Study on the Efficacy of Treatment by Radiotherapy and Pembrolizumab in Newly Diagnosed Metastatic Head & Neck Cancers

Pembrolizumab 200 mg every 3 weeks until disease progression (as confirmed according to response evaluation criteria in solid tumors version 1.1 (RECIST v1.1)) or unacceptable toxicity. The treatment of pembrolizumab should not be delayed because of radiotherapy planning.

Depending on the choice of radiotherapy timing: * Before 3 cycles of pembrolizumab with or without chemotherapy : radiotherapy could start at any time between one week after the first administration of pembrolizumab and the first day of the 3rd cycle. * After 3 cycles of pembrolizumab with or without chemotherapy : radiotherapy could start at any time after 3rd cycle (C3D1) and up to a maximum of 4 weeks after the 6th cycle of pembrolizumab. Dose/fraction of radiotherapy: 54 Gy/18 fractions (recommended schedule) or 70Gy/33-35 fractions or other curative dose/fraction schedules with shorter duration and biologically equivalent dose of at least 60Gy at the discretion of local investigators, in the head and neck region. The volume of RT will include only involved loco-regional tumor region and no prophylactic neck volume will be necessary. Other cycles of pembrolizumab will be administered during and after radiotherapy.

If the investigator decide to add chemotherapy with pembrolizumab, and depending on the radiotherapy timing: * Start of radiotherapy planned before 3rd cycle: Chemotherapy could be delayed after the end of radiotherapy and start from cycle 3 or 4 of pembrolizumab. Administration of chemotherapy can be delayed in case of non resolved grade 3 or higher toxicity from radiotherapy. * Radiotherapy planned after 3rd cycle: Chemotherapy should start at the same time of pembrolizumab. Chemotherapy will combine carboplatin AUC 5mg/mL/min or cisplatin 100mg/m² every 3 weeks with 5-FU 1000mg/m²/j during 4 days every 3 weeks for a maximum of 6 cycles