ESTRO 2025 - Abstract Book

S3540

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

ESTRO 2025

Keywords: RBE, MKM, radiosensitivity

References: Inaniwa, T., Furukawa, T., Kase, Y., Matsufuji, N., Toshito, T., Matsumoto, Y., Furusawa, Y., & Noda, K. (2010). Treatment planning for a scanned carbon beam with a modified microdosimetric kinetic model. Phys Med Biol , 55 (22), 6721-6737. https://doi.org/10.1088/0031-9155/55/22/008 Chen, Y., Li, J., Li, C., Qiu, R., & Wu, Z. (2017). A modified microdosimetric kinetic model for relative biological effectiveness calculation. Phys Med Biol , 63 (1), 015008. https://doi.org/10.1088/1361-6560/aa9a68

3704

Digital Poster LET- optimized proton therapy for sparing optic structures in sinonasal cancer Miriam Paetzel 1,2 , Camilla H Stokkevåg 1,3 , Camilla G Boer 1 , Jon E Dale 1 , Marianne Brydøy 1 , Kristian Ytre-Hauge 3 , Grete M Engeseth 1,2 1 Department of Oncology and Medical Physics, Haukeland University Hospital, Bergen, Norway. 2 Faculty of Health Sciences, Oslo Metropolitan University, Oslo, Norway. 3 Department of Physics and Technology, University of Bergen, Bergen, Norway Purpose/Objective: Variations in relative biological effectiveness (RBE) and linear energy transfer (LET) in proton therapy could lead to unexpected toxicity in critical organs at risk (OARs). The aim of this study was to improve the quality of intensity modulated proton therapy (IMPT) for sinonasal cancer patients by reducing dose-average LET (LETd) to OARs through LET-optimization. Material/Methods: Reference (Ref) IMPT plans were generated in the RayStation treatment planning system for 9 patients. The prescribed dose was 64-70 GyRBE (RBE=1.1), robustly optimized to the CTV. LET-optimized (LETopt) plans were generated based on the 9 Ref plans using the same dose constraints, while adding LET objectives (LETd<3kev/µm at doses >30GyRBE) to the optic chiasm and the optic nerves. Dose calculations were performed (RBE1.1) using robustness- settings of 0.2cm for setup and 3.5% for range uncertainties for both optimization- and evaluation. Ref and LETopt plans were compared by evaluating LET- and variable RBE dose distributions to the optic OARs based on McNamara, Wedenberg, Carabe, Rørvik (1) and Lyngholm (2) (α/β=3). Results: The prescribed dose criteria were met for both Ref and LETopt plans (RBE1.1). Median LETdmax was reduced from 3.6 keV/µm in Ref to 2,9 keV/µm in LETopt for the optic chiasm, and from 3.9 keV/µm in Ref to 2,9 keV/µm in LETopt for the optic nerves (Fig.1). Across all RBE-models and all patients, dose reduction in the LETopt compared to Ref ranged from 1.7-5.0 GyRBE (median 2.2) for the optic chiasm, from 2.0-4.0 GyRBE (median 2.5) for the left optic nerve and from 3.9-5.7 GyRBE (median 4.5) for the right optic nerve (Fig.2). RBE-calculated doses for the optic chiasm using the RBE-models by McNamara, Wedenberg and Carabe were 57.3 GyRBE, 57.9 GyRBE and 56.1 GyRBE for Ref, whereas 55.6 GyRBE, 52.9 GyRBE and 53.9 GyRBE for LETopt, respectively. RBE-calculated doses to the left optic nerve using Rørvik and Lyngholm were 58.9 GyRBE and 56.8 GyRBE for Ref, whereas 54.9 GyRBE and 54.5 GyRBE for LETopt, respectively (Fig.2). RBE-calculated doses in the Ref plans exceeded a threshold of 55 GyRBE to the optic OARs in 80% of cases, whereas LETopt plans achieved doses within this threshold in 60% of the cases.