S121
Brachytherapy - Physics
ESTRO 2026
1 Beaulieu, L., Carlsson Tedgren, A., Carrier JF, et al., Report of the Task Group 186 on model - based dose calculation methods in brachytherapy beyond the TG - 43 formalism: current status and recommendations for clinical implementation. Medical physics, 2012. 39(10): p. 6208-6236Wg2 Beaulieu, L., Ballester, F., Granero, D., et al., AAPM WGDCAB Report 372: A joint AAPM, ESTRO, ABG, and ABS report on commissioning of model - based dose calculation algorithms in brachytherapy. Medical physics, 2023. 50(8): p. e946-e960.3 Moghadam, N., Akbari, F., Jarvis S, Martinov M, Thibodeau M, Zhang C, Cygler J, Thomson R M., A graphical user interface for Monte Carlo dose calculations for brachytherapy with egs_brachy. Medical Physics, 2025. Updates and validation of RapidBrachyMCTPS v3.0, a comprehensive and flexible Monte Carlo-based brachytherapy treatment planning system Jonathan Kalinowski 1,2 , Hossein Jafarzadeh 1,2 , Farhood Farahnak 2 , Shirin Abbasinejad Enger 1,2 1 Medical Physics Unit, Department of Oncology, McGill University, Montréal, Canada. 2 Lady Davis Institute for Medical Resarch, Jewish General Hospital, Montréal, Canada Digital Poster Highlight 1301 Purpose/Objective: RapidBrachyMCTPS is a brachytherapy treatment planning system built on the Geant4-based RapidBrachyMC dose calculation engine [1,2]. For nearly a decade, RapidBrachyMCTPS has served as a platform for advanced brachytherapy dosimetry research, including the development of intensity- modulated brachytherapy applicators, deep learning- based dose prediction algorithms, and novel dosimetry phantoms. In this work, we present the latest developments of RapidBrachyMCTPS v3.0 and the validation of its dose calculations. Material/Methods: Major updates to RapidBrachyMCTPS v3.0 centered around improving its simulation efficiency and overall user experience. Key changes include the simplification of the simulation world geometry, standardization of all material definitions, an optimized nrrd-file interface to reduce read/write times, and enhanced applicator modelling capabilities. The CADMesh interface was introduced for direct import of applicator model files in triangular surface mesh or tetrahedral mesh representations. RapidBrachyMCTPS v3.0 was benchmarked according to the recommendations of Report 372 of the AAPM Working Group on Model-Based Dose Calculation Algorithms in Brachytherapy [3], which specifies four water phantom-based test cases to validate the
accuracy of dose calculations against reference dose distributions. Per Report #372 recommendations, dose distributions were compared via examination of local and global dose differences, representing the percentage dose difference in each voxel relative to the dose in the voxel and to a fixed dose value in the reference dose distribution, respectively. The same four test cases were also used to characterize the simulation efficiency gains over RapidBrachyMCTPS v2.0. Results: Figure 1 shows an example comparison of local and global differences between RapidBrachyMCTPS v3.0 and the reference dose distribution for Test Case 3. Dose difference metrics across all test cases are presented in Table 1. In general, RapidBrachyMCTPS dose calculations agreed with reference data, with mean local percent differences within ±0.11%, and all root-mean-square dose differences within 0.4%. Ma et al. established previously that the total uncertainty on these comparisons is on the order of 0.4% [4]. Timing data indicated a speedup factor for Test Case 2 and 3 simulations of 1.66±0.02 and 1.63±0.03, respectively for RapidBrachyMCTPS v3.0 compared to v2.0. In Test Case 4, which features an applicator, a total speedup factor of up to 1.61±0.02 times was achieved when importing the applicator as a tetrahedral mesh.
Conclusion: RapidBrachyMCTPS v3.0 offers users a streamlined and improved user experience for Monte Carlo-based dosimetry. The accuracy of dose calculations was verified according to the latest published commissioning guidelines, and simulations were found to run over 1.5 times faster.
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