BackgroundEarly-stage lung cancer, primarily treated with surgery, often occur in poor surgical candidates (impaired respiratory function, prior thoracic surgery, severe comorbidities). Stereotactic ablative radiotherapy (SABR) is a non-invasive alternative that provides comparable local control. This technique is particularly relevant for surgically resectable metachronous lung cancer, in patients unable to undergo surgery.. The objective of this study is to evaluate the clinical outcome of patients treated with SABR for stage I metachronous lung cancer (MLC) versus stage I primary lung cancer (PLC).Patients and methods137 patients treated with SABR for stage I non-small cell lung cancer were retrospectively reviewed, of which 28 (20.4%) were MLC and 109 (79.6%) were PLC. Cohorts were evaluated for differences in overall survival (OS), progression-free survival (PFS), metastasis-free survival, local control (LC), and toxicity.ResultsAfter SABR, patients treated for MLC have comparable median age (76.6 vs 78.6,p= 0.2), 3-year LC (83.6% vs. 72.6%,p= 0.2), PFS (68.7% vs. 50.9%,p= 0.9), and OS (78.6% vs. 52.1%,p= 0.9) as PLC, along with similar rates of total (54.1% vs. 42.9%,p= 0.6) and grade 3 + toxicity (3.7% vs. 3.6%,p= 0.9). Previous treatment of MLC patients was either surgery (21/28, 75%) or SABR (7/28, 25%). The median follow-up was 53 months.ConclusionSABR is a safe and effective approach for localized metachronous lung cancer.