¯=54$\bar{Z}=54$) and Al2O3 (Z¯=11$\bar{Z}=11$) were included for comparison in selected scenarios. We determined absorbed‐dose energy dependence of crystals relative to water under different scatter conditions (calibration phantom 12 × 12 × 30 cm3, characterization phantoms 20 × 20 × 20 cm3, 30 × 30 × 30 cm3, 40 × 40 × 40 cm3, and patient‐like elliptic phantom 40 × 30 × 25 cm3). To mimic irradiation conditions during prostate treatments, we evaluated whether the presence of pelvic bones and calcifications affect ZnSe response. ZnSe detector design influence was also investigated. Results: In contrast to low‐Z organic and medium‐Z inorganic scintillators, ZnSe and CsI media have substantially greater absorbed‐dose energy dependence relative to water. The response was phantom‐size dependent and changed by 11% between limited‐ and full‐scatter conditions for ZnSe, but not for Al2O3. For a given phantom size, a part of the absorbed‐dose energy dependence of ZnSe is caused not due to in‐phantom scatter but due to source anisotropy. Thus, the absorbed‐dose energy dependence of high‐Z scintillators is a function of not only the radial distance but also the polar angle. Pelvic bones did not affect ZnSe response, whereas large and intermediate size calcifications reduced it by 9% and 5%, respectively, when placed midway between the source and the detector. Conclusions: Unlike currently prevalent low‐ and medium‐Z scintillators, high‐Z crystals are sensitive to characterization and in vivo measurement conditions. However, good agreement between MC data for ZnSe in the present study and experimental data for ZnSe:O by Jørgensen et al. (2021) suggests that detector signal is proportional to the average absorbed dose to the detector cavity. This enables an easy correction for non‐TG43‐like scenarios (e.g., patient sizes and calcifications) through MC simulations. Such information should be provided to the clinic by the detector vendors. [ABSTRACT FROM AUTHOR]

Purpose: High dose–rate brachytherapy is a method of radiotherapy for cancer treatment in which the radiation source is placed within the body. In addition to give a high enough dose to a tumor, it is also important to spare nearby healthy organs [organs at risk (OAR)]. Dose plans are commonly evaluated using the so‐called dosimetric indices; for the tumor, the portion of the structure that receives a sufficiently high dose is calculated, while for OAR it is instead the portion of the structure that receives a sufficiently low dose that is of interest. Models that include dosimetric indices are referred to as dose–volume models (DVMs) and have received much interest recently. Such models do not take the dose to the coldest (least irradiated) volume of the tumor into account, which is a distinct weakness since research indicates that the treatment effect can be largely impaired by tumor underdosage even to small volumes. Therefore, our aim is to extend a DVM to also consider the dose to the coldest volume. Methods: An improved DVM for dose planning is proposed. In addition to optimizing with respect to dosimetric indices, this model also takes mean dose to the coldest volume of the tumor into account. Results: Our extended model has been evaluated against a standard DVM in ten prostate geometries. Our results show that the dose to the coldest volume could be increased, while also computing times for the dose planning were improved. Conclusion: While the proposed model yields dose plans similar to other models in most aspects, it fulfils its purpose of increasing the dose to cold tumor volumes. An additional benefit is shorter solution times, and especially for clinically relevant times (of minutes) we show major improvements in tumour dosimetric indices. [ABSTRACT FROM AUTHOR]

