SBRT +/- Pembrolizumab in Patients With Local-Regionally Recurrent or Second Primary Head and Neck Carcinoma

Safety Run-In: To evaluate the safety of the addition of pembrolizumab (anti PD-1 immunotherapy) to re-irradiation with SBRT for patients with recurrent or new second primary head and neck squamous cell carcinoma (HNSCC). Phase II: To compare progression-free survival (PFS) for patients with recurrent or new second primary head and neck squamous cell carcinoma with SBRT re-irradiation with or without pembrolizumab. OUTLINE: Safety Run-In: Patients receive SBRT over 2 weeks and then receive pembrolizumab every 3 weeks for up to 2 years. Phase II: Patients are randomized to 1 of 2 treatment arms. ARM I: Patients receive SBRT over 2 weeks and then receive pembrolizumab every 3 weeks for up to 2 years. ARM II: Patients receive SBRT over 2 weeks. Arm II patients who experience progressive disease within 2 years after the start of SBRT will be allowed to cross over to receive pembrolizumab for up to 2 years. After the completion of study treatment, patients are followed up every 6 months for 3 years.