Histologic and Dose Predictors of Local Failure for Non-Spine Bone Metastases Treated with Stereotactic Body Radiation Therapy

Objectives: : Stereotactic body radiation therapy (SBRT) is increasingly utilized to achieve pain relief and durable local control of bone metastases. However, real-world evidence regarding the efficacy of SBRT specifically for non-spine bone metastases (NSBM) remains limited. We investigate factors that impact local failure following SBRT for NSBM, including histology and biologically effective dose (BED), in one of the largest cohorts to date with extended follow-up time.

Methods: : We conducted a retrospective review of patients with one or more metastatic lesions in non-spine bone treated with SBRT between 2019 and 2024. Data collected included primary tumor histology, total prescription dose, number of fractions, and treatment modality. BED10 was calculated to facilitate comparison of prescriptions. Local failure (LF) was defined as disease progression at the prior treatment site, as determined by radiologist reports based on CT, NM Bone Scan MRI, and/or PET. Age at treatment initiation and time to LF were calculated based on the SBRT start date, with the last follow-up defined as the most recent healthcare provider visit or imaging appointment. Patients without follow-up after completing SBRT were excluded. Kaplan-Meier curves were used to demonstrate local control rates over time. The Wilcoxon rank-sum test compared LF rates based on histologic and dosimetric factors.

Results: : A total of 259 SBRT courses in 194 patients were included in the analysis. The median age at SBRT was 66 years (range: 15-94). The most common histologies included prostate adenocarcinoma (48.3%), non-small cell lung cancer (10.8%), renal cell carcinoma (9.7%), sarcoma (5.8%), breast adenocarcinoma (5.0%), thyroid carcinoma (5.0%), liver or bile duct carcinoma (4.6%), and urothelial carcinoma (3.5%). The median time to last follow-up imaging and last appointment was 17.8 months (range: 0.5-66 months) and 21.8 months (0.5-67 months), respectively. There were 44 local failures (12.7%), with a median time to failure of 6 months (range: 0.5-52.0 months). Median total dose, number of fractions, and BED10 were 3000 cGy (range: 600-4750 cGy), 5 fractions (range: 1-6), and 5557.5 cGy (range: 960-9118 cGy), respectively. Neither total dose nor BED10 differed between SBRT courses that resulted in local failure and those that did not (p=0.93, p=0.34). Hepatobiliary histology was associated with LF, and renal histology reached near significance (p=0.05, p=0.07). Despite receiving significantly higher median total dose (3500 cGy, p=0.003) and BED10 (6457.5 cGy, p=0.01), renal tumors accounted for 18.2% of all LFs while representing only 9.6% of total treatments.

Conclusion(s):: In one of the largest studies to date, with extended follow-up time, renal and hepatobiliary tumors are found to experience disproportionately higher rates of LF in patients with non-spine bone metastasis, even when treated with escalated radiation doses. Radioresistant tumor histologies may benefit from further dose escalation and optimization of SBRT protocols to achieve durable local control.