ESTRO 2025 - Abstract Book

S2910

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

impact of this variability on OAR dose distributions [2], with calculations performed using two different radiation types, photons and protons.

Material/Methods: The 4D-CT and DIBH-CT scans of a patient with gastric MALT were shared with 11 radiation oncologists from 11 different centers. Each oncologist performed a blinded ITV and CTV delineation on both 4D-CT and DIBH-CT series, respectively. According to international guidelines [3], a consensus-based ITV ref and CTV ref were then defined by all participants and used as reference volumes. Inter-operator variability was assessed using the relative volume (RV), DICE-similarity index (DICE-index) and Hausdorff distance (HD) for each contouring. A VMAT plan on DIBH-CT (VMAT DIBH ) and both VMAT and IMPT plans on 4D-CT (VMAT 4DCT and IMPT 4DCT ) were robustly optimized with a prescribed dose of 2400 cGy in 12 daily fractions using the same optimization goals and priorities for ITV ref , CTV ref and OARs. To assess the impact of inter-operator variability on OARs, VMAT DIBH and IMPT 4DCT plans were evaluated under the contouring worst-case scenarios. Results: As expected, all CTVs contoured on DIBH-CT showed reduced volumes compared to ITVs contoured on 4D-CT. Median RV and DICE-index values increase from 0.86 and 0.85 for ITVs to 0.97 and 0.93 for CTVs, respectively. In contrast, HD values were similar across ITVs and CTVs of both CT series (Table 1).