2648 - 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

S. Zayed¹, M. E. Ruschin¹, E. Atenafu², D. Dinakaran¹, H. Chen¹, J. Detsky¹, H. Soliman¹, S. D. Myrehaug¹, A. Sahgal¹, and C. L. Tseng¹; ¹Department of Radiation Oncology, Odette Cancer Centre, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, ON, Canada, ²Department of Biostatistics, University Health Network, University of Toronto, Toronto, ON, Canada

Purpose/Objective(s): The relationship between spine SBRT outcomes and the extent of malignant epidural disease (MED) compression of the cauda equina has yet to be reported. Our objective was to determine clinical, anatomic and dosimetric predictive factors for local failure (LF) and overall survival (OS) in a cohort of patients with spine metastases and epidural disease at the level of the cauda equina. Materials/

Methods: Consecutive patients with cauda equina MED treated with SBRT, between January 1, 2008 and July 1, 2023, were identified and retrospectively reviewed from a prospectively maintained institutional database. MED parameters including linear dimensions, surface area, and volume ratios relative to the spinal canal, lumbar stenosis grading systems, clock position and various dosimetric factors were analyzed for their predictive value for LF post-SBRT and OS. 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-five individual spinal segments (79 patients) with cauda equina MED were identified, of which 69 (73%) were intact and 26 (27%) post-op. Forty-one (43.2%) received 24 Gy in 2 fractions (fr), 27 (28.4%) received 30 Gy in 4 fr, 22 (23.2%) received 28 Gy in 2 fr and 5 (5.2%) received 30 Gy in 5 fr. Median follow-up and median time to LF were 16.3 (IQR 6.7-32.0) months and 4.7 (IQR 2.2-12.1) months, respectively. The cumulative incidence of LF at 6, 12, and 24 months was 12.7%, 12.7% and 17.4% respectively. OS at 6, 12, and 24 months was 79.6%, 58.4% and 40.7% respectively. In the intact cohort only (69/95), factors predictive of a higher risk of LF on MVA were chemotherapy naive (HR 8.197, 95% CI 2.198-30.303, P=0.0017), MED greater than one third of the circumference of the spinal canal (HR 9.632, 95% CI 1.863-49.806, P=0.0069), and lower V50Gy Equivalent Dose in 2Gy Fractions (EQD2) to the MED volume (HR 1.049, 95% CI 1.018-1.081, P=0.0014). These factors retained significance on MVA for LF when combined with the post-op (26/95) cohort (P=0.0047, 0.0272, 0.0024, respectively). In the intact cohort only, MVA identified oligometastatic disease (HR 0.381, 95% CI 0.190-0.764, P=0.0065) and MED limited to a single spinal level (HR 0.405, 95% CI 0.182-0.902, P=0.0269) as prognostic for OS. Oligometastatic disease and MED limited to a single spinal level also retained significance as predictors for OS on MVA when combined with the post-op (26/95) cohort (P<0.0001, 0.0182, respectively).
Conclusion: We identified chemotherapy naive, MED encompassing more than one third of the circumference of the spinal canal, and lower V50Gy EQD2 to the epidural disease volume, as novel predictive factors for higher LF post-SBRT for MED at the cauda equina. Consideration for more aggressive management with decompressive surgery in those with MED spanning greater than a third of the spinal canal circumference and dose escalation to the thecal sac organ-at-risk may improve outcomes.