Curative-Intent Cryoablation for Bone Metastases: Study Protocol for BOREALIS, a Prospective Multicentre Observational Investigation

PURPOSE: Cryoablation (CA) is a well-established, safe and effective palliative treatment for painful bone metastases (BM). Data exploring its curative potential (i.e. radical clearing of the target lesion) is however very scarce, mostly coming from small retrospective studies. The BOne metastases REgistry for patients undergoing cryoAbLation with curatIve purpoSe (BOREALIS) aims to address this shortcoming by collecting prospective, multicentric real-world data on the outcomes of curative-intent CA for bone metastases. METHODS: BOREALIS is an observational cohort study designed to follow for 1 year up to 50 participants with BM (< 5 cm) from oligometastatic or bone oligo-progressive disease treated with curative-intent CA. The primary outcome measure is the 12-months local progression-free survival rate, assessed per lesion using the newly developed BOREALIS tumour response criteria. Secondary outcome measures include complete response rate, fracture-free, progression-free, and overall survival; safety; patient-reported pain; and health-related quality of life (HRQoL). The study will also evaluate the feasibility of a comprehensive procedure integrating, when indicated, biopsy and bone consolidation within the same CA session. Results will be reported using mainly descriptive statistics. Patient enrolment opened in April 2025. CONCLUSION: BOREALIS intends to provide insights into the effectiveness of curative-intent CA for BM to inform the development of future clinical guidelines. BOREALIS is registered on ClinicalTrial.gov (NCT06480955).