Purpose: Experimental dosimetry of high‐dose‐rate (HDR) 192Ir brachytherapy (BT) sources is complicated due to high dose and dose‐rate gradients, and softening of photon energy spectrum with depth. A single crystal synthetic diamond detector microDiamond (PTW 60019, Freiburg, Germany) has a small active volume, high sensitivity, direct readout, and nearly water‐equivalent active volume. The purpose of this study was to evaluate the suitability of microDiamond detectors for the determination of absorbed dose to water around HDR 192Ir BT sources. Three microDiamond detectors were used, allowing for the comparison of their properties. Methods: In‐phantom measurements were performed using microSelectron and VariSource iX HDR 192Ir BT treatment units. Their treatment planning systems (TPSs), Oncentra (v. 4.3) and BrachyVision (v. 13.6), respectively, were used to create irradiation plans for a cubic PMMA phantom with the microDiamond positioned at one of three source‐to‐detector distances (SDDs) (1.5, 2.5, and 5.5 cm) at a time. The source was stepped in increments of 0.5 cm over a total length of 6 cm to yield absorbed dose of 2 Gy at the nominal reference‐point of the detector. Detectors were calibrated in 60Co beam in terms of absorbed dose to water, and Monte Carlo (MC) calculated beam quality correction factors were applied to account for absorbed‐dose energy dependence. Phantom correction factors were applied to account for differences in dimensions between the measurement phantom and a water phantom used for absorbed dose calculations made with a TPS. The same measurements were made with all three of the detectors. Additionally, dose‐rate dependence and stability of the detectors were evaluated in 60Co beam. Results: The percentage differences between experimentally determined and TPS‐calculated absorbed doses to water were from −1.3% to +2.9%. The values agreed to within experimental uncertainties, which were from 1.9% to 4.3% (k = 2) depending on the detector, SDD and treatment delivery unit. No dose‐rate or intrinsic energy dependence corrections were applied. All microDiamonds were comparable in terms of preirradiation dose, stability of the readings and energy response, and showed a good agreement. Conclusions: The results indicate that the microDiamond is potentially suitable for the determination of absorbed dose to water around HDR 192Ir BT sources and may be used for independent verification of TPS's calculations, as well as for QA measurements of HDR 192Ir BT treatment delivery units at clinical sites. [ABSTRACT FROM AUTHOR]

Purpose: In order to facilitate a smooth transition for brachytherapy dose calculations from the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) formalism to model-based dose calculation algorithms (MBDCAs), treatment planning systems (TPSs) using a MBDCA require a set of well-defined test case plans characterized by Monte Carlo (MC) methods. This also permits direct dose comparison to TG-43 reference data. Such test case plans should be made available for use in the software commissioning process performed by clinical end users. To this end, a hypothetical, generic high-dose rate (HDR) 192Ir source and a virtual water phantom were designed, which can be imported into a TPS. Methods: A hypothetical, generic HDR 192Ir source was designed based on commercially available sources as well as a virtual, cubic water phantom that can be imported into any TPS in DICOM format. The dose distribution of the generic 192Ir source when placed at the center of the cubic phantom, and away from the center under altered scatter conditions, was evaluated using two commercial MBDCAs [Oncentra® Brachy with advanced collapsed-cone engine (ACE) and BrachyVision ACUROS™ ]. Dose comparisons were performed using state-of-the-art MC codes for radiation transport, including ALGEBRA, BrachyDose, GEANT4, MCNP5, MCNP6, and PENELOPE2008. The methodologies adhered to recommendations in the AAPM TG-229 report on high-energy brachytherapy source dosimetry. TG-43 dosimetry parameters, an along-away dose-rate table, and primary and scatter separated (PSS) data were obtained. The virtual water phantom of (201)³ voxels (1 mm sides) was used to evaluate the calculated dose distributions. Two test case plans involving a single position of the generic HDR 192Ir source in this phantom were prepared: (i) source centered in the phantom and (ii) source displaced 7 cm laterally from the center. Datasets were independently produced by different investigators. MC results were then compared against dose calculated using TG-43 and MBDCA methods. Results: TG-43 and PSS datasets were generated for the generic source, the PSS data for use with theACE algorithm. The dose-rate constant values obtained from seven MC simulations, performed independently using different codes, were in excellent agreement, yielding an average of 1.1109±0.0004 cGy/(h U) (k = 1, Type A uncertainty). MC calculated dose-rate distributions for the two plans were also found to be in excellent agreement, with differences within type A uncertainties. Differences between commercial MBDCA and MC results were test, position, and calculation parameter dependent. On average, however, these differences were within 1% for ACUROS and 2% for ACE at clinically relevant distances. Conclusions: A hypothetical, generic HDR 192Ir source was designed and implemented in two commercially available TPSs employing different MBDCAs. Reference dose distributions for this source were benchmarked and used for the evaluation of MBDCA calculations employing a virtual, cubic water phantom in the form of a CT DICOM image series. The implementation of a generic source of identical design in all TPSs using MBDCAs is an important step toward supporting univocal commissioning procedures and direct comparisons between TPSs. [ABSTRACT FROM AUTHOR]

