Inclusion Criteria:

• * Age 18 years and older
• * Written informed consent and any locally-required authorization obtained from the patients prior to performing any protocol-related procedures, including screening evaluations
• * Pathologically (histologically or cytologically) proven diagnosis of cancer of the head and neck (nasopharynx, oral cavity, oropharynx, hypopharynx, larynx, or unknown primary)
• * Locally recurrent or metastatic HNC not deemed amenable to curative-intent salvage therapy, in whom at least six months have passed since their prior RT, if received
• * Must have evaluable lesion per RECIST v1.1
• * Patients agree to provide their smoking history prior to registration
• * ECOG performance status of 0-2

Exclusion Criteria:

• * Prior radiotherapy to the region of the study cancer within less than 6 months
• * Patients who have received prior radiation therapy and who, in the opinion of the treating radiation oncologist, cannot be reirradiated safely without excess risk of severe toxicity given prior radiation dose to critical structures.
• * Patients with known contraindications to radiotherapy, including inherited syndromes associated with hypersensitivity to ionizing radiation (e.g., Ataxia-Telangiectasia, Nijmegen Breakage Syndrome)
• * Female patients who are pregnant