Spine stereotactic radiosurgery (SSRS) offers high rates of local control in a critical anatomic area by delivering precise, ablative doses of radiation for treatment of spine metastases. However, the dose tolerance of the spinal cord (SC) after SSRS with relation to radiation myelopathy (RM) is not well-described.We reviewed patients who underwent single fraction, de novo SSRS from 2012-2017 and received12 Gy Dmax to the SC, defined using MRI-CT fusion without PRV expansion. The standard SC constraint was D0.01cc ≤ 12 Gy. Local control was estimated with the Kaplan-Meier method. Bayesian analysis was used to compute posterior probabilities for RM.A total of 146 SSRS treatments among 132 patients were included. The median SC Dmax was 12.6 Gy (range, 12.1-17.1 Gy). The SC Dmax was12 and13 Gy for 109 (75%) treatments, ≥13 and14 Gy for 28 (19%) treatments, and ≥14 Gy for 9 (6%) treatments. The 1-year local control rate was 94%. With a median follow-up time of 42 months, there were zero (0) RM events observed. Assuming a prior 4.3% risk of RM, the true rate of RM for SC Dmax of ≤14 Gy was computed as1% with 98% probability.In one of the largest series of patients treated with single fraction, de novo SSRS, there were no cases of RM observed with a median follow-up of 42 months. These data support safe relaxation of MRI-defined SC dose up to D0.01cc ≤ 12 Gy, which corresponds to1% risk of RM.
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