The charge of Task Group 186 (TG-186) is to provide guidance for early adopters of model-based dose calculation algorithms (MBDCAs) for brachytherapy (BT) dose calculations to ensure practice uniformity. Contrary to external beam radiotherapy, heterogeneity correction algorithms have only recently been made available to the BT community. Yet, BT dose calculation accuracy is highly dependent on scatter conditions and photoelectric effect cross-sections relative to water. In specific situations, differences between the current water-based BT dose calculation formalism (TG-43) and MBDCAs can lead to differences in calculated doses exceeding a factor of 10. MBDCAs raise three major issues that are not addressed by current guidance documents: (1) MBDCA calculated doses are sensitive to the dose specification medium, resulting in energy-dependent differences between dose calculated to water in a homogeneous water geometry (TG-43), dose calculated to the local medium in the heterogeneous medium, and the intermediate scenario of dose calculated to a small volume of water in the heterogeneous medium. (2) MBDCA doses are sensitive to voxel-by-voxel interaction cross sections. Neither conventional single-energy CT nor ICRU/ICRP tissue composition compilations provide useful guidance for the task of assigning interaction cross sections to each voxel. (3) Since each patient-source-applicator combination is unique, having reference data for each possible combination to benchmark MBDCAs is an impractical strategy. Hence, a new commissioning process is required. TG-186 addresses in detail the above issues through the literature review and provides explicit recommendations based on the current state of knowledge. TG-43-based dose prescription and dose calculation remain in effect, with MBDCA dose reporting performed in parallel when available. In using MBDCAs, it is recommended that the radiation transport should be performed in the heterogeneous medium and, at minimum, the dose to the local medium be reported along with the TG-43 calculated doses. Assignments of voxel-by-voxel cross sections represent a particular challenge. Electron density information is readily extracted from CT imaging, but cannot be used to distinguish between different materials having the same density. Therefore, a recommendation is made to use a number of standardized materials to maintain uniformity across institutions. Sensitivity analysis shows that this recommendation offers increased accuracy over TG-43. MBDCA commissioning will share commonalities with current TG-43-based systems, but in addition there will be algorithm-specific tasks. Two levels of commissioning are recommended: reproducing TG-43 dose parameters and testing the advanced capabilities of MBDCAs. For validation of heterogeneity and scatter conditions, MBDCAs should mimic the 3D dose distributions from reference virtual geometries. Potential changes in BT dose prescriptions and MBDCA limitations are discussed. When data required for full MBDCA implementation are insufficient, interim recommendations are made and potential areas of research are identified. Application of TG-186 guidance should retain practice uniformity in transitioning from the TG-43 to the MBDCA approach. [ABSTRACT FROM AUTHOR]

Purpose: Experimental radiation dosimetry with thermoluminescent dosimeters (TLDs), calibrated in a 60Co or megavoltage (MV) photon beam, is recommended by AAPM TG-43U1for verification of Monte Carlo calculated absorbed doses around brachytherapy sources. However, it has been shown by Carlsson Tedgren et al. [Med. Phys. 38, 5539-5550 (2011)] that for TLDs of LiF:Mg,Ti, detector response was 4% higher in a 137Cs beam than in a 60Co one. The aim of this work was to investigate if similar over-response exists when measuring absorbed dose to water around 192Ir sources, using LiF:Mg,Ti dosimeters calibrated in a 6 MV photon beam. Methods: LiF dosimeters were calibrated to measure absorbed dose to water in a 6 MV photon beam and used to measure absorbed dose to water at distances of 3, 5, and 7 cm from a clinical high dose rate (HDR) 192Ir source in a polymethylmethacrylate (PMMA) phantom. Measured values were compared to values of absorbed dose to water calculated using a treatment planning system (TPS) including corrections for the difference in energy absorption properties between calibration quality and the quality in the users' 192Ir beam and for the use of a PMMA phantom instead of the water phantom underlying dose calculations in the TPS. Results: Measured absorbed doses to water around the 192Ir source were overestimated by 5% compared to those calculated by the TPS. Corresponding absorbed doses to water measured in a previous work with lithium formate electron paramagnetic resonance (EPR) dosimeters by Antonovic et al. [Med. Phys. 36, 2236-2247 (2009)], using the same irradiation setup and calibration procedure as in this work, were 2% lower than those calculated by the TPS. The results obtained in the measurements in this work and those obtained using the EPR lithium formate dosimeters were, within the expanded (k = 2) uncertainty, in agreement with the values derived by the TPS. The discrepancy between the results using LiF:Mg,Ti TLDs and the EPR lithium formate dosimeters was, however, statistically significant and in agreement with the difference in relative detector responses found for the two detector systems by Carlsson Tedgren et al. [Med. Phys. 38, 5539-5550 (2011)] and by Adolfsson et al. [Med. Phys. 37, 4946-4959 (2010)]. Conclusions: When calibrated in 60Co or MV photon beams, correction for the linear energy transfer (LET) dependence of LiF:Mg,Ti detector response will be needed as to measure absorbed doses to water in a 192Ir beam with highest accuracy. Such corrections will depend on the manufacturing process (MTS-N Poland or Harshaw TLD-100) and details of the annealing and read-out schemes used. [ABSTRACT FROM AUTHOR]

