6574 Background: DURTRERAD is a randomized phase II study evaluating feasibility and efficacy of durvalumab (anti-PD-L1) vs. durvalumab and tremelimumab (anti-CTLA-4) in combination with radiotherapy as primary treatment for locally advanced HPV negative HNSCC. (NCT03624231). Concurrent chemo-RT with a platinum-based regimen is considered the standard treatment, although efficacy and long-term toxicity are not satisfactory. Combining immunotherapy with RT might result in improved efficacy with limited long-term toxicity. Methods: The phase II study planned to enroll 120 pts, 60 pts (1:1) in each treatment arm. Treatment with DT (1500mg/75 mg, arm DT),or D (1500mg, arm D) both in combination with RT (70Gy) was considered to be feasible if less than 10% of the patients treated will discontinue treatment due to on-treatment toxicities. A first interim analysis for feasibility and efficacy was planned after randomisation of 20 patients. Results: So far 23 patients have been screened, 16 patients have been randomised and started their allocated treatment, 10 in arm D and 6 in arm DT. Of 10 patients in arm D 1 patient stopped infusional treatment due to immune related toxicity. Out of 6 patients in the DT arm, however, 5 patients stopped treatment due to treatment related AEs, 2 pts due to immune related toxicity with one Grade 5 AE. Three patients stopped due to non-immune related AE. The grade 5 AE prompted the interim analysis, which revealed non-feasibility as well as safety-issues of the DT+radiotherapy combination . As a result, the DT arm was prematurely terminated. Conclusions: Even though in the recurrent/metastatic setting DT was not associated with increased toxicity, DT in combination with RT was not feasible in our poor prognostic, vulnerable patient cohort of advanced HPV negative unresectable HNSCC, warranting early disclosure of these results. No increase in toxicity was observed in the D monotherapy arm, and the trial continued with D monotherapy in combination with RT. Clinical trial information: NCT03624231 .