ESTRO 2025 - Abstract Book

S3502

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

ESTRO 2025

1 Radiotherapy, Erasmus MC Cancer Institute, Rotterdam, Netherlands. 2 Medical Physics & Informatics, HollandPTC, Delft, Netherlands. 3 Radiation Oncology, Leiden University Medical Center, Leiden, Netherlands. 4 Radiation Science and Technology, Delft University of Technology, Delft, Netherlands Purpose/Objective: In the Netherlands, proton therapy target and OAR dose are evaluated using the voxel-wise minimum and maximum dose distribution of 28 scenarios. Although calibrated against PTV-based photon plans [1], this approach lacks a statistically robust foundation and is inherently conservative. With recent advancements in simulating a large number of scenarios, probabilistic evaluation has become feasible [2]. In this study, we investigated whether probabilistic evaluation with an IMPT replanning strategy improves OAR sparing or enhances robust target coverage in cases where it was initially compromised. Material/Methods: The 22 neuro-oncological patients were treated at HollandPTC in 2023 with prescribed doses of 50.4 GyRBE (16/22) or 59.4 GyRBE (6/22), delivered in 1.8 GyRBE fractions. Of the 22 patients, 18 met their original target goals (group A), while in 4, target coverage was compromised to spare OARs (group B). The probabilistic goal for the CTV was calibrated to be consistent with PTV-based photon plans, resulting in D 99.8%,CTV =0.95 D pres with a 90% confidence level. The probabilistic OAR constraints were set to meet the constraints in [3] with a 95% confidence level. For both groups, the clinical plans were re-optimized in RayStation, keeping the clinical objectives and constraints, but reducing robustness for the CTV (group A) to precisely meet the probabilistic goal, or for the dose-limiting OARs (group B) to approach but never exceed the constraints. For the original and re-optimized plans, polynomial chaos expansion was applied to simulate 10,000 fractionated treatments, deriving probability distributions for relevant DVH parameters. Results: For group A, re-optimization resulted in a population median decrease of 8.2 (range: 0.4–20.8) GyRBE for the sum of OAR DVHs listed in figure 1. For the D 0.03cc,Brainstem_Core reductions up to 3.3 GyRBE were achieved. For the D 0.03cc,OpticNrv and the D 0.03cc,OpticChiasm reductions up to 4.6 and 3.6 GyRBE were found. The D 99.8%,CTV decreased with a median of 1.1 (range: 0.3–1.9) GyRBE without compromising clinical goal achievement. For group B, re-optimization resulted in a population median increase of 2.7 (range: 1.3–6.8) GyRBE in the D 99.8%,CTV (figure 2). The population median of the V95%,CTV improved from 97.4% to 99.1%. For one patient, the probabilistic goal for the CTV was achieved after re-optimization. The dose limiting OAR DVH values increased with a median of 1.4 (range: -0.2–6.9) GyRBE without exceeding the clinical constraints.