Background: Stereotactic ablative radiotherapy (SABR), which delivers high biologically effective radiation doses, can kill cancer cells, release tumor-associated antigens, and activate tumor-specific T cells, thereby functioning as a cancer-specific vaccine in situ. The combination of the immune-triggering effects of ionizing radiation with immune check point PD-1inhibitor may leverage the effects of radiotherapy, transforming what was once considered a local therapy to a novel systemic treatment. Further, the combined effects of local control plus systemic control may improve cure rate in early stage NSCLC. Methods: This is a randomized phase II trial (NCT03110978) designed to study SABR (biological effective dose > 100 Gy) with or without concurrent and adjuvant Nivolumab for total of 7 doses in early stage or isolated recurrent NSCLC. Inclusion criteria: stage I disease (tumor≤5cm, N0M0) OR selected cases of stage IIa disease (tumor > 5cm but≤7cm, N0M0), including multiple primary tumors, OR isolated lung-parenchymal recurrent or persistent NSCLC suitable for SABR. Tumor tissue /blood/stool samples will be collected before/during/after treatment and at the time of recurrence. Primary endpoints: Event-free survival , secondary malignancy and death; Secondary endpoints: overall survival; toxicity; exploratory analyses of potential predictive markers and immunologic mechanisms of action. Statistical design: It is considered significant with a decrease of the 4-year cumulative event rate from 46% to 23%. Assuming a one-sided type I error rate of 0.05, an accrual rate of 3.5 patients per month, and an additional 20 months of follow-up, a study with 70 patients in each arm will have 85% power to detect an improvement of 23% in 4-year EFS rate. One interim analysis will be done to allow early termination of the trial should evidence at that time reveal that I-SABR is superior to SABR-only or that no difference is found between the two treatment arms. Up to January 2018, 20 of planned 140 patients have been enrolled, met with anticipated enrollment rate. Clinical trial information: NCT03110978