To Study Perioperative PD-1 Inhibitor and Cesium-131 Interstitial Brachytherapy in patients with locally recurrent HNSCC eligible for salvage surgery.

This study is a non-randomized, single-arm, multi-institutional phase 1b/II study including patients with locally recurrent head and neck cancer eligible for resection. Patients who are identified to be eligible for surgical resection will be screened and consented to receive one 200mg IV dose of pembrolizumab followed by salvage surgery. At the time of surgery, they will have Cesium131 seeds implanted. Two to six weeks after surgery, subjects will be started on 200 mg IV of pembrolizumab to be continued every 3 weeks for 6 months. The phase 1b part will enroll 3 patients for a safety run-in, followed by the phase 2 part if no DLTs are seen in these 3 patients. Phase 2 will be used to estimate the 2-year DFS in comparison to a benchmark selected based on a historical control. For correlative studies, archived tissue and surgical specimen will be evaluated by H&E and IHC for immune phenotype. Blood samples will also be collected as shown in schema above for correlative studies (blood will be collected for correlatives and biomarkers before and after the first of pembrolizumab, before starting adjuvant pembrolizumab and at completion of 6 month course of adjuvant pembrolizumab). Patients will be followed approximately every 3 weeks during treatment for the first 6 months, then monthly for 3 months following treatment and then every 3 months thereafter for 24 months for disease free survival (DFS) and overall survival (OS).

Inclusion Criteria:

• locally recurrent HNSCC and be eligible for salvage surgery
• If patient received radiation therapy in the past, they should have recovered from the acute toxicity to \