ESTRO 2025 - Abstract Book

S3560

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

ESTRO 2025

Results: The maximum variation reported in M95 among all the cubes was -1.98%, with an average of -0.07% ± 1.1%. Maximum variations of 0.8 mm were observed in R90 and R80, with mean deviations of -0.51 ± 0.31 mm and -0.5 ± 0.25 mm, respectively, all within a 1 mm tolerance. The maximum DDF variation was 0.2 mm. Output variation averaged -0.87% ± 0.01%, with a maximum of -2.32%. Penumbra deviations were within 2 mm, and field size differences were within 1 mm, demonstrating strong agreement between TPS and measurements. Conclusion: The validated PBS proton therapy beam data model demonstrated high accuracy for small target treatments in a homogeneous medium, with all variations within clinically acceptable tolerances, supporting its reliability for clinical use. Further thorough validations involving inhomogeneous medium and clinical scenarios are recommended. References: 1. Jonathan B. Farr et al. Clinical commissioning of intensity-modulated proton therapy systems: Report of AAPM Task Group 185, https://doi.org/10.1002/mp.14546 2. Farr JB, Moskvin V, Lukose RC, Tuomanen S, Tsiamas P, Yao W. Development, commissioning, and evaluation of a new intensity modu-lated minibeam proton therapy system. Med Phys. 2018;45(9):4227–4237. 3. Arjomandy B, Taylor P, Ainsley C, et al. Report of the AAPM TaskGroup 224: Proton Machine QA. College Park, MD: American Association of Physicists in Medicine; 2017. Keywords: Proton therapy, TPS beam modelling and validation Poster Discussion From Dosimetric Promise to Clinical Reality: Challenges of SHArc Radiotherapy for hypoxia Filipa Baltazar 1,2,3 , Thomas Tessonnier 1,2 , Jakob Liermann 1,4,5 , Andrea Mairani 1,6 1 Department of Radiation Oncology, Heidelberg Ion-Beam Therapy Center (HIT), Heidelberg, Germany. 2 Clinical Cooperation Unit Radiation Oncology, German Cancer Research Center, Heidelberg, Germany. 3 Medical Faculty, University of Heidelberg, Heidelberg, Germany. 4 Medical Faculty, Heidelberg University, University of Heidelberg, Heidelberg, Germany. 5 Department of Radiation Oncology, Heidelberg University Hospital, Heidelberg, Germany. 6 Clinical Cooperation Unit Translational Radiation Oncology, German Cancer Research Center, Heidelberg, Germany Purpose/Objective: Spot-Hadron Arc (SHArc) radiotherapy shows promise for treating hypoxic tumors, such as pancreatic cancer, by enhancing dose-averaged Linear Energy Transfer (LETd) in the tumor's hypoxic core, potentially overcoming tumor radioresistance. 1,2 While prior studies highlight the dosimetric potential of ion arc therapy, its clinical applicability remains limited by technical and practical constraints. This study evaluates SHArc's dosimetric benefits under these constraints, focusing on dose distribution, LETd optimization, and plan robustness. Material/Methods: Robust optimized plans with carbon ions were created for pancreatic cancer patients using the clinically available beam angles for carbon ions at the Heidelberg Ion Therapy (HIT) center. To limit total irradiation time, two energy layer (EL) selection strategies were tested: 1. Central EL: retaining seven energy layers per beam, focused on the tumor’s center; 2. MU-Based EL: retaining the energy layers contributing the most monitor units (MUs) following initial optimization. The percentage of ELs retained was chosen to ensure irradiation times comparable to those achieved with the Central EL approach. The planning strategy also included LET optimization, with a minimum LETd of 50 keV/µm for the Gross Tumor Volume (GTV), which could be increased depending on the specifics of each plan. After optimization, plan robustness was evaluated by separately assessing position and range uncertainties (5mm, 4175