Purpose: This prospective observational study aims to investigate which proportion of patients with bone metastases referred for External Beam Radiation Therapy (EBRT) would be able to undergo Magnetic Resonance-guided High Intensity Focused Ultrasound (MR-HIFU) as alternative to EBRT, and to examine reasons for ineligibility.

Materials and Methods: Adult patients with nonspinal bone metastases referred to four radiotherapy departments were included. Local, multidisciplinary teams assessed which patients would be eligible for MR-HIFU. The main reason(s) for ineligibility were categorized as patient-related or lesion-related. A random subsample of 30 ineligible patients were analyzed in detail to identify all reasons of ineligibility.

Results: Overall, 57 of 741 (8%) nonspinal bone lesions were eligible for MR-HIFU as alternative to EBRT. In total, 153 lesions (21%) in 130 patients were ineligible because of patient-related factors, including curative treatment intent for oligo-metastatic disease (10%), and poor performance status (8%). Of the remaining 588 bone metastases in 526 patients, 531 lesions (470 patients) were ineligible because of lesion-related factors, including ‘lesion too extensive/advanced’ (29%), ‘(impending) pathological fracture’ (15%), ‘no moderate/severe pain from target lesion’ (11%). Proportion of ineligibility varied between centers from 70% to 96%, which was mainly attributable to differences in patient-related factors. Within the random subsample of 30 ineligible patients, 27 patients had multiple reasons for ineligibility.

Conclusion: A small proportion of patients, referred for EBRT of bone metastases, would be eligible to undergo MR-HIFU as alternative palliation option. Taken together, patients presenting with small, localized lesions in nonspinal regions, primarily seeking pain relief without additional treatment goals are the most promising candidates for this therapy. These factors could be used to triage patients eligible for MR-HIFU, thereby reducing unnecessary screening efforts, enhancing patient selection, and ultimately improve patient management strategies by optimizing the use of MR-HIFU as a treatment option.