Breast cancer spine metastases treated with stereotactic body radiation therapy: patient outcomes and predictors.

Background/purpose: Spine stereotactic body radiation therapy (SBRT) is increasingly utilized for oligometastatic and symptomatic breast cancer spinal metastases (BCSM), yet primary site-specific outcomes remain lacking. This study evaluates outcomes of SBRT for BCSM, focusing on predictors of local failure (LF), vertebral compression fraction (VCF) and overall survival (OS).

Materials/methods: We retrospectively analyzed 409 BCSM in 168 patients treated with SBRT between 2008 and 2022. Receptor status was grouped based on ER+/Her2-, HER2+, and ER-/HER2-. Follow-up included full-spine magnetic resonance imaging (MRI) and clinical assessment every 3-6 months post-SBRT. The primary endpoint was radiological LF, and secondary endpoints were OS and VCF.

Results: Median follow-up was 33 months (range, 3.3-123 months), most were ECOG 0-1 (95%), neurologically intact (94%), polymetastatic (> 5 metastases, 45%), and ER+/HER2- (79%). Of 409 segments, most had no prior radiation or surgery (76%), were SINS stable (60%) and treated with 24-28 Gy/2 fractions (73%). Five-year LF, OS and VCF rates were 14%, 45% and 11%, respectively. On multivariable analyses, lower LF rates were associated with < 2 lines of prior systemic therapy, low/no grade epidural disease, and greater equivalent dose in 2 Gy fractions (α/β = 2) to the spinal cord/thecal sac (HR = 0.97 per 1 Gy increase, 95% CI: 0.949-0.995, p = 0.019). ER-/HER2-, liver and lung metastases, and > 2 systemic therapy lines predicted worse OS. Baseline VCF and deformity were associated with a higher VCF risk.

Conclusion: Treatment intensification in those heavily systemically pretreated or with high-grade epidural disease may optimize long-term LF rates independent of molecular status.