ESTRO 2025 - Abstract Book

S3524

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

ESTRO 2025

cumulative dose sparing of the heart and lungs; selecting between protons and photons for lung cancer reirradiation could be based on these OARs.

Keywords: Reirradiation, proton vs. photon, lung cancer

References: [1] Andratschke N, Willmann J, Appelt AL, Alyamani N, Balermpas P, Baumert BG, Hurkmans C, Høyer M, Langendijk JA, Kaidar-Person O, van der Linden Y, Meattini I, Niyazi M, Reynaert N, De Ruysscher D, Tanadini-Lang S, Hoskin P, Poortmans P, Nieder C. European Society for Radiotherapy and Oncology and European Organisation for Research and Treatment of Cancer consensus on re-irradiation: definition, reporting, and clinical decision making. Lancet Oncol. 2022 Oct;23(10):e469-e478.

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Mini-Oral LET optimized proton arc plans reduce cerebrum and visual OAR dose for low grade glioma patients H.R. van de Glind, B.A. de Jong, H.L. van der Weide, J.A. Langendijk, S. Both, E.W. Korevaar, D. Wagenaar Department of Radiation Oncology, University Medical Center Groningen, Groningen, Netherlands Purpose/Objective: Intensity Modulated Proton Therapy (IMPT) is standard of care in low grade glioma (LGG), but managing higher RBE at the distal end of the Bragg peak remains a challenge. In our clinic, we use the McNamara model to evaluate variable RBE-weighted dose (D McN ) [1]. LET optimization may help to optimize D McN . An emerging treatment technique that might improve both dose (D RBE=1.1 ) and D McN is proton arc therapy (PAT). The aim of this study was to test the hypothesis that combining PAT with LET optimization (PAT-LET) in LGG patients improve D RBE=1.1 and D McN to visual OARs (optic nerves, chiasm) and cerebrum compared to IMPT without LET optimization. Material/Methods: The study population was composed of 10 LGG patients treated with IMPT between November 2021 and November 2022 with a 50.40 Gy RBE=1.1 prescription dose and a CTV located within 6 mm of visual OARs. Plans were robustly optimized for 3 mm positional and 3% density error and meeting clinical objectives. The PAT-LET setup involved a 360-degree gantry rotation without table rotation, using 30 equally spaced beams with 360 energy layers, as shown optimal in prior studies [2]. A full gantry rotation allows the optimizer to select the ideal angles for each case. A method was developed to convert D McN (α/β= 2) constraints to LET objectives in the current TPS (RayStation 2023B), as shown in Figure 1a. IMPT and PAT-LET plans were compared regarding doses to visual OARs, cerebrum, and target coverage. Results: Figure 1b shows that plans with lower cerebrum dose (V2Gy RBE=1.1 ) had higher D McN , forming a Pareto front. Figure 1c highlights extreme examples of these effects, showing that D McN can be lowered at the cost of a low dose bath. Figure 2 compares PAT-LET and IMPT plans, which had similar CTV coverage and robustness. On average, D RBE=1.1,0.03cc improved by 4.6% (chiasm), and 6.0% (optic nerves). D MCN,0.03cc improved by 4.1% (chiasm), and 5.0% (optic nerves). D 3cc in the cerebrum was consistent, but Figure 2c shows that V2Gy RBE=1.1 – V40Gy RBE=1.1 improved, with the largest reduction at V10Gy RBE=1.1 (average 27.2%). PAT-LET reduced mean D RBE=1.1 to OARs (Figure 2d), with significant average reductions of 7.2 Gy RBE=1.1 for the pituitary and 2.6 Gy RBE=1.1 for the cerebrum.