Purpose: Dose plans generated with optimization models hitherto used in high-dose-rate (HDR) brachytherapy have shown a tendency to yield longer dwell times than manually optimized plans. Concern has been raised for the corresponding undesired hot spots, and various methods to mitigate these have been developed. The hypotheses upon this work is based are (a) that one cause for the long dwell times is the use of objective functions comprising simple linear penalties and (b) that alternative penalties, as these are piecewise linear, would lead to reduced length of individual dwell times. Methods: The characteristics of the linear penalties and the piecewise linear penalties are analyzed mathematically. Experimental comparisons between the two types of penalties are carried out retrospectively for a set of prostate cancer patients. Results: When the two types of penalties are compared, significant changes can be seen in the dwell times, while most dose-volume parameters do not differ significantly. On average, total dwell times were reduced by 4.2%, with a reduction of maximum dwell times by 25%, when the alternative penalties were used. Conclusions: The use of linear penalties in optimization models for HDR brachytherapy is one cause for the undesired long dwell times that arise in mathematically optimized plans. By introducing alternative penalties, a significant reduction in dwell times can be achieved for HDR brachytherapy dose plans. Although various measures for mitigating the long dwell times are already available, the observation that linear penalties contribute to their appearance is of fundamental interest. [ABSTRACT FROM AUTHOR]

Background Purpose Methods Results Conclusions Model‐based dose calculation algorithms (MBDCA), such as the Advanced Collapsed cone Engine (ACE) in Oncentra Brachy® can be used to overcome the limitations of the TG‐43 formalism. ACE is a point kernel superposition algorithm that calculates the total dose separated into primary, first‐scatter, and multiple‐scatter dose. Albeit ACE yields accurate results under most circumstances, several studies have reported underestimations of the dose to cortical bone. These underestimations are likely caused by approximations in the handling of multiple‐scatter dose for non‐water media. Such would result in noticeable deviations where the multiple‐scatter is a considerable fraction of the total dose, that is, at greater distances from the source.To improve and test the accuracy of the multiple‐scatter dose component in the ACE algorithm to remedy its inaccuracy for non‐water geometries.A careful analysis of the transport and absorption of the multiple‐scatter energy fluence revealed an inconsistency in the scaling of energy absorption ratios for non‐water media of the multiple‐scatter kernel. We implemented an updated algorithm version, ACEcorr, and tested it for three different geometries. All had a single 192Ir‐source at the center of a cubic water phantom with a box‐shaped heterogeneity of either cortical bone or air, positioned at different distances from the source. Dose distributions for the three cases were calculated with ACE and ACEcorr and compared to Monte Carlo simulations, using the percentage dose difference ratio as figure‐of‐merit. All dose calculation methods scored separately the dose deposited by primary, first‐scattered, and multiple‐scattered photons.The accuracy of the updated algorithm ACEcorr was superior to ACE. In the cortical bone heterogeneity, the mean percentage dose difference ratio for the total dose improved from −11.7%$ - 11.7{\mathrm{\% }}$ to −2.2%$ - 2.2{\mathrm{\% }}$ (in the worst case) by our update. Less impact was seen in the air heterogeneity, where both ACE and ACEcorr deviated less than 2% from the Monte Carlo results. The algorithm update mainly concerns the multiple‐scattered dose component, but an accompanying data processing update also had a small effect (≤$ \le $0.5% difference) on the primary and first‐scattered dose. The calculation times were not affected.The updates to ACE improved the accuracy of multiple‐scatter dose calculation for non‐water media, without increasing calculation times. [ABSTRACT FROM AUTHOR]

• Most Radiotherapy Departments in Europe have HDR-PDR brachytherapy equipment with Ir-192 or Co-60. • The present GEC-ESTRO ACROP Recommendations provide guidance on the calibration of such sources. • Practical aspects and issues not specifically accounted for in well-accepted societal recommendations are included. • The aim is to provide a European-wide standard in HDR-PDR BT source calibration at the hospital level. The vast majority of radiotherapy departments in Europe using brachytherapy (BT) perform temporary implants of high- or pulsed-dose rate (HDR-PDR) sources with photon energies higher than 50 keV. Such techniques are successfully applied to diverse pathologies and clinical scenarios. These recommendations are the result of Working Package 21 (WP-21) initiated within the BRAchytherapy PHYsics Quality Assurance System (BRAPHYQS) GEC-ESTRO working group with a focus on HDR-PDR source calibration. They provide guidance on the calibration of such sources, including practical aspects and issues not specifically accounted for in well-accepted societal recommendations, complementing the BRAPHYQS WP-18 Report dedicated to low energy BT photon emitting sources (seeds). The aim of this report is to provide a European-wide standard in HDR-PDR BT source calibration at the hospital level to maintain high quality patient treatments. [ABSTRACT FROM AUTHOR]

