Simple SummaryAnatomical variations may distort the carbon-ion beam, leading to dose degradation during treatment. Thus, a robust-beam arrangement is important for patients with pancreatic cancer with carbon-ion radiotherapy. Our study aimed to investigate the angular dependency of water equivalent thickness (WET) variation for pancreatic cancer and to evaluate the robustness of accumulated doses with WET-based beam configurations. We found that posterior oblique beams in the supine position and anteroposterior beams in the prone position were the most robust to WET changes, and the robustness of the accumulated dose was significantly improved by using WET-based beam configurations. The findings may provide a reference for robust beam selection in clinical practice.AbstractCarbon-ion radiotherapy (CIRT) is one of the most effective radiotherapeutic modalities. This study aimed to select robust-beam configurations (BC) by water equivalent thickness (WET) analysis in passive CIRT for pancreatic cancer. The study analyzed 110 computed tomography (CT) images and 600 dose distributions of eight patients with pancreatic cancer. The robustness in the beam range was evaluated using both planning and daily CT images, and two robust BCs for the rotating gantry and fixed port were selected. The planned, daily, and accumulated doses were calculated and compared after bone matching (BM) and tumor matching (TM). The dose-volume parameters for the target and organs at risk (OARs) were evaluated. Posterior oblique beams (120–240°) in the supine position and anteroposterior beams (0° and 180°) in the prone position were the most robust to WET changes. The mean CTV V95% reductions with TM were −3.8% and −5.2% with the BC for gantry and the BC for fixed ports, respectively. Despite ensuring robustness, the dose to the OARs increased slightly with WET-based BCs but remained below the dose constraint. The robustness of dose distribution can be improved by BCs that are robust to ΔWET. Robust BC with TM improves the accuracy of passive CIRT for pancreatic cancer.