2477 - Repeat Stereotactic Radiosurgery in Patients with Spinal Metastases Recurrent Following Initial Radiosurgery

J. Abi Jaoude¹, C. F. Chuang¹, D. Klebaner¹, A. Pratapneni¹, T. Kollipara¹, K. A. Kumar Jr², D. K. Fujimoto¹, M. G. Mendoza¹, D. Park³, A. Persad³, Y. Hori³, E. Rahimy¹, I. C. Gibbs¹, S. D. Chang³, E. Pollom¹, and S. G. Soltys¹; ¹Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA, ²Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, ³Department of Neurosurgery, Stanford University School of Medicine, Stanford, CA

Purpose/Objective(s): Stereotactic radiosurgery (SRS) is the standard of care treatment in selected patients with spinal metastases. However, limited data exist on the safety and efficacy of repeat SRS to the same spinal level that was previously treated with SRS. We hypothesized that multiple courses of SRS to the same spinal level can provide safe and effective local tumor control. Materials/

Methods: We conducted an IRB-approved retrospective review of patients treated with repeat SRS for tumors locally recurrent following initial SRS to the same spinal level. We estimated the cumulative incidence of local failure (LF) at 1- and 2-years from repeat SRS with death as competing risk. The total cumulative spinal cord physical dose and EQD2, with an a/b of 2 were calculated through deformable registration of the first and subsequent treatment plans using MIM.

Results: In total, 24 patients with 24 recurrent spinal metastases were treated with repeat SRS. The median time from initial to repeat SRS was 14.2 months (IQR 9.0-22.7 months). After repeat SRS, median follow up time was 19 months. The 1- and 2-year cumulative incidences of LF after repeat SRS were 10% (95% Confidence Interval (CI) 1.6%-28%) and 20% (95%CI 5.9%-40%), respectively. No patient developed radiation myelopathy. From our cohort, preliminary analyses of the cumulative DVH data in 10 patients have been completed. The median EQD2 for cord Dmax, D0.03, and D0.35 were 71.5 Gy2, 63.8 Gy2, and 56.8 Gy2 for the first course of SRS and 53.8 Gy2, 42.1 Gy2 and 35.9 Gy2 for the repeat course of SRS, respectively. The median cumulative doses for both courses were 113.5 Gy2 Dmax, 96.4 Gy2 D0.03, 57.3 Gy2 D0.35. With 50% forgiveness of dose from the first course of SRS, the median sum Dmax, D0.03, and D0.35 were 85.5 Gy2, 69.9 Gy2, and 46.2 Gy2. Of note, simple addition of the spinal cord values from the plans (with 50% forgiveness of dose from the first SRS course), as opposed to deformable registration of the actual plans, overestimated the cumulative spinal cord doses: median Dmax, D0.03, and D0.35 were 94.8 Gy2, 82.5 Gy2, and 52.9 Gy2 respectively.

Conclusion: With a 1-year local failure of 10%, repeat SRS for local progression at the same spinal level as previous SRS appears to be an effective treatment option for patients. With a median cumulative cord Dmax EQD2 of 113.5 Gy2, no myelopathy was seen. Analyses of spinal cord dosimetry with deformable registration for the entire cohort is ongoing.