S120
Brachytherapy - Physics
ESTRO 2026
Canada. 2 Département de physique, de génie physique et d’optique et Centre de recherche sur le cancer, Université Laval, Québec, Canada. 3 Département de Physique, Université de Montréal, Montréal, Canada. 4 Département de Radio-oncologie, Centre Hospitalier de l'Université de Montréal (CHUM), Montréal, Canada. 5 Department of Surgery, Division of Radiation Oncology, BC Cancer Kelowna, Kelowna, Canada. 6 Department of Medical Physics, BC Cancer, Kelowna, Canada. 7 Centre de recherche sur le cancer, Département de Radio-Oncologie et Centre de recherche du CHU de Québec, Université Laval, Québec, Canada. 8 Service de physique médicale et radioprotection, et Axe Oncologie du CRCHU de Québec, CHU de Québec, Université Laval, Québec, Canada. 9 Department of Medical Physics, Arthur Child Comprehensive Cancer Centre, Calgary, Canada. 10 Department of Medical Physics, The Ottawa Hospital Cancer Centre, Ottawa, Canada Purpose/Objective: To evaluate inter-institutional variability in treatment planning practices and dosimetric outcomes for permanent implant prostate brachytherapy (PIPB) using TG-1861,2–based Monte Carlo (MC) dose calculations. The objective was to characterize real- world planning patterns, identify factors influencing dose distributions, and establish benchmark dosimetric ranges to support clinical quality assurance
patients. Both loose and stranded seed attachment methods were applied. The average number of implanted seeds varied considerably among centers, ranging from 52 ± 10 to 97 ± 15. Significant inter- institutional variability was found for most dosimetric endpoints (p < 0.01) calculated with egs_brachy. Results of Kruskal–Wallis test indicated that for target coverage, PTV D90 and D99 showed moderate variability ( ε² = 0.13 and 0.10), while V100 exhibited large variability ( ε² =0.14). Among OARs, bladder dose metrics demonstrated the greatest variability across the four centers, followed by rectal and urethral doses. Weak correlations were observed between PTV D90 and treatment parameters, including seed type and strength, and attachment method, suggesting that dosimetric differences are more strongly influenced by institutional technique and planning philosophy than by individual treatment parameters.
and standardization. Material/Methods:
Retrospective data from four Canadian centers (>2500 patients) were analyzed. The prescription dose was 144-145 Gy across centers, calculated with the traditional water-based TG-43 protocol. All post- implant plans were recalculated with the freely- available MC code egs_brachy through its graphical user interface, eb_gui3. The eb_gui software enables patient data in DICOM format to be converted into patient-specific MC simulations which account for tissue heterogeneities and inter-seed-attenuation. A database captured planning parameters and TG-186– based dose–volume metrics for the prostate and organs at risk. Inter-institutional differences were evaluated using Kruskal–Wallis with Dunn’s post-hoc tests; epsilon-squared ( ε² ) quantified effect size; Pearson correlations assessed associations between pan parameters and PTV D90. Results: All participating centers used a combined approach, whereby a preliminary plan was developed using ultrasound imaging before surgery and subsequently refined and finalized intraoperatively using updated ultrasound images. Across the four centers, a total of five different seed types were used to treat patients, with some centers using the same seed types and others employing more than one seed type between
Conclusion: This first large-scale TG-186 Monte Carlo analysis revealed noticeable inter-institutional dosimetric variability. Benchmark ranges support planning standardization. Ongoing work will explore potential dose–effect correlations between TG-186-based parameters and clinical outcomes to clarify how dosimetric differences may influence tumor control and toxicity in prostate brachytherapy. Keywords: brachytherapy, prostate, Monte-Carlo References:
Made with FlippingBook - Share PDF online