Impact of reduced robustness settings on the necessity for proton therapy treatment plan adaptation in head-and-neck cancer patients

Background and purpose: Robust optimization mitigates the effects of range and setup uncertainties in intensity–modulated proton therapy (IMPT) through robustness settings. However, it increases the dose to organs-at-risk (OARs). This study investigated how reducing range robustness (RR) and setup robustness (SR) settings impacts plan adaptation frequency in IMPT of head-and-neck cancer (HNC) patients. Material and methods: A cohort of twenty-six HNC patients treated with IMPT were retrospectively analyzed. Original plans used RR/SR settings of 3%/3 mm. For each patient, four plans were generated with reduced robustness settings. All plans were robustly evaluated on weekly repeat computed tomography scans (rCTs) across 28 error scenarios, applying 2.7–3.2% range and 1 mm setup uncertainty. The failure rate of the voxel-wise minimum D98%≥94% criterion was used as a surrogate for plan adaptation frequency. Differences and equivalence in failure rates between settings were assessed using both two-proportion z-test and Two-One Side Test. Results: Fixing SR at 3 mm, each 1% reduction in RR setting reduced the mean OAR dose by 0.45 Gy (RBE) and normal tissue complication probability (NTCP) by 1.1%-points at a 3.5%-points increase in adaptation triggering rate. Implementing a RR of 3%, each 1 mm reduction in SR settings reduced the OAR dose by 1.0 Gy (RBE) and NTCP by 2.2%-points at a 7.0%-points increase in adaptation triggering rate. Conclusions: Reducing robustness margins decreased OAR dose and NTCP with a manageable increase in plan adaptation; however, larger reductions would require adaptive workflows and implementation of range uncertainty mitigation strategies before safe clinical implementation.