Background: Concurrent platinum-based chemo-RT is the standard treatment for LA-HNSCC, despite of limited efficacy in poor-risk patients and long-term toxicity. Targeting PD-L1 with D has activity in recurrent/metastatic (RM)-HNSCC, and the combination with the CTLA4 targeting agent T was considered to augment the immunotherapy efficacy of a chemotherapy-free regimen. Thus DURTRERAD was designed as a randomized two-arm phase II study to investigate, whether D or DT combined with RT was feasible in the patient population and a 1-year progression free survival rate of 50% was reached. Methods: The phase II study planned to enroll 60 patients in each treatment arm. Key selection criteria were patients with advanced HPV negative unresectable HNSCC planned for definitive radiotherapy. Treatments: First dose of immunotherapy 14 days prior to start of RT (70Gy over 7 weeks). D (1500mg q 3 weeks x 13) or DT (D: 1500mg q 3 weeks x 13 + T: 4 x 75 mg in weeks 5,9,13,17). Planned treatment duration was 51 weeks in both arms. Treatment was considered feasible in case of <10% treatment discontinuations. An interim analysis for feasibility was planned after treatment of 6 patients per arm and a futility analysis was performed after treatment of 18 patients, resulting in premature closure of the trial. Results: Patient accrual in 4 centers in Germany from 9/2018 - 9/2020. The interim feasibility analysis of DT revealed that 5/6 patients stopped treatment due to 1 grade 5 and 1 grade 4 immune related AE and 3 non-immune related AEs, respectively. Thus, the DT arm was prematurely terminated, and only the D monotherapy arm continued. Futility analysis of D monotherapy: One of 12 patients stopped treatment due to immune related toxicity, and 7 patients experienced disease progression on treatment or died within the first year. The median PFS was 39 weeks, the 1 year PFS rate was 42%. Conclusions: Even though D was previously proven as an active agent in RM-HNSCC, and there DT was not associated with increased toxicity, DT in combination with RT was not feasible in our poor prognostic, vulnerable patient cohort of HPV negative LA-HNSCC. D monotherapy in combination with RT was well tolerated, but revealed a low level of efficacy, when administered in the trial schedule, starting 2 weeks prior to RT. Similar to results in lung cancer, start of immunotherapy after completion of radiotherapy may be of interest. Clinical trial information: NCT03624231.