Development and validation of a recursive partitioning analysis–based pretreatment decision-making tool identifying ideal candidates for spine stereotactic body radiation therapy

Background: This study was aimed at developing and validating a decision-making tool predictive of overall survival (OS) for patients receiving stereotactic body radiation therapy (SBRT) for spinal metastases. Methods: Three hundred sixty-one patients at one institution were used for the training set, and 182 at a second institution were used for external validation. Treatments most commonly involved one or three fractions of spine SBRT. Exclusion criteria included proton therapy and benign histologies. Results: The final model consisted of the following variables and scores: Spinal Instability Neoplastic Score (SINS) ≥ 6 (1), time from primary diagnosis < 21 months (1), Eastern Cooperative Oncology Group (ECOG) performance status = 1 (1) or ECOG performance status > 1 (2), and >1 organ system involved (1). Each variable was an independent predictor of OS (p <.001), and each 1-point increase in the score was associated with a hazard ratio of 2.01 (95% confidence interval [CI], 1.79–2.25; p <.0001). The concordance value was 0.75 (95% CI, 0.71–0.78). The scores were discretized into three groups—favorable (score = 0–1), intermediate (score = 2), and poor survival (score = 3–5)—with 2-year OS rates of 84% (95% CI, 79%–90%), 46% (95% CI, 36%–59%), and 21% (95% CI, 14%–32%), respectively (p <.0001 for each). In the external validation set (182 patients), the score was also predictive of OS (p <.0001). Increasing SINS<zaq;6> was predictive of decreased OS as a continuous variable (p <.0001). Conclusions: This novel score is proposed as a decision-making tool to help to optimize patient selection for spine SBRT. SINS may be an independent predictor of OS.