Stereotactic Body Radiotherapy for Spinal Metastases: Outcomes and Development of a Risk Assessment Score to Predict Local Tumor Progression

BACKGROUND AND OBJECTIVES: Stereotactic body radiotherapy (SBRT) has emerged as an effective treatment modality for spinal metastases. However, high-powered studies evaluating clinical and radiographic outcomes, and prognostic risk factors for local tumor progression (LTP), remain underreported. The objective of this study was to evaluate local tumor control, adverse radiation effects, pain response, and overall survival after SBRT for spinal metastases and to develop an internally derived LTP prediction tool to guide personalized patient management. METHODS: A prospectively maintained database of 936 SBRT treatments (600 patients) from 2001-2024 for spinal metastases at a quaternary referral center was analyzed. Single-fraction and multifraction SBRT with median prescribed doses of 16 Gy (IQR: 15-17) and 24 Gy (IQR: 24-27), respectively, were included. RESULTS: The median follow-up and overall survival were 8 months (IQR: 2-22) and 11 months (IQR: 3-28), respectively. There were 129 (13.8%) LTPs. Local tumor control rates at 1 year and 3 years were 84.2% (95% CI: 81.1%-87.3%) and 75.1% (95% CI: 70.8%-79.5%). An LTP risk scoring system was developed using high-fidelity machine learning models, with scores summed on a 0-15 scale using treatment characteristics (spinal instability neoplastic score >6 [4 points], lytic lesion [4 points], radiographic spinal misalignment [3 points], no prior chemotherapy [2 points], and polymetastatic disease [2 points]). Crude LTP incidences in low-risk (LTP score: 0-5), intermediate-risk (LTP score: 6-8), and high-risk groups (LTP score: 9-15) were 2.4%, 10.2%, and 35.5%, respectively. Stratified survival analyses demonstrated significant LTP differences between all risk groups (log-rank and Gray test, P < .001). Pain response or stability at 1, 3, and 6 months after SBRT was 94.4%, 90.6%, and 84.3%, respectively. The crude risk of grade II or III adverse radiation effects was 12.6%. CONCLUSION: This large clinical cohort investigation demonstrates that SBRT is safe and effective for spinal metastases. Risk stratification using clinical and radiographic variables may help inform patient selection to optimize outcomes.