In patients with oligometastatic disease (OMD) treated with stereotactic body radiation therapy (SBRT), those who develop brain metastases (BrM) may have poor outcomes. We aimed to investigate variables associated with BrM development in this population.

Patients with ≤ 5 extracranial metastases from solid tumors treated with SBRT from 2008 to 2016 at Sunnybrook Odette Cancer Centre were included. We investigated the association between covariates and CIBrM (cumulative incidence of BrM) using Fine-Gray analysis, and progression-free survival (PFS) and overall survival (OS) using Cox regression. We investigated the association between extracranial progression and CIBrM using time-based conditional analysis.

Among 404 patients, the most common primary sites were lung, colorectal, prostate, breast and kidney. Median follow-up was 49 months. Median PFS was 25 months. Median OS was 70 months. 58 patients developed BrM, and 5-year CIBrM was 16%. On multivariable analysis, number of extracranial metastases, location of metastases, total planning target volume (PTV), and time from primary diagnosis to OMD were not associated with CIBrM, although several of these variables were associated with extracranial PFS and OS. Primary site was associated with CIBrM, with colorectal and prostate cancer associated with lower CIBrM compared to lung cancer. Widespread extracranial progression (≥ 5 sites) within 24, 36, 48 and 60 months of OMD diagnosis was independently associated with higher CIBrM.

In patients with OMD treated with SBRT, baseline variables related to extracranial disease burden and distribution were not associated with BrM development, while primary site and widespread extracranial progression were associated with BrM development.