30 GY IN 4 STEREOTACTIC BODY RADIOTHERAPY (SBRT) FRACTIONS FOR COMPLEX SPINAL METASTASES: MATURE OUTCOMES SUPPORTING THIS NOVEL REGIMEN

We designed a 30 Gy in 4 fractions stereotactic body radiotherapy (SBRT) protocol, as an alternative option to our standard 2-fraction approach, for primarily large volume, multilevel, or previously radiated spinal metastases. We report imaging-based outcomes of this novel fractionation scheme. MATERIALS AND METHODS: Retrospective analysis of all patients who underwent 30 Gy/4 fractions from 2010-2021 identified from an institutional database. Primary outcomes were magnetic resonance-based vertebral compression fracture (VCF) and local failure (LF). RESULTS: We reviewed 245 treated segments in 116 patients. The median number of consecutive segments within the treatment volume was 2 (range, 1-6) and clinical target volume (CTV) was 126.2 cc (range, 10.4-863.5). Fifty-four percent had received at least one prior course of radiotherapy. The cumulative incidence of LF was 10.7% (95% CI 7.1-15.2) at 1 year and 16% (95% CI 11.5-21.2) at 2 years. The cumulative incidence of VCF was 7.3% (95% CI 4.4-11.2) at 1 and 11.2% (95% CI 7.5-15.8) at 2 years. On multivariable analysis, age ≥68 years (p=0.038), CTV volume ≥72 cc (p=0.021), and no prior surgery (p=0.021) predicted for VCF. The risk of VCF for CTV volumes <72 cc/≥72 cc was 1.8%/14.6% at 2 years. No case of radiation-induced myelopathy was observed. CONCLUSION: 30 Gy in 4 fractions was safe and efficacious despite a population at increased risk of toxicity. The lower risk of VCF in previously stabilized segments highlights the potential for a multi-modal treatment approach for complex metastases, especially for those with CTV volume ≥72 cc.