S1783
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
APSS, Trento, Italy. 4 Faculty of Medicine, University of Bern, Bern, Switzerland. 5 Department of Oncology – Laboratory of Experimental Radiotherapy, KU Leuven – University of Leuven, Leuven, Belgium. 6 Department of Radiation Oncology, University Hospitals Leuven, Leuven, Belgium. 7 Particle Therapy Interuniversity Centre Leuven (PARTICLE) Proton Therapy Centre, Department of Radiation Oncology, University Hospital Leuven, Leuven, Belgium. 8 Department of Oncology, Aarhus University Hospital, Aarhus, Denmark Purpose/Objective: Proton therapy’s (PT) biological effect depends on dose and linear energy transfer (LET), which rises at the distal edge, where the relative biological effectiveness (RBE) may exceed the fixed 1.1 used for treatment planning [1]. In the international randomized phase III PROTECT trial for oesophageal cancer patients [2], posterior fields often result in high-LET regions in the heart, resulting in increased RBE [3]. The McNamara variable-RBE model links dose- averaged LET (LETd) and tissue radiosensitivity to RBE, enabling biologically weighted dose estimates [1,4]. This study compares variable RBE (McNamara) whole- heart dose estimates with constant RBE of 1.1 doses throughout the treatment. Material/Methods: Fifteen consecutive patients enrolled in the PROTECT trial were treated with 2-3 posterior PT fields delivering 50.4 Gy(RBE=1.1) in 28 fractions under predefined target-coverage and robustness criteria [5]. One patient discontinued treatment due to oesophageal perforation. The remaining 14 completed proton treatment. The approved plan on planning CT (pCT) was recalculated on weekly surveillance CTs (sCTs). When protocol checks indicated relevant dosimetric deviations, an adaptive replan was generated and used for subsequent fractions. For every CT scan, Monte Carlo dose (RayStation v2023B, Research Laboratories; 2mm grid; 0.5% statistical uncertainty) and LETd were retrospectively calculated; McNamara variable-RBE doses ( α / β =2Gy and 3Gy; late- responding normal-tissue range) were reconstructed. Analyses were confined to the whole heart. Mean heart dose (MHD), V25Gy and V40Gy, and D1cm3 were extracted from original plan and weekly recalculations and benchmarked against PROTECT thresholds. Results: A distal-edge LETd hotspot adjacent to the heart, with McNamara doses exceeding RBE=1.1 was seen on pCT and sCTs (Figure 1). Cardiac metrics were stable over time: V25Gy, V40Gy, and MHD remained within PROTECT thresholds at all assessed weeks (Figure 2). Pooled across pCT, weekly sCTs and replans, McNamara yielded higher values than RBE=1.1: MHD increased on average (±SD) by 2.5±0.9Gy RBE ( α / β =2Gy) and 1.8±0.6Gy RBE ( α / β =3Gy); D1cm3 by
Conclusion: This work presents a practical translational pathway for RMs in carbon ion therapy. Through physical dose verification, in vitro biological assessment, and patient- specific planning, we demonstrate that these modulators can reliably deliver conformal physical and RBE-weighted doses using monoenergetic beams. Clinically, this approach may shorten treatment times, reduce motion sensitivity, and support future clinical adoption of UHDR and FLASH carbon ion therapy. Keywords: Range modulator, RBE dose, Carbon ion therapy Impact of LET effect on dose distribution in proton therapy for oesophageal cancer patients in the PROTECT trial (NCT05055648) Sarah Eckholdt 1,2 , Anne Vestergaard 1 , Francesco Fracchiolla 3,4 , Gilles Defraene 5 , Karin Haustermans 5,6 , Kenneth Poels 6,7 , Lone Hoffmann 2,8 , Maria Fuglsang Jensen 1 , Marianne Nordsmark 2,8 , Robin De Roover 6,7 , Hanna Rahbek Mortensen 1,2 1 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark. 2 Department of Clinical Medicine, Faculty of Health Sciences, Aarhus University, Aarhus, Denmark. 3 UO Fisica Sanitaria, Poster Discussion 4797
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