S1766
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
Hospital, Lund, Sweden. 2 Department of Medical Physics, The Skandion Clinic, Uppsala, Sweden. 3 Medical Radiation Physics, Department of Clinical Sciences Lund, Lund University, Lund, Sweden Purpose/Objective: In photon therapy the planning target volume (PTV) margin concept commonly used assumes that the dose distribution is unchanged by geometrical uncertainties. This implies that a geometrical expansion of the clinical target volume (CTV) to the PTV is sufficient for robust target dose coverage [1]. In proton therapy however, this assumption is not valid because of the sensitivity to density changes in the beam path due to set-up errors. Therefore, robust optimisation and robust evaluation where various uncertainty scenarios in relation to the nominal plan are simulated is often applied to ensure adequate target dose coverage [2]. There is, however, no consensus on how to evaluate the robustness of proton plans [3]. In our clinic, the treatment choice for each patient (protons or photons) is based on a DVH comparison between the nominal photon and proton plan, under the premise that only the proton plan requires robustness evaluation. The aim of this study was therefore to retrospectively compare the robustness of photon and proton plans and thereby extending the comparison beyond nominal dose distributions. Material/Methods: In total 23 patients were included, of whom nine had brain tumours, nine had head-and-neck (H&N) tumours and five had pelvic tumours. All patients were treated with proton therapy, either single field uniform dose (SFUD) or multi-field optimisation (MFO), but had a clinically acceptable volumetric modulated arc therapy (VMAT) photon plan prepared for comparison. Uncertainty scenario plans with isocentric displacement corresponding to the PTV margin was generated for both treatment types and a 3.5% range uncertainty was added to the proton uncertainty plans. This resulted in 12 uncertainty plans for each proton plan, of which at least 10 had to meet the criterion D98% > 95% to be considered clinically acceptable. The robustness was compared between the techniques by evaluating the CTV dose coverage. Results: The robust evaluation showed sufficient dose coverage to the CTV in more than 90% of the plans for both protons and photons (Figure 1) and the mean CTV D98% was above 95% in all cases (Table 1). The H&N plans were the least robust, and D98% was below 95% for four uncertainty plans in both the proton and photon plans (Figure 1).
Conclusion: Shoot-through enhanced proton arcs achieve promising results: for all but the smallest targets, the dose fall-off compared to VMAT is similar in the high- dose region but better in the low-dose region. References: [1] Engwall, Erik, et al. "Shoot - through layers in upright proton arcs unlock advantages in plan quality and range verification." Medical Physics 52.9 (2025): e18051.[2] Sundström, Johan, et al. "Partitioning of multiple brain metastases improves dose gradients in single - isocenter radiosurgery." Medical Physics 52.10 (2025): e18117.[3] Wase, Viktor, et al. "Optimizing the traversal time for gantry trajectories for proton arc therapy treatment plans." Physics in Medicine & Biology 69.6 (2024): 065007. Keywords: transmission protons, single-isocenter SRS Digital Poster 4052 Evaluation of Plan Robustness: Comparing Robustly Optimised Proton and PTV Optimised Photon Plans Annika Mannerberg 1,2 , Lars Weber 1,3 , Marika Enmark 1,2 1 Radiation Physics, Department of Hematology, Oncology, and Radiation Physics, Skåne University
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