SABR Protocol Deviations Linked to Poorer Survival in Oligometastatic Head and Neck Cancer

PURPOSE: The OMET GORTEC 2014-04 trial showed prolonged survival with multisite Stereotactic Ablative Body Radiation therapy (SABR) in patients with oligometastatic head and neck cancer. However, deviations from radiation therapy protocol can be a confounding factor in clinical trials. The impact of SABR quality assurance by individual case review (ICR) of protocol deviations and quality metrics on survival and local control at SABR-treated oligometastases was assessed. METHODS AND MATERIALS: The SABR planning and delivery protocol was standardized based on the oligometastasis number/size/nearby critical structures (organs at risk [OARs]) across SABR techniques. ICR assessed oligometastatic topological complexity, indication, delineation (targets, OARs), image guidance, prescription, target coverage, and OAR doses. An exploratory analysis of quantitative dosimetric indices was also performed. RESULTS: Among 69 patients (98 metastases, unique 58.0%, lung-only 82.6%), there was no imbalance of deviations between the trial arms. Median follow-up was 55.3 months. Median overall survival was 61.7 months (IQR, 41.1-Not reached) in patients (N = 19) without deviation, 50.9 months (IQR, 32.1-Not reached) in patients (N = 36) with at least a minor deviation, and 20.9 months (IQR, 12.1-81.7) in those (N = 12) with at least a major deviation. Among patients with a major deviation, the conformity number (P < .001) and modified gradient index (P = .01) deviated more from their optimal values. CONCLUSIONS: Deviations from protocol by ICR were evenly distributed between arms. Major deviations were mostly related to the cumulative size of the oligometastases and OAR proximity and were associated with poorer survival in this SABR trial.