S1747
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
Conclusion: Inter-center variation in SPR and range prediction decreased substantially since the initial run. The efforts undertaken for harmonization of CT-based SPR prediction within the EPTN proved to have a relevant impact. Despite this progress, systematic differences remain, leaving room for further improvement of SPR prediction accuracy. It is important to acknowledge, that our study methodology does not cover all components of the overall range uncertainty budget. Therefore, a one-to-one translation of these results into a safety margin reduction is not warranted. Instead, a center-specific assessment of the full uncertainty budget remains necessary. References: [1] Peters N, Wohlfahrt P, Dahlgren CV, De Marzi L, Ellerbrock M, Fracchiolla F, et al. Experimental assessment of inter-centre variation in stopping-power and range prediction in particle therapy. Radiother Oncol 2021;163:7–13. https://doi.org/10.1016/j.radonc.2021.07.019.[2] Peter s N, Taasti VT, Ackermann B, Bolsi A, Dahlgren CV, Ellerbrock M, et al. Consensus guide on CT-based prediction of stopping-power ratio using a Hounsfield look-up table for proton therapy. Radiother Oncol 2023;184:109675. https://doi.org/10.1016/j.radonc.2023.109675. Keywords: SPR and range prediction, computed tomography Single-institution, multi-year retrospective analysis of adaptive planning in a proton and carbon-ion therapy facility: replanning rates and causes Amaglio Beatrice 1 , Alessandro Vai 1 , Silvia Molinelli 1 , Alfredo Mirandola 1 , Giuseppe Magro 1 , Stefania Russo 1 , Alessia Bazani 1 , Eleonora Rossi 1 , Amelia Barcellini 2,3 , Francesca Colombo 1 , Maria Rosaria Fiore 2 , Alberto Iannalfi 2 , Sara Lillo 2,3 , Barbara Vischioni 2 , Viviana Vitolo 2 , Mario Ciocca 1 , Ester Orlandi 2,4 1 Medical Physics Department, CNAO, Pavia, Italy. 2 Radiation Oncology Unit, Clinical Department, CNAO, Pavia, Italy. 3 Department of Internal Medicine and Therapeutics, University of Pavia, Pavia, Italy. 4 Dept.of Clinical, surgical, Diagnostic and Pediatric Sciences, University of Pavia, Pavia, Italy Purpose/Objective: Plan adaptation in particle therapy (PT) is essential to maintain treatment quality and represents a major workload component whose frequency and triggers should be thoroughly investigated. This study reports a single-institution, multi-year retrospective analysis of plan adaptation in pencil beam scanning proton and Proffered Paper 2937
carbon-ion (C-ion) therapy, assessing how clinical, anatomical, and dosimetric factors influence the need for replanning across different anatomical sites. Material/Methods: Patients treated between January 2022 and May 2025 were retrospectively analyzed. Plans were optimized using the same treatment planning system with consistent, site-specific robust optimization strategies. Each patient underwent at least one re-evaluation CT scan during treatment, either scheduled or triggered by daily pre-treatment imaging. Apart from pediatric cases (13.9% of the full cohort), patients were grouped by anatomical site: brain (24.0%), head and neck (H&N, 32.0%), thorax/abdomen (4.2%), gynecological (5.0%), and pelvis/spine (21.6%) (Figure 1). The replan rate was calculated for the overall cohort and for each site. The main triggers for replanning were classified as anatomical variations, target and/or organ-at-risk (OAR) dose discrepancies, or setup-related issues. Pediatric cases and sites with higher prevalence, i.e. H&N and pelvis (respectively 31.3% and 21.6% of the full cohort), or higher replan frequency (gynecological) were further analyzed for the most recent two-year period.
Results: Among 1,442 patients treated with protons (728 pts, 50.5%) or C-ions (714 pts, 49.5%), 498 pts (34.5%) required at least one plan adaptation (protons: 247 pts, C-ions: 251 pts). The replan rate varied markedly by site: brain (4.6%), H&N (42.7%), thorax/abdomen (45.2%), gynecological (62.5%), pediatric (37.2%), and pelvis (46.0%). An increasing trend over time was observed for H&N, thorax/abdomen, and pelvic cases, while other sites remained relatively stable (Figure 2). The predominant causes for plan adaptation were anatomical changes and dosimetric deviations. Relative to H&N, gynecological, and pelvic subsite groups, respectively 68.8%, 78.8%, and 57.1% of the adapted patients were replanned due to these combined motivations. For H&N adaptations were driven by dosimetric changes (44.4%) and target or sinonasal cavity filling variations (33.3%); in gynecological cases, target under-coverage and OAR overdoses were the leading factors (68.8%); while in pelvic cases both dosimetric (49%) and setup-related
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