Long Term Clinical Outcomes of Multi-Level Spinal SBRT for Vertebral Metastases

Currently, sSBRT is commonly limited to solitary, 2 contiguous or 3 non-contiguous vertebral levels because of concerns regarding setup accuracy, efficacy, and toxicity when 2-3 non-contiguous levels are treated. There is, however, a paucity of data regarding the clinical outcomes of multilevel sSBRT. LC and toxicity are particularly important endpoints to ensure that standard clinical target volume (CTV), planning target volume (PTV), and planning organ-at-risk volume (PRV) expansions are appropriate in the multilevel setting. We sought to compare efficacy and toxicities of single- vs multilevel (≥2-5) treatment in a cohort of patients with malignant spinal tumors. Radioresistant histologies, Bilsky grade 0 to 1 epidural spinal cord compression,5 and a life expectancy >6 mo. are treated by SBRT. Single level and Multi-level spinal SBRT are defined as sSBRT delivered to a malignant lesion spanning single and ≥2 contiguous vertebral levels. All patients underwent treatment rigid immobilization for C1-D4 targets / body fix immobilization for D5 and below targets. For intact cases, GTVs were delineated using the registered T1-weighted precontrast and postcontrast MRI sequences, with reference to additional diagnostic studies (PET and perfusion MRI). CTVs were expanded 2mm circumferentially to create PTVs, with the goal of≥95% PTV coverage with ≥100% of the prescription dose. For cord delineation done on either CT myelography / T2-weighted MRI fused to the planning CT with 1-mm cord PRV. The same PTV and PRV expansions were used regardless of surgical status or number of treated vertebral levels. All patients underwent multi-isocenter VMAT planning using dual arcs and treatment delivered on C-Arm Linac using 6 DOF robotics with pretreatment, Intrafraction and posttreatment imaging. We analyzed consecutive cases of sSBRT treated from 2013 to 2017. Time-to event outcomes for single-level and multilevel cases were compared using mixed effect Cox models and differences in toxicity rates were evaluated using linear mixed effect models. All models incorporate a patient-level random intercept to account for any within patient. There were 101 single-level and 84 multilevel sSBRT cases (2-5 continuous vertebral levels). One-year LC was 95% vs 85%, respectively. After adjusting for baseline covariates, dose delivered, and accounting for within-patient correlation, there was no significant difference in time to local failure (hazard ratio, HR 1.79 [0.59-5.4]; P = .30). Pain improved in 83.5% of the 139 initially symptomatic tumors. There were no significant differences in grade 2+ acute or late toxicities between single-level and multilevel sSBRT Multilevel sSBRT provides high rates of LC, similar to single-level treatment, without need for larger planning volume margins. Efforts to improve prognostication and case selection for multilevel sSBRT are warranted to ensure that the benefits of improved LC over palliative radiation are justified.