ESTRO 2025 - Abstract Book

S3464

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTRO 2025

Conclusion: Using an MRF with our proton therapy system significantly improves efficiency by reducing beam-on time, number of energy layers and spots, with minimal and clinically insignificant dosimetric impact.

Keywords: mini ridge filter, proton therapy, MRF

References: O'Grady F, Janson M, Rao AD, et al. The use of a mini-ridge filter with cyclotron-based pencil beam scanning proton therapy. Med Phys. 2023; 50: 1999–2008. https://doi.org/10.1002/mp.16254 Courneyea, L., Beltran, C., Tseung, H.S.W.C., Yu, J. and Herman, M.G. (2014), Optimizing mini-ridge filter thickness to reduce proton treatment times in a spot-scanning synchrotron system. Med. Phys., 41: 061713. https://doi.org/10.1118/1.4876276

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Mini-Oral Dose-averaged LET optimized carbon-ion radiotherapy for sacral chordoma: preliminary results of clinical application Silvia Molinelli 1 , Alessia Bazani 1 , Marco Rotondi 1 , Agnieszka Chalaszczyk 1 , Eleonora Rossi 1 , Stefania Russo 1 , Elettra Ferrari 2 , Maria Rosaria Fiore 1 , Ester Orlandi 3,1 , Mario Ciocca 1 1 Clinical department, CNAO National Center for Oncological Hadrontherapy, Pavia, Italy. 2 Radiotherapy department, University of Milan, Milan, Italy. 3 Department of Clinical, Surgical, Diagnostic and Pediatric Sciences, University of Pavia, Pavia, Italy Purpose/Objective: In a previous retrospective analysis of carbon ion therapy (CIRT) outcomes for large sacral chordoma patients we highlighted the potential correlation between tumor relapse and suboptimal dose-averaged linear energy transfer (LETd) distribution in the target. We then investigated the maximum minimum LETd (LETd-min) to the GTV able to guarantee the same plan quality and robustness of purely dose-optimized plans (plan-D). In this work we report on the feasibility and inter-fraction robustness of the fisrt 20 sacral chordoma patients, treated with (LETd)-optimized CIRT plans (plan-DL). Material/Methods: We optimized treatment plans to a prescribed relative biological effectiveness (RBE)-weighted dose of 67.2 Gy(RBE), according to the microdosimetric kinetic model [1], delivered in 16 fractions, with a sequential boost approach. Once plan-D was clinically acceptable, a constraint on LETd-min to the GTV was added to the objective function (plan-DL). The optimal LETd-min guaranteed compliance with clinical goals in the nominal and robust scenarios, making plan-D and plan-DL comparable, by definition, in terms of robust organs at risk dose and target coverage on the planning CT. We collected the resulting difference in LETd-min to the GTV. Re-evaluation CT scans, performed twice during treatment course, served as a basis to evaluate inter-fraction variation of target coverage (dose to 98% and 2% of the volume (D98%; D2%)), for the reference plan-D and the delivered plan-DL. Statistically significant differences between dosimetric criteria were estimated with a Mann-Withney test, with a significance level of 5% (p<0.05). Skin, intestinal and genito-urinary toxicity was evaluated, according to CTCAE (v.5), during treatment and at first follow-up. Results: The median GTV volume was 169.2cc (range: 90.6–348.0); the median CTV volume was 757.5cc (range: 256.6 1364.6). The median GTV LETd-min constraint, applied to clinically delivered plans-DL, was 43 keV/um (range: 40-46). The resulting median GTV LETd-min increased of 33.0%, from 32.1 keV/um (plan-D) to 42.7 keV/um (plan-DL) (Figure 1-2). Inter-fraction robustness of target coverage was comparable (p>0.05), with mean D98% and D2% variations,