Radiation Myelitis Risk After Hypofractionated Spine Stereotactic Body Radiation Therapy.

Importance: Stereotactic body radiation therapy (SBRT) for spinal metastases improves symptomatic outcomes and local control compared to conventional radiotherapy. Treatment failure most often occurs within the epidural space, where dose is constrained by the risk of radiation myelitis (RM). Current constraints designed to prevent RM after spine SBRT are derived from limited data. Objective: To characterize the risk of RM after spine SBRT and to update the dosimetric constraints for preventing it. Design, Setting, and Participants: This cohort study was conducted in a single tertiary cancer care center with patients treated for spinal metastases from 2014 to 2023. All included participants had undergone spine SBRT, had a minimum of 1-month follow-up with magnetic resonance imaging (MRI), a maximal cord dose to a voxel (Dmax) greater than 0 Gy, and no overlapping prior radiotherapy. In all, 2051 patients received SBRT to 2835 spinal metastases (levels C1-L2) during the study period. Exposures: Three-fraction spine SBRT to a prescription dose of 27 to 36 Gy. Main Outcomes and Measures: RM defined as radiographic evidence of spinal cord injury in the treatment field, classified as grade (G) 1 to G4 or G3 to G4 per the Common Terminology Criteria for Adverse Events, version 5.0. Multiple dosimetric parameters of the true spinal cord structure were assessed for an association with risk of RM to determine the important covariates associated with this toxicity. Results: The analysis included 1423 patients (mean [SD] age, 61.6 [12.9] years; 695 [48.8%] females and 728 [51.1%] males) who received SBRT for 1904 spinal metastases. Among them, 30 cases of RM were identified, 19 of which were classified as G3 to G4. Two years after SBRT, the rate of G1 to G4 RM was 1.8% (95% CI, 1.2%-2.5%) and the rate of G3 to G4 RM was 1.1% (95% CI, 0.7%-1.7%). The minimum dose to the 0.1 cm3 of spinal cord receiving the greatest dose (D0.1cc) was the most important covariate on univariable cause-specific hazards regression for RM (for G3 to G4: hazard ratio, 2.14; 95% CI, 1.68-2.72; P < .001). A true cord D0.1cc of 19.1 Gy and Dmax of 20.8 Gy estimated a 1.0% risk (95% CI, 0.3%-1.6% and 0.4%-1.6%, respectively) of G3 to G4 RM 2 years after SBRT. Conclusions and Relevance: The findings of this cohort study indicate that a cord (myelogram or MRI-derived) D0.1cc constraint of 19.1 Gy and a Dmax constraint of 20.8 Gy correspond with a 1.0% risk of G3 to G4 RM at 2 years.