Prognostic factors for local failure and overall survival in patients with epidural disease at the cauda equina following stereotactic body radiotherapy: a clinical, anatomic and dosimetric analysis.

Purpose: The relationship between spine SBRT outcomes and the extent of epidural cauda equina compression (ECEC) by malignant epidural disease has yet to be reported. Our objective was to determine clinical, anatomic and dosimetric factors that predict for local failure (LF) and overall survival (OS) specific to ECEC.

Methods: Consecutive patients with ECEC treated with spine SBRT were retrospectively reviewed. ECEC parameters including anatomic measurements, lumbar stenosis grading, anatomic disease position and various dosimetric factors, were analyzed for their prognostic utility. Covariates with a p-value ≤ 0.20 on univariate analysis were selected for multivariable analysis (MVA), and those statistically significant (p < 0.05) were included in the final model.

Results: Ninety-four spinal segments (79 patients) with ECEC were identified, 69 (73%) were intact (non-operated) and 25 (27%) post-operative. MVA identified a significantly lower risk of LF with systemic-therapy prior to SBRT (HR 0.164, p = 0.0005), ECEC encompassing ≤ 1/3 of the circumference of the spinal canal (HR 0.216, p = 0.0194) and an epidural disease volume < 0.45cm³ (HR 0.000, p < 0.001). In the intact cohort, a greater volume of epidural disease receiving 50 Gy (V50Gy) as an Equivalent Dose in 2 Gy Fractions (EQD2) using an α/β = 10 was associated with a lower LF (HR 0.909, p = 0.0122). Oligometastatic disease and ECEC limited to a single spinal-level were significant prognostic factors for OS in both cohorts.

Conclusion: The volume and circumferential extent of epidural disease within the spinal canal are important factors for LF in ECEC treated by SBRT, suggesting a possible therapeutic role for surgical cytoreduction to enable optimal treatment planning.