Role of epidural disease in local control of spinal metastases treated with stereotactic body radiation therapy

Spinal metastases can be contained in the bone or have epidural spread. Whether the extent of epidural involvement changes tumor response to therapy is unknown. The decision of when to treat disease progression with focal radiation therapy with or without surgery vs. systemic therapy is debated. The present study compared outcomes and local tumor control after stereotactic body radiation therapy (SBRT) between patients with spine metastases localized to the bone (Bilsky 0) vs. patients with mild epidural spread (Bilsky 1). A retrospective analysis of a prospectively maintained database of adult oncological patients who underwent SBRT to the spine at a single, large, tertiary care facility from August 2010 to January 2021 was performed. Patients with Bilsky grades 1a, 1b and 1c were grouped and compared. Approximately half (53.7%) of the 255 patients identified had Bilsky grade 1 epidural disease. Of the 311 spine treatment sites, 86 (27.7%) had a radiosensitive histology, 116 (37.3%) had intermediate radiosensitivity and 109 (35.0%) had a radioresistant histology. Patients with Bilsky grade 1 were more predisposed to receive surgery followed by SBRT compared with those with Bilsky grade 0 (21.0% vs. 6.3%; P=0.0002). Patients with Bilsky grade 0 compression had 92.0% local control at 12 months and 85.8% local control at 24 months; patients with Bilsky grade 1 compression had 85.6% local control at 12 months and 77.6% local control at 24 months. Biologically effective dose and infield progression between patients presenting with Bilsky grade 0 and 1 compression were not statistically different. Local control rates did not differ significantly between Bilsky grade 0 and grade 1 patients following treatment with spinal SBRT. However, patients with grade 1 disease were more likely to receive surgery before SBRT. Overall, evidence indicates that patients may benefit from treatment with SBRT before epidural disease progresses to requiring separation surgery.