Background: The Radiation Therapy Oncology Group (RTOG) protocol 0618 was a phase II trial utilizing SBRT to treat early stage non-small cell lung cancer in operable patients (pts). Methods: All pts were deemed operable by a thoracic surgeon utilizing specific criteria. Pts with biopsy proven peripheral T1-T3, N0, M0 tumors were eligible. The prescription dose was 18 Gy X 3 fractions delivered in 1½-2 weeks. The primary endpoint was 2-year primary tumor control (PTC, avoidance of in-field (INF) and marginal failure (MF)) with overall and progression free survival (OS, PFS), adverse events (AE), local (LF), regional (RF), and distant failure (DF) as secondary endpoints. Early surgical salvage was directed as part of protocol design in the event of LF after SBRT. Results: The study opened December 2007 and closed May 2010 after accruing a total of 33 pts. Of 26 evaluable pts, 23 had T1, and 3 had T2 tumors. Median age was 72 years. Median FEV₁, DLCO at enrollment were 72%, 68% predicted, respectively. 4 pts (16%) had SBRT related grade 3 AEs while 0 had grade 4-5 AEs. Median follow-up was 25 months. 2 pts have been scored with INF (11.7 and 12.4 months post SBRT) and 1 with MF (32.5 months post SBRT) giving an estimated 2-year primary tumor failure rate of 7.7% (95% CI: 0.0%, 18.1%). 2-year estimates of LF (primary tumor plus involved lobe failure), RF, and DF are 19.2% (95% CI: 3.7%, 34.7%), 11.7% (95% CI: 0.0%, 24.5%), and 15.4% (95% CI: 1.2%, 29.6%), respectively. Only one patient was eligible for attempted surgical salvage and underwent lobectomy 1.2 years post SBRT complicated by a grade 4 cardiac arrhythmia. 2-year estimates of PFS and OS are 65.4% (95% CI: 44.0%, 80.3%) and 84.4% (95% CI: 63.7%, 93.9%), respectively. Conclusions: SBRT given appears to be associated with a high rate of PTC, moderate treatment related morbidity, and infrequent need for surgical salvage in operable early stage lung cancer pts with peripheral lesions. These results support ongoing enrollment into the ACOSOG Z4099-RTOG 1021 trial comparing SBRT to sublobar resection in high risk operable pts. The project was supported by RTOG grant U10 CA21661, CCOP grant U10 CA37422, and ATC U24 CA81647 from the National Cancer Institute. Clinical trial information: NCT00551369.