PURPOSE: A Benchmark Case (BC) was performed as part of the quality assurance process of the randomized phase 2 GORTEC 2014-14 OMET study, testing the possibility of multisite stereotactic radiation therapy (SBRT) alone in oligometastatic head and neck squamous cell carcinoma (HNSCC) as an alternative to systemic treatment and SBRT. MATERIAL AND METHODS: Compliance of the investigating centers with the prescription, delineation, planning and evaluation recommendations available in the research protocol was assessed. In addition, classical dosimetric analysis was supplemented by quantitative geometric analysis using conformation indices. RESULTS: Twenty centers participated in the BC analysis. Among them, four major deviations (MaD) were reported in two centers. Two (10%) centers in MaD had omitted the satellite tumor nodule and secondarily validated after revision. Their respective DICE indexes were 0.37 and 0 and use of extracranial SBRT devices suboptimal There were significant residual heterogeneities between participating centers, including those with a similar SBRT equipment, with impact of plan quality using standard indicators and geometric indices. CONCLUSION: A priori QA using a BC conditioning the participation of the clinical investigation centers showed deviations from good SBRT practice and led to the exclusion of one out of the twenty participating centers. The majority of centers have demonstrated rigorous compliance with the research protocol. The use of quality indexes adds a complementary approach to improve assessment of plan quality.