Stereotactic Body Radiation Therapy for Oligometastatic Disease: A Multi-Institutional Retrospective Survival Analysis

Abstract

Stereotactic body radiation therapy (SBRT) is an emerging treatment for patients with extracranial oligometastatic disease, with multiple studies demonstrating promising rates of overall survival (OS). However, selection of patients who will benefit most remains a challenge. This multi-institutional retrospective analysis sought to identify clinical and demographic factors associated with improved OS in patients treated with SBRT for oligometastatic disease. We reviewed a multi-institutional database of patients treated with SBRT to all sites of metastasis with curative intent. Patients with all primary tumor types and histologies were included. Entry criteria included patients with ≤5 extracranial metastatic lesions in ≤3 sites, minimum 6-month follow-up, curative intent treatment of primary disease, and histologic diagnosis of primary. Factors associated with OS were examined using univariate Cox proportional hazards model. We then used multivariate Cox regression including covariates reaching p<0.10 on univariate analysis to identify factors that remained associated with improved OS. A total of 371 patients treated from 2006 to 2015 met entry criteria. Median follow-up was 1.5 years (range, 0.5 – 9). Median OS was 3.1 years (95% CI, 2.8 – 3.5), and 2-year OS was 67.3% (95% CI, 61.7% - 72.3%). On univariate analysis, Karnofsky Performance Score (KPS) > 80 (HR 0.42, p<0.001), diagnosis of SBRT-treated metastasis >60 days after primary diagnosis (HR 0.57, p=0.006), prior local therapy for SBRT-treated metastasis including surgery (67%), conventional radiotherapy (30%), or radiofrequency ablation (3%) (HR 0.49, p=0.02), time from primary diagnosis to SBRT-treated metastasis (HR 0.996, p=0.04), and colorectal, breast, prostate, and kidney primary compared to lung primary (p<0.05) were associated with improved OS. Solitary metastasis also showed a trend for improved OS (HR 0.69, p=0.08). On multivariate analysis, KPS > 80 (HR 0.44, p=0.001), solitary metastasis (HR 0.53, p=0.02), prior local therapy (HR 0.45, p=0.03), and colorectal, breast, and kidney primary compared to lung primary (p<0.01) were associated with improved OS. Patients with a total of 0-1, 2, or ≥3 favorable predictors from the multivariate model had 2-year OS of 42.6% (95% CI, 26.3% - 57.9%), 60.8% (49.7% - 70.3%), and 83.3% (72.5% - 90.2%), respectively. Using a large multi-institutional cohort of patients who received SBRT for extracranial oligometastatic disease, we found that KPS>80, solitary metastasis, prior local therapy, and primary tumor organ were associated with improved OS. These variables were used to create a prognostic model that predicts the longest survival after metastasis-directed SBRT. Future validation studies are needed.