Adjuvant Nivolumab After Salvage Resection in Head and Neck Cancer Patients Previously Treated With Definitive Therapy

Inclusion Criteria: * Prior definitive therapy with radiation (with or without prior surgical resection and/or chemotherapy) who have underwent salvage resection with curative intent and have no other curable options * Pre-operative scans including chest imaging preferably PET/CT and CT neck w/contrast. MRI neck is acceptable if contrast contraindicated. * Able to provide archived biopsy or resected tissue. * Adequate performance status and labs. Exclusion Criteria: * Patients who did not receive at least radiotherapy as prior definitive treatment. * Patients who have gross measurable residual disease after surgery or those who underwent surgery for palliative purposes i.e. for symptom control. * Has nasopharyngeal or sinonasal carcinoma. * Has confirmed metastatic disease.