Simple SummaryThe study retrospectively analysed the clinical proton treatment strategy for pancreas patients with small moving tumours. Seventeen hypofractionated proton treatment plans were analysed based on 4D dose calculations, which gives insight into the interplay of beam and organ motion. The results showed that the synchrotron-based dose delivery employing horizontal and vertical beams and robust optimisation was robust against intra-fractional movements up to 3.7 mm. However, outliers were observed in some patients, indicating the need for continuous monitoring during clinical practice to identify patient cases with more significant deviations. The research serves as a basis for future treatment strategies for patients with larger motion amplitudes and the transition towards carbon ion treatments.AbstractMotion compensation strategies in particle therapy depend on the anatomy, motion amplitude and underlying beam delivery technology. This retrospective study on pancreas patients with small moving tumours analysed existing treatment concepts and serves as a basis for future treatment strategies for patients with larger motion amplitudes as well as the transition towards carbon ion treatments. The dose distributions of 17 hypofractionated proton treatment plans were analysed using 4D dose tracking (4DDT). The recalculation of clinical treatment plans employing robust optimisation for mitigating different organ fillings was performed on phased-based 4D computed tomography (4DCT) data considering the accelerator (pulsed scanned pencil beams delivered by a synchrotron) and the breathing-time structure. The analysis confirmed the robustness of the included treatment plans concerning the interplay of beam and organ motion. The median deterioration ofD50%(ΔD50%) for the clinical target volume (CTV) and the planning target volume (PTV) was below 2%, while the only outlier was observed for ΔD98%with −35.1%. The average gamma pass rate over all treatment plans (2%/ 2 mm) was 88.8% ± 8.3, while treatment plans for motion amplitudes larger than 1 mm performed worse. For organs at risk (OARs), the median ΔD2%was below 3%, but for single patients, essential changes, e.g., up to 160% for the stomach were observed. The hypofractionated proton treatment for pancreas patients based on robust treatment plan optimisation and 2 to 4 horizontal and vertical beams showed to be robust against intra-fractional movements up to 3.7 mm. It could be demonstrated that the patient’s orientation did not influence the motion sensitivity. The identified outliers showed the need for continuous 4DDT calculations in clinical practice to identify patient cases with more significant deviations.