Magnetic Resonance Imaging Frequency After Stereotactic Body Radiation Therapy for Spine Metastases.

ABSTRACT

PURPOSE: Stereotactic body radiation therapy (SBRT) is increasingly being used to treat spine metastases. Current post-SBRT imaging surveillance strategies in this patient population may benefit from a more data-driven and personalized approach. The objective of this study was to develop risk-stratified post-SBRT magnetic resonance imaging (MRI) surveillance strategies using quantitative methods. METHODS AND MATERIALS Adult patients with bony spine metastases treated with SBRT between 2008 and 2021 and who had at least 2 follow-up spine MRIs were reviewed retrospectively. A recursive partitioning analysis model was developed to separate patients into different risk categories for post-SBRT progression anywhere within the spine. Imaging intervals were derived for each risk category using parametric survival regression based on multiple expected spine progression rates per scan. RESULTS: A total of 446 patients and 1039 vertebral segments were included. Cumulative incidence of spine progression was 19.2% at 1 year, 26.7% at 2 years, and 35.3% at 4 years. The internally validated risk stratification model was able to divide patients into 3 risk categories based on epidural disease, paraspinal disease, and Spinal Instability Neoplastic Score category. The 4-year risk of spine progression was 23.4%, 39.0%, and 51.8%, respectively, for the low-, intermediate-, and high-risk groups. Using an expected per-scan spine progression rate of 3.75%, the low-risk group would require follow-up scans every 6.0 months (95% CI, 4.9-7.6) and the intermediate-risk group would require surveillance every 3.1 months (95% CI, 2.6-3.7). At an expected spine progression rate of 5%, the high-risk group would require surveillance every 1.3 months (95% CI, 1.1-1.6) during the first 13.2 months after SBRT and every 5.9 months thereafter (95% CI, 2.8-12.3). CONCLUSIONS: Data-driven follow-up MRI surveillance intervals at a range of expected spine progression rates have been determined for patients at different risks of spine progression based on an internally validated, single-institution risk stratification model.