Background: Combining local ablative and systemic therapies in patients with oligometastatic NSCLC leads to improved overall survival (OS) and progression-free survival (PFS). The potential immunostimulatory effects of ablating all visible disease with SBRT in combination with dual immune checkpoint inhibition has prompted interest, but the toxicity and benefit are unknown. Methods: We conducted a phase Ib study to investigate the safety of SBRT, with doses between 30 and 50 Gy in five fractions to all sites of disease, followed by durvalumab 1500 mg IV in combination with tremelimumab 75 mg IV every 4 weeks x 4 cycles, followed by durvalumab maintenance until progression. Eligible patients had 1-6 extracranial metastatic sites, allowing multiple metastases per location, with all lesions suitable for SBRT, ECOG performance status 0-1, no actionable driver mutation, and no prior immunotherapy. The primary endpoint was safety of this combination. Secondary endpoints include PFS and OS. Dose-limiting toxicities (DLTs) (any Grade ≥ 3 toxicity) were evaluated from the first administration of SBRT until 28 days post start of durvalumab and tremelimumab. Baseline tumor mutational burden, PD-L1 expression on post-SBRT biopsy and circulating tumor cells will be correlated with outcomes. In this first cohort analysis, we assess the safety and outcomes of the first 17 patients. Results: From 2/2018-2/2021, the first 17 pts were enrolled. Characteristics of those enrolled included: median age 68 years, female/male 4/13, squamous/non-squamous 2/15, median number of non-central nervous system (CNS) metastatic sites 2 (1-5), median number of non-CNS metastatic lesions 2 (1-9), CNS involvement 6/17 (35.3%), previous treatment 4/17 (23.5%). DLTs were seen in 2/17 (11.8%) patients; DLTs were autoimmune hepatitis and autoimmune pancreatitis. Most treatment-related adverse events (TRAEs) were grade (G) 1/2. TRAEs included: all TRAEs n = 188, 88.2% (of patients); G 3 n = 17, 29.4%; G 4 n = 1, 5.8%. There were no treatment-related deaths. Five patients discontinued treatment due to grade 3/4 immune related adverse events (IRAE). At a median follow up of 20 months 11/17 (64.7%) patients are alive with 10/17 (58.8%) with no evidence of disease (NED). Six of 17 (35.2%) patients experienced disease progression and 4/17 (23.5%) patients died of disease progression. Median PFS and OS are not yet reached. Conclusions: There were no unexpected safety signals in the cohort of patients enrolled. The incidence of grade ≥ 3 IRAEs is similar to the treatment of advanced NSCLC and no additional toxicity was observed with the addition of SBRT. Clinical outcomes look promising with median OS and PFS not yet reached at 20 months median follow up. The study continues to enroll a second cohort and results will be updated. Clinical trial information: NCT03275597