Background: Cancers can be recognized by the immune system and immunotherapy (ICI) has become an important treatment for lung cancer. Compared to chemotherapy, combining an anti-CTLA-4 and PD-L1 inhibitor increases response- and survival rates independent of PD-L1 status. We hypothesized that adding stereotactic radiotherapy (SBRT) to increase the release of cancer cell antigens might improve treatment results even further. Because of accumulating tumor heterogeneity it may be important to irradiate the primary tumor (addressing trunk mutations), rather than its metastases (branch mutations). We assessed toxicity of adding SBRT to (dual) ICI. Methods: In 3 sequential cohorts, ICI regimes combined with SBRT were studied in stage IIIB/IV NSCLC patients progressing on chemotherapy. All patients were irradiated on an FDG avid part of the primary tumor (1x20 Gy on 9cc) 1 week after the 1^st dose of ICI; in case of dual ICI the 1^st dose of the 2^nd ICI was administered within 1 week following SBRT. Following treatments both ICI were combined. The 1^st cohort (n = 3) received durvalumab (D) monotherapy for up to 1 year. The 2^nd cohort starting with tremelimumab (T) and the 3^rd cohort starting with D (both n = 6) received a combination of D and T (4 cycles) followed by D monotherapy for up to 1 year. The 2^nd and 3^rd cohort only proceeded when dose limiting toxicities (DLT) occurred in ≤33% of cases in the previous cohort. DLT was defined as any grade ≥3 toxicity that occurred during the first 8 weeks of therapy. Treatment related adverse events (TRAEs) were categorized using NCI CTCAE version 4.3. Descriptive statistics were used to summarize baseline characteristics and TRAEs. Results: Fifteen patients were included from June 2018 to November 2020. Last study visit was in September 2021. Baseline characteristics of the groups were comparable. The median irradiated tumor volume was 9.13 cc (8.98-9.60 cc) with a mean lung dose of 0.44 Gy (0.14-0.73 Gy). The V20 was nihil (0-0.1%) and the V5 1.8% (0-4%). There was 1 DLT in cohort 3 (colitis grade 3). There was 1 low grade TRAE (pneumonitis grade 2) in cohort 1. In cohort 2, 6 low grade TRAEs (CTC 1-2) were observed and 2 high grade (CTC 3) in 1 patient, who discontinued treatment. In cohort 3, 4 low grade and 1 high grade TRAE occurred. Median progression free survival was 2 months, median overall survival 10 months. Conclusions: No additional toxicity was observed by adding SBRT to the primary tumor to (dual) ICI. Clinical trial information: 2017-002797-39.