ESTRO 2023 - Abstract Book

S1716

Digital Posters

ESTRO 2023

Conclusion To establish a common framework within which RBE estimated using each model can be directly measured, we designed microdosimetric estimation methods for each model based on a single measurable input, which were almost always able to predict true RBE to within ±5% accuracy. Based on the results of this study, evaluation of the modeled RBE value based on microdosimetric measurements, is reasonable. This work provides a framework for an intercomparison system, which will be capable of delivering measurement-based feedback on the clinical implementation of RBE models to allow for practical comparison of RBE across both institutions and models.

PO-1955 PBS proton arc therapy may improve focal boosting compared to VMAT for localised prostate SBRT

R. De Roover 1,2 , C. Draulans 3 , K. Poels 1,2 , E. Engwall 4 , V. Wase 4 , O. Marthin 4 , J. Sundström 4 , S. Isebaert 1,2,3 , U. van der Heide 5 , T. Depuydt 1,2,3 , K. Haustermans 6,2,3 1 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium; 2 University Hospitals Leuven, Particle Therapy Interuniversity Center Leuven – PARTICLE, Leuven, Belgium; 3 KU Leuven, Department of Oncology, Leuven, Belgium; 4 RaySearch Laboratories, Research and Development, Stockholm, Sweden; 5 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands; 6 University Hospitals Leuven, Department of Radiation Oncology, , Leuven, Belgium Purpose or Objective In the phase III FLAME trial a dose response relationship to the intraprostatic GTV D98% has been shown leading to improved biochemical disease-free survival for localised prostate cancer (PCa). In the current study, a novel treatment planning system prototype build for PBS proton arc therapy optimization is explored to investigate whether PBS proton arc therapy could improve focal dose escalation to the GTV while maintaining similar or reduced dose to the critical organs at risk (OAR) compared to contemporary photon VMAT. Materials and Methods Fifteen men with localised PCa previously treated in the hypo-FLAME and hypo-FLAME 2.0 trials were replanned using VMAT, bilateral opposing IMPT and PBS proton arc therapy (PAT). The prescribed dose was 35 Gy to the prostate (CTVp1) and 30 Gy to the seminal vesicles (CTVp2) in 5 fractions. The focal boost was up to 50 Gy if the dose to the OAR permitted so. Both photon and proton plans were robustly optimized in a research version of RayStation 11B using 5 mm setup uncertainty. For proton plans 3% range uncertainty was included. VMAT plans were generated for 6MV FFF on a Varian TrueBeam STx and consisted of two full arcs with collimator angles 10° and 80°. Proton plans were generated for an IBA ProteusONE compact gantry. Bilateral opposing IMPT plans consisted of two beams at gantry angle 90° for couch angles 0° and 180°. PAT plans consisted of two partial arcs going from 100° to 325° gantry angle for couch angles 0° and 180°. The gantry angle spacing between two consecutive energy layers was set at 4° resulting in 34 energy layers per arc, each at a different gantry angle. The focal boost dose (GTV D98%), the target coverage (CTVp1 and CTVp2 D98%) and dose to the critical OAR (rectum, bladder and urethra) were compared for the nominal scenario. Robust evaluation (5mm-(3%)) was performed to compare the voxelwise minimum (VWmin) focal boost and target dose and the voxelwise maximum maximum dose (D0.03cc) to the critical OAR. Statistical testing comprised of a Friedman test followed by Wilcoxon signed-rank tests. Results PAT plans achieved higher GTV D98% in the nominal scenario compared to VMAT with a median difference of 0.7 Gy (p<0.01) and >3 Gy for 3/15 patients (Fig 1A). In addition, PAT plans had higher VWmin GTV D98% compared to VMAT. IMPT also