Fluorodeoxyglucose-positron emission tomography (FDG-PET) scans may be able to detect early-stage non-small cell lung cancer (NSCLC) patients who are at high risk of treatment failure after stereotactic body radiation therapy (SBRT). A retrospective study examined patients with early-stage NSCLC who were ineligible for or refused surgery and were instead treated with SBRT. Patients with lower FDG-PET readings prior to SBRT treatment survived longer, and those whose FDG-PET readings changed more after SBRT were less likely to experience treatment failure. FDG-PET scans may therefore help identify which patients are at lower or higher risk of recurrence; high-risk patients may opt for additional treatment and/or more frequent surveillance after treatment. FDG-PET has shown similar predictive value in early-stage NSCLC treated with surgery.