Stereotactic Body Radiation Therapy for Non-Spine Bone Metastases: Local Control and Fracture Risk

Abstract

<h3>Purpose/Objective(s)</h3> Stereotactic Body Radiation Therapy (SBRT) is increasingly being utilized to treat non-spine bone metastases (NSBM) with both palliative and radical intent. There remains limited data on the long-term outcomes and toxicities and, therefore, we report mature results from our institutional database. <h3>Materials/Methods</h3> Patients with NSBM who were treated with SBRT were identified from 2011 to 2021. The primary endpoint was radiographic local failure (LF). Secondary endpoints included rate of in-field pathologic fracture (PF), overall survival (OS), and late (>3 months post-SBRT) grade ≥3 toxicity (Common Terminology Criteria for Adverse Events, version 5.0). Competing risks analysis was used to assess rates of LF and PF. OS was estimated using the Kaplan-Meier method. Univariable regression (UVR) and multivariable regression (MVR) were performed to investigate predictive factors for LF and PF. <h3>Results</h3> A total of 373 patients with 505 NSBM were included in this study. Primary indications for SBRT included oligometastatic disease (52.6%), pain relief (23.8%), and oligoprogression (19.7%). Median follow-up was 26.5 months (range: 3.0-128.3 months). The most common primary histologies included prostate (33.7%), RCC (20.7%), breast (16.9%), and lung (14.8%). The two most common NSBM sites were pelvic (42.3%) and rib (30.5%). Of all NSBM, 217 (42.7%) were lytic and 230 (45.3%) were symptomatic. The most common dose prescriptions were 35 Gy in 5 fractions (33.3%) and 24 Gy in 2 fractions (25.2%). The cumulative incidence of LF at 6, 12, and 24 months were 5.7%, 7.9%, and 12.6%, respectively. The cumulative incidence of PF at 6, 12, and 24 months were 3.8%, 6.1%, and 10.9%, respectively. There were 3 instances of late grade ≥3 toxicity: 1 grade 3 osteonecrosis, 1 grade 3 peripheral neuropathy, and 1 grade 5 bowel fistula. Median OS was 53.9 months (95% confidence interval: 45.3-63.2 months). Lytic NSBM (HR=2.20; p<0.01), symptomatic NSBM (HR=2.15; p<0.01), and those with a larger PTV (HR=1.01; p=0.04) were at higher risk of LF on MVR. Lytic lesions (HR=3.43; p<0.01), mixed (lytic/sclerotic) lesions (HR=2.70; p=0.04), and rib metastases (HR=2.68; p<0.01) were at a higher risk of PF on MVR. <h3>Conclusion</h3> SBRT is an effective modality to treat NSBM with high rates of radiographic local control with an acceptable rate of fracture. We identify predictors of both LF and PF that can serve to inform practice.