Local failure and vertebral body fracture risk using multifraction stereotactic body radiation therapy for spine metastases

PURPOSE: Single-fraction radiation surgery for spine metastases is highly effective. However, a high rate (20-39%) of vertebral body fracture (VBF) has been associated with large, single-fraction doses. We report our experience using multifraction stereotactic body radiation therapy (SBRT). METHODS AND MATERIALS: All patients who were treated with multifraction SBRT for spine metastases at our institution between 2009 and 2017 were retrospectively analyzed. SBRT was delivered in 2 to 5 fractions using the Cyberknife System (Accuray, Sunnyvale, CA). Patients were followed clinically and with magnetic resonance imaging every 3 to 6 months. Local control, complications (including VBF), and overall survival were evaluated. Patient, disease, and treatment variables were analyzed for a statistical association with outcomes. RESULTS: A total of 83 patients were treated to 98 spine lesions with a median follow-up of 7.6 months. Histologies included non-small cell lung cancer (NSCLC; 24%), renal cell carcinoma (RCC; 18%), and breast cancer (12%). Surgery or vertebroplasty were performed before SBRT in 21% of cases. Patients received a median SBRT dose of 24 Gy in a median of 3 fractions. Local control was 93% at 6 months and 84% at 1 year. Higher prescribed dose, higher biologic effective dose, higher minimum dose to 90% of the planning target volume, tumor histology, and smaller tumor volume predicted improved local control. The cumulative dose was 23 Gy versus 26 Gy for patients with and without failure (P = .02), higher biologic effective dose 39 Gy versus 46 Gy, (P = .01), and higher minimum dose to 90% of the planning target volume 23 Gy versus 26 Gy (P = .03). VBF occurred in 4.2% of all cases and 5.3% of those without surgery or vertebroplasty prior to SBRT. Only preexisting VBF predicted risk of post-SBRT VBF (P < .01). CONCLUSIONS: Multifraction SBRT results in a high local control rate for metastatic spinal disease with a low VBF rate, which suggests a favorable therapeutic ratio compared with single-fraction SBRT.