Standardising definitions, endpoints, and reporting parameters for clinical studies of stereotactic body radiation therapy in bone metastases: An international Delphi consensus

Background Stereotactic body radiation therapy (SBRT) is increasingly used for bone metastases, but inconsistent endpoint definitions and reporting hinder evidence synthesis and clinical application. This study aimed to establish international consensus recommendations for standardised endpoints, definitions, and reporting parameters in SBRT studies for bone metastases. Methods A systematic review of prospective SBRT studies (2014–2024) informed a three-round modified Delphi consensus process conducted from 2024 to 2025. Consensus was predefined as at least 75% agreement. Candidate items were refined through iterative online surveys, qualitative feedback, and a final prioritisation vote by an international multidisciplinary expert panel. Results Of 114 invited experts, 82 from 20 countries participated in at least one Delphi round. Review of 58 prospective studies showed substantial variability in endpoint definitions, pain and toxicity assessment, radiological response criteria, and timing of outcome evaluation. The panel endorsed 46 reporting items (41 required, 5 recommended) and prioritised core endpoint sets across three clinical contexts: oligometastatic, oligoprogressive, and asymptomatic high-risk bone metastases. Strong consensus was reached for definitions of vertebral compression fracture (92%), time to salvage local therapy (93%), duration of pain response (92%), and time to local progression (90%); pain flare achieved 89% agreement. A revised clinical response framework (C-BRAC), introducing a stable disease category, achieved 91% agreement and was recommended for exploratory use alongside existing criteria. Conclusions These recommendations provide a structured framework for designing and reporting SBRT studies in bone metastases and may improve consistency, comparability, and future guideline development.