Abstract: Methods: The authors have assumed a theoretical Tm-170 cylindrical source encapsulated with stainless steel and typical dimensions taken from the currently available HDR Ir-192 brachytherapy sources. The dose-rate distribution was calculated for this source using the GEANT4 Monte Carlo (MC) code considering both photon and electron Tm-170 spectra. The AAPM TG-43 U1 brachytherapy dosimetry parameters were derived. To study general properties of Tm-170 encapsulated sources, spherical sources encapsulated with stainless steel and platinum were also studied. Moreover, the influence of small variations in the active core and capsule dimensions on the dosimetric characteristics was assessed. Treatment times required for a Tm-170 source were compared to those for Ir-192 and Yb-169 for the same contained activity. Results: Due to the energetic beta spectrum and the large electron yield, the bremsstrahlung contribution to the dose was of the same order of magnitude as from the emitted gammas and characteristic x rays. Moreover, the electron spectrum contribution to the dose was significant up to 4 mm from the source center compared to the photon contribution. The dose-rate constant Lambda of the cylindrical source was 1.23 cGy h(-1) U-1. The behavior of the radial dose function showed promise for applications in brachytherapy. Due to the electron spectrum, the anisotropy was large for r < 6 mm. Variations in manufacturing tolerances did not significantly influence the final dosimetry data when expressed in cGy h(-1) U-1. For typical capsule dimensions, maximum reference dose rates of about 0.2, 10, and 2 Gy min(-1) would then be obtained for Tm-170, Ir-192, and Yb-169, respectively, resulting in treatment times greater than those for HDR Ir-192 brachytherapy. Conclusions: The dosimetric characteristics of source designs exploiting the low photon energy of Tm-170 were studied for potential application in HDR-brachytherapy. Dose-rate distributions were obtained for cylindrical and simplified spherical Tm-170 source designs (stainless steel and platinum capsule materials) using MC calculations. Despite the high activity of Tm-170, calculated treatment times were much longer than for Ir-192.

Abstract: PurposeTo provide the first clinical test case for commissioning of Ir-192 brachytherapy model-based dose calculation algorithms (MBDCAs) according to the AAPM TG-186 report workflow. Acquisition and Validation MethodsA computational patient phantom model was generated from a clinical multi-catheter Ir-192 HDR breast brachytherapy case. Regions of interest (ROIs) were contoured and digitized on the patient CT images and the model was written to a series of DICOM CT images using MATLAB. The model was imported into two commercial treatment planning systems (TPSs) currently incorporating an MBDCA. Identical treatment plans were prepared using a generic Ir-192 HDR source and the TG-43-based algorithm of each TPS. This was followed by dose to medium in medium calculations using the MBDCA option of each TPS. Monte Carlo (MC) simulation was performed in the model using three different codes and information parsed from the treatment plan exported in DICOM radiation therapy (RT) format. Results were found to agree within statistical uncertainty and the dataset with the lowest uncertainty was assigned as the reference MC dose distribution. Data Format and Usage NotesThe dataset is available online at ,. Files include the treatment plan for each TPS in DICOM RT format, reference MC dose data in RT Dose format, as well as a guide for database users and all files necessary to repeat the MC simulations. Potential ApplicationsThe dataset facilitates the commissioning of brachytherapy MBDCAs using TPS embedded tools and establishes a methodology for the development of future clinical test cases. It is also useful to non-MBDCA adopters for intercomparing MBDCAs and exploring their benefits and limitations, as well as to brachytherapy researchers in need of a dosimetric and/or a DICOM RT information parsing benchmark. Limitations include specificity in terms of radionuclide, source model, clinical scenario, and MBDCA version used for its preparation.

Abstract: Background Low-energy x-ray beams used in electronic brachytherapy (eBT) present significant dosimetric challenges due to their high depth-dose gradients, the dependence of detector response on materials, and the lack of standardized dose-to-water references. These challenges have driven the need for Monte Carlo (MC) simulations to ensure accurate dosimetry. However, discrepancies in the physics models used by different MC systems have raised concerns about their dosimetric consistency, particularly in modeling bremsstrahlung interactions. Purpose To assess the dosimetric impact of using different physics approaches in three state-of-the-art MC systems for eBT, focusing on the disagreements observed when different MC methods are used to evaluate bremsstrahlung interactions. Methods The MC studies of the Axxent S700, the Esteya, and the INTRABEAM eBT systems were performed using two EGSnrc applications (egsbrachy and egskerma), TOPAS, and PENELOPE-2018 (PEN18). The fluence spectra and depth doses were compared for simplified x-ray tube models, which maintain the target mode (transmission or reflection), the target material and thickness, and the surface applicators' source-to-surface distance. An extra simulation was made to evaluate the utility of the simplified models as proxies in predicting the most important characteristics of an accurate applicator's simulation (detailed model of INTRABEAM's 30 mm surface applicator). The EGSnrc applications and PEN18 utilized their default bremsstrahlung angular emission approaches. TOPAS used two physics lists: g4em-livermore (TOPAS(liv)) and g4em-penelope (TOPAS(pen)). Results The most significant differences between MC codes were observed for the transmission target mode. The bremsstrahlung component of the fluence spectra differed by about 15% on average, comparing PEN18, EGSnrc applications, and TOPAS(liv), with PEN18's fluences consistently lower. EGSnrc and PEN18 agreed within 3% for their characteristic spectrum components. However, PEN18's characteristic lines overreached TOPAS(liv)'s by 40%. Those spectral characteristics generated depth dose differences, where PEN18, on average, scored 9% lower than EGSnrc and TOPAS(liv). Considering TOPAS(pen) in the transmission mode, PEN18's fluence spectrum presented a lower bremsstrahlung (5%) but a higher characteristic component (10%); these spectral differences compensated, generating depth dose differences within 1% average. In the reflection target mode, EGSnrc and PEN18 agreed within 4% for the bremsstrahlung and characteristic components of the fluence spectra. With TOPAS(pen) in the reflection mode, PEN18 presents 12% lower fluences in the bremsstrahlung component but 6% higher characteristic lines. This spectral behavior diminished the depth dose differences up to 3%. Conclusion This work found considerable disagreements between three state-of-the-art MC systems commonly used in medical applications when simulating bremsstrahlung in eBT. The differences arose when the bremsstrahlung angular distribution and the atomic relaxation processes in the target became relevant. More theoretical and experimental studies are necessary to evaluate the impact of these differences on related calculations.