Deviation from consensus contouring guidelines predicts inferior local control after spine stereotactic body radiotherapy

Background and purpose: To analyze the impact of target delineation on local control (LC) after stereotactic body radiotherapy (SBRT) for spine metastasis. Materials and methods: Patients with de novo metastasis of the spine treated with SBRT, excluding those with prostate or hematologic malignancies, were retrospectively reviewed. Deviations from consensus contouring guidelines included incomplete coverage of involved vertebral compartments, omission of adjacent compartments, or unnecessary circumferential coverage. Univariable and multivariable Cox proportional hazard analyses were performed using death as a competing risk. Results: 283 patients with 360 discrete lesions were included with a median follow up of 14.6 months (range 1.2–131.3). The prescription dose was 24–27 Gy in 2–3 fractions for the majority of lesions. Median survival after SBRT was 18.3 months (95 % confidence interval [CI]: 14.8–22.8). The 1 and 2-year local control (LC) rates were 81.1 % (95 % CI: 75.5–85.6 %) and 70.6 % (95 % CI: 63.2–76.8 %), respectively. In total, 60 deviations (16.7 %) from consensus contouring guidelines were identified. Deviation from guidelines was associated with inferior LC (1-year LC 63.0 % vs 85.5 %, p < 0.001). Gastrointestinal primary, epidural extension, and paraspinal extension were all associated with inferior LC on univariable analyses. After adjusting for confounding factors, deviation from guidelines was the strongest predictor of inferior LC (HR 3.52, 95 % CI: 2.11–5.86, p < 0.001). Among guideline-compliant treatments, progressions were mainly in field (61 %) and/or epidural (49 %), while marginal (42 %) and/or epidural progressions (58 %) were most common for those with deviations. Conclusions: Adherence to consensus contouring guidelines for spine SBRT is associated with superior LC and fewer marginal misses.