Clinical 3D/4D cumulative proton dose assessment methods for thoracic tumours with large motion

•This study describes the first clinical experiences with the IMPT treatment of large moving target volumes in terms of robustness.•Comprehensive 3D and 4D robustness evaluation methods were explored to assess target coverage and OAR dose in the clinical IMPT treatment of large movers.•Adequate course-wise target dose coverage was assured for IMPT for intra-thoracic tumours with large motion (10–17 mm).•The influence of anatomical changes, setup and range errors was larger than the effect of respiratory motion on target and OAR dose robustness in fractionated IMPT treatments.•Offline adaptation based on weekly imaging seems sufficient to ensure adequate course-wise target coverage.