ESTRO 2023 - Abstract Book

S658

Monday 15 May 2023

ESTRO 2023

respectively. In the subgroups analysis based on cancer types, the OS and PFS were not significantly between delineations ( Figure 2 ).

Conclusion Modified margin pelvic nodal delineation reduced the dose-volume to the small bowel, and lowered the GI toxicity in adjuvant pelvic radiotherapy. The survival outcomes were similar between delineations. Modified margin delineation may achieve optimized therapeutic ratio in patients with gynecological cancer. OC-0787 Proton-based RT enables target dose escalation in oesophageal cancer with limited impact on OAR dose P. Klinker 1 , S. Visser 1 , E. Korevaar 1 , M. Dieters 1 , C. Ijsbrandy 1 , J. Beukema 1 , J. Langendijk 1 , C. Muijs 1 1 University Medical Centre Groningen, Radiation Oncology, Groningen, The Netherlands Purpose or Objective Oesophageal cancer (EC) patients with a clinical complete response after neo-adjuvant chemoradiotherapy (nCRT) might be candidates for organ preservation. Thomas et al. (2019) demonstrated that higher radiation dose improves the rate of major pathologic response. The ARTDECO trial (2021) showed a trend towards better local control in the high dose arm in definitive setting. However, dose escalation using photon-based RT failed to result in improved overall survival. This might be related to higher normal tissue dose and consequently higher toxicity risks. The aim of this in silico planning study was to evaluate the impact of dose escalation to the tumour regarding organs at risk (OARs) dose in nCRT for EC patients. Materials and Methods For this study, we used the planning CT scans of 16 EC patients, who were treated with proton nCRT between April and July 2022 and who provided informed consent for the use of their data. The target dose was escalated from 41.4 Gy (current standard) to a total dose of 50.4 Gy by re-scaling the original clinical treatment plan. We investigated the impact of dose escalation on OARs dose for both the proton (IMPT) and the back-up photon (VMAT) plans, and differences in OARs dose were tested using the Wilcoxon signed rank test. Results For both PhRT and PRT, the OARs dose increased significantly by escalating the dose to the target. In the escalated PhRT plans, the mean heart dose (MHD) and Lung V5 exceeded the constraints in 6 (37.5%) and 4 (25%) patients. None of the escalated PRT plans exceeded the normal tissue constraints. Only the left kidney dose was relatively high in 3 patients (mean 10-17 Gy). The absolute differences in OARs dose between the escalated and standard treatment plans were significantly higher in the PhRT plans as compared to the PRT plans. The MHD increased on average by 4.3 Gy, while in PRT the increase was only 1.9 Gy on average (Table 1). Moreover, the mean dose to heart and lungs remained significantly lower in the escalated proton plans compared to the non-escalated photon plans.