• Recommendations to assure harmonized and high-quality seed calibration in European clinics. • To provide a European wide high standard in LE-LDR source calibration at end-user level. • Level of agreement between the measured source strength by the user and the vendor value, actions to undertake when the difference exceeds a certain level. • Recalibration of ionization chambers to maintain the long-term stability of their calibration factors. • Knowledge on multi-seed inserts and their relationship to a single (calibration) seed setup. Prostate brachytherapy treatment using permanent implantation of low-energy (LE) low-dose rate (LDR) sources is successfully and widely applied in Europe. In addition, seeds are used in other tumour sites, such as ophthalmic tumours, implanted temporarily. The calibration issues for LE-LDR photon emitting sources are specific and different from other sources used in brachytherapy. In this report, the BRAPHYQS (BRAchytherapy PHYsics Quality assurance System) working group of GEC-ESTRO, has developed the present recommendations to assure harmonized and high-quality seed calibration in European clinics. There are practical aspects for which a clarification/procedure is needed, including aspects not specifically accounted for in currently existing AAPM and ESTRO societal recommendations. The aim of this report has been to provide a European wide standard in LE-LDR source calibration at end-user level, in order to keep brachytherapy treatments with high safety and quality levels. The recommendations herein reflect the guidance to the ESTRO brachytherapy users and describe the procedures in a clinic or hospital to ensure the correct calibration of LE-LDR seeds. [ABSTRACT FROM AUTHOR]

Purpose Establishment of an end-to-end system for the brachytherapy (BT) dosimetric chain could be valuable in clinical quality assurance. Here, the development of such a system using MOSFET (metal oxide semiconductor field effect transistor) detectors and experience gained during 2 years of use are reported with focus on the performance of the MOSFET detectors. Methods and Materials A bolus phantom was constructed with two implants, mimicking prostate and head & neck treatments, using steel needles and plastic catheters to guide the 192 Ir source and house the MOSFET detectors. The phantom was taken through the BT treatment chain from image acquisition to dose evaluation. During the 2-year evaluation-period, delivered doses were verified a total of 56 times using MOSFET detectors which had been calibrated in an external 60 Co beam. An initial experimental investigation on beam quality differences between 192 Ir and 60 Co is reported. Results The standard deviation in repeated MOSFET measurements was below 3% in the six measurement points with dose levels above 2 Gy. MOSFET measurements overestimated treatment planning system doses by 2–7%. Distance-dependent experimental beam quality correction factors derived in a phantom of similar size as that used for end-to-end tests applied on a time-resolved measurement improved the agreement. Conclusions MOSFET detectors provide values stable over time and function well for use as detectors for end-to-end quality assurance purposes in 192 Ir BT. Beam quality correction factors should address not only distance from source but also phantom dimensions. [ABSTRACT FROM AUTHOR]

Purpose At St. Erik Eye Hospital in Stockholm, Sweden, ocular tumors of apical height above 6 mm are treated with brachytherapy, using iodine-125 seeds attached to a gold alloy plaque while the treatment planning is performed assuming homogeneous water surroundings. The aim of this work was to investigate the dose-modifying effects of the plaque and the seed fixating silicone rubber glue. Methods and Materials The impact of the gold plaque and silicone rubber glue was studied with the Monte Carlo N-particle transport code, version 5. Results For the 2 cm most proximal to the plaque surface along the plaque's central axis, the eyeball received 104.6–93.0% of the dose in all-water conditions. Conclusions The 0.3 mm thick layer of silicone rubber glue, used for seed fixation, attenuates photons little enough to allow characteristic X-rays from the gold alloy plaque to reach the eyeball. Close to the plaque, the dose rates were higher with the plaque and glue present, than in homogeneous water conditions. This is in contrast to what has been reported for more commonly used eye plaques, demonstrating the importance of investigating the dosimetry of individual treatment systems. [ABSTRACT FROM AUTHOR]