S1767
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
dose side effects, caused by amplification of proton dose by high dose-averaged linear energy transfer (LETd) at the end of range. Compared to IMPT, PAT plans exhibit higher variable RBE dose and LETd values, potentially increasing the risk of high-dose- related side effects, such as radiation ulcers and soft- tissue necrosis. This study investigated if LETd optimization in PAT can reduce LETd values in high dose regions. Material/Methods: For six OPC patients previously treated with IMPT, with prescribed doses of 7000 cGy(RBE=1.1) and 5425 cGy(RBE=1.1) to the primary target and prophylactic lymph nodal area, respectively, static PAT plans employing 30 gantry angles, 420 energy layers, and two anterior oblique range-shifted fields were robustly optimized using 3 mm/3% setup and range uncertainty settings in RayStation. Additional LETd optimized PAT plans were created, aiming to reduce LETd values <5.0 keV/ μ m in regions exceeding 6500 cGy(RBE=1.1). The McNamara variable RBE model with α / β = 2Gy estimated variable RBE dose (DMcN), conform the Dutch approach for variable RBE evaluation. Maximum DRBE=1.1 and DMcN points in PAT and IMPT plans, as well as LETd values at the corresponding locations, were evaluated. Results: Table 1 shows the maximum DRBE=1.1 was, on average, lower(p=0.09) in PAT plans than in IMPT plans: 7408 cGy(RBE=1.1) versus 7576 cGy(RBE=1.1), respectively. In contrast, the maximum DMcN was 3.4% higher: 8946 cGy(RBE=McN) versus 8657 cGy(RBE=McN)(p=0.09) in the PAT plans. LETd values at the location of maximum DMcN were on average 46.6% higher(p=0.03) with 5.8 keV/ μ m in PAT plans compared to 4.0 keV/ μ m in IMPT plans.LETd optimized PAT plans showed a reduction(p=0.09) in maximum DMcN compared to IMPT, with on average 8472 cGy(RBE=McN), while maintaining similar(p=0.27) LETd values 4.0 keV/ μ m at the location of maximum DMcN. Figure 1 illustrates maximum DMcN in the oral cavity.
Conclusion: There was no large difference in robustness between the proton and photon plans. We intend to continue this work by including more patients and also evaluating other factors such as the effect of SFUD/MFO or range shifter. References: 1. Sterpin, E., et al. “Robustness evaluation of pencil beam scanning proton therapy treatment planning: A systematic review.” Radiotherapy and Oncology, vol. 197, June 2024, p. 110365. https://doi.org/10.1016/j.radonc.2024.1103652. Korevaar, Erik W., et al. “Practical Robustness Evaluation in Radiotherapy – a Photon and Proton- proof Alternative to PTV-based Plan Evaluation.” Radiotherapy and Oncology, vol. 141, Sept. 2019, pp. 267–74. https://doi.org/10.1016/j.radonc.2019.08.0053. Siwako ti, Krishmita, et al. “Application of Photon-Derived Worst-Case Robustness Criteria to Proton Therapy Planning.” International Journal of Particle Therapy, vol. 15, Feb. 2025, p. 100740. https://doi.org/10.1016/j.ijpt.2025.100740 Keywords: Proton Therapy, Robustness, Protons vs Photons Digital Poster 4120 LETd optimization reduces potential risk for high- dose side effects in proton arc therapy for oropharyngeal cancer patients. Bas Adriaan de Jong, Dirk Wagenaar, Erik W. Korevaar, Jeffrey Free, Roel J.H.M. Steenbakkers, Johannes A. Langendijk Department of radiation oncology, University medical center Groningen, Groningen, Netherlands Purpose/Objective: Recent studies have shown proton arc therapy (PAT) planning can reduce healthy tissue clinical dose (DRBE=1.1) in oropharyngeal cancer (OPC) patients. Variable relative biological effectiveness (RBE) models enable the identification of regions at risk for high-
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