S119
Brachytherapy - Physics
ESTRO 2026
1 Carleton Laboratory for Radiotherapy Physics, Physics Department, Carleton University, Ottawa, Canada. 2 Medical Physics Laboratory, Medical School, National and Kapodistrian, University of Athens, Athens, Greece. 3 Département de physique, de génie physique et d’optique, et Centre de recherche sur le cancer, Université Laval, Quebec, Canada. 4 Service de physique médicale et radioprotection, et Axe Oncologie du CRCHU de Québec, CHU de Québec, , Université Laval, Quebec, Canada. 5 Instituto de Seguridad Industrial, Radiofísica y Medioambiental (ISIRYM), Universitat Politècnica de València, València, Spain. 6 Departamento de Física Atómica, Molecular y Nuclear, Universitat de Valencia (UV), Burjassot, Spain. 7 Instituto de Física Corpuscular, IFIC (UV-CSIC), Burjassot, Spain Purpose/Objective: The objective is to develop Monte-Carlo (MC)-based test cases for commissioning Model-Based Dose Calculation Algorithms (MBDCAs) for permanent implant prostate brachytherapy (PIPB) using I-125 seeds. This development follows the recommendations1 of the joint Working Group of the American Association of Physicists in Medicine (AAPM), the European Society for Radiotherapy and Oncology (ESTRO), and the Australasian Brachytherapy Group (ABG) on MBDCAs in Brachytherapy and TG 1862. Material/Methods: Six test cases were designed using I-125 OncoSeed model 6711, ranging from a single-seed in a water phantom to 58 seeds in a CT-based full-tissue model including calcifications. Simulations were independently performed using EGSnrc (egs_brachy with eb_gui), MCNP6 (BrachyGuide), and Penelope(PenRed). The absorbed dose was computed as collision kerma per voxel, and air-kerma strength factors (S ₖ ) were determined using NIST WAFAC geometry. Local ( Δ DLOCAL(%)) and global ( Δ DGLOBAL(%)) dose difference ratios were calculated voxel-by-voxel, with eb_gui results used as reference data. Dose–volume histograms (DVHs) were also compared for prostate, urethra, bladder, and rectum. Results: Across all test cases, S ₖ values agreed within 0.6%between codes. The mean Δ DLOCAL was below 1.12% for MCNPand3.09% for PenRed, with Δ DGLOBALconsistently centered around zero, confirming strong agreement among codes. Differences were largest near seed ends and low-dose regions, but negligible in clinically relevant zones. DVH analysis showed excellent agreement for bladder and rectum, and minor deviations for target and urethra.
Conclusion: This study introduces a validated set of MC-based PIPB test cases encompassing realistic anatomical and dosimetric conditions. These datasets serve as essential references for commissioning and quality assurance of emerging MBDCAs, bridging the gap between conventional TG-43-based planning and patient-specific dose modeling. The framework establishes reproducible, benchmark datasets for future MBDCA development, including treatment planning systems, in permanent implant prostate brachytherapy. Keywords: Implant prostate brachy, Model-based calculation References: 1 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. e946.2 Report of the Task Group 186 on model - based dose calculation methods in brachytherapy beyond the TG - 43 formalism. Medical physics, 2012. 39(10): p. 6208 Uncovering planning patterns and dosimetric trends in permanent prostate brachytherapy: A comprehensive Monte Carlo–based multi- institutional analysis Fatemeh Akbari 1 , Samuel Ouellet 1,2 , Sandra Parent 3,4 , Raman Dhoot 5 , Matthew Muscat 6 , Damien Carignan 7 , Marie-Claude Lavallée 2,8 , Tyler Meyer 9 , Joanna E Cygler 10 , Eric Vigneault 7 , Jean-François Carrier 3,4 , Juanita Crook 5 , Luc Beaulieu 2,8 , Rowan M Thomson 1 1 Carleton Laboratory for Radiotherapy Physics, Department of Physics, Carleton University, Ottawa, Digital Poster Highlight 971
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