Oligometastatic Disease State Is Associated with Improved Local Control in Patients Undergoing Three or Five Fraction Spine Stereotactic Body Radiotherapy

Background The objective of our study was to describe the local control (LC) outcomes with 3- or 5-fraction stereotactic body radiotherapy (SBRT) to the spine in patients with oligometastatic (≤5 systemic metastases) versus polymetastatic disease (>5 metastases). Methods We retrospectively reviewed the outcomes of patients who had undergone SBRT for spinal metastases. No patients had undergone previous surgical intervention or had spinal cord compression. All patients were treated with 3-fraction (median dose, 27 Gy; range, 24–30 Gy) or 5-fraction (median dose, 35 Gy; range, 25–40 Gy) SBRT. The Kaplan-Meier method and Spine Response Assessment in Neuro-Oncology criteria were used to determine LC. Results We included 61 patients with a total of 72 distinct SBRT targets who had been treated from August 2007 to June 2017. The median follow-up period was 13.58 months. We treated 20 targets and 52 targets with 3 and 5 fractions, respectively. Thirteen patients (18.1%) had undergone previous RT to the SBRT area. Twenty patients (35% of the distinct SBRT targets) had an oligometastatic disease state. The 1-year LC rate was 83% for the entire cohort. On univariable analysis, polymetastases (1-year LC, 73.8% vs. 100%; P  = 0.07) showed a trend toward worse LC. On multivariable analysis, patients with an oligometastatic state (hazard ratio, 0.21; P  = 0.04) had improved LC. Conclusions Our study was hypothesis-generating in that patients with an oligometastatic disease state appear to have improved LC after SBRT, suggesting a biological advantage exists with local therapy for this group of patients not seen for patients with polymetastatic disease.