Inclusion Criteria:

          -  Advanced, recurrent or metastatic head and neck squamous cell carcinoma, as defined by
             clinical or pathological diagnosis of any of the following:

          -  Locally advanced head and neck squamous cell carcinoma not suitable for curative local
             treatment.

          -  Locally recurrent head and neck squamous cell carcinoma not suitable for curative
             local treatment within or outside a previously irradiated tissue.

          -  Metastatic head and neck squamous cell carcinoma.

          -  Target site in the head and neck region amenable to quad-shot palliative radiotherapy,
             for which palliative radiotherapy is recommended, as determined by the treating
             radiation oncologist.

          -  Age 18 years or greater at time of registration.

          -  ECOG Performance Status of 0-2.

          -  Women of child-bearing potential and men must agree to use adequate contraception
             (hormonal or barrier method of birth control; abstinence) prior to study entry and for
             the duration of study participation. Should a woman become pregnant or suspect she is
             pregnant while participating in this study, she should inform her treating physician
             immediately.

          -  Ability to understand and the willingness to sign an IRB-approved informed consent
             document (either directly or via a legally authorized representative).

          -  Willingness to provide blood and saliva samples for exploratory research purposes.

          -  Organ and Marrow Function as defined below: Absolute neutrophil count (ANC) ≥ 1.5 x
             109/L, platelet count ≥ 100 x 109/L, hemoglobin ≥ 9.0 g/dL, serum bilirubin ≤ 1.5 x
             ULN (institutional upper limit of normal), AST and ALT ≤ 2.5 x ULN (institutional
             upper limit of normal), serum creatinine CL>40 mL/min by the Cockcroft-Gault formula
             (Cockcroft and Gault 1976) or by 24-hour urine collection for determination of
             creatinine clearance:

        MALES: Creatinine CL (mL/min) = Weight (kg) x (140 - Age) (divided by) 72 x serum
        creatinine (mg/dL).

        FEMALES: Creatinine CL (mL/min) = Weight (kg) x (140 - Age) x 0.85 (divided by) 72 x serum
        creatinine (mg/dL)

        Exclusion Criteria:

          -  Radiation therapy to the head and neck region within 30 days of registration.

          -  Prior radiotherapy to the head and neck that precludes safe delivery of study
             radiotherapy, as determined by the treating radiation oncologist.

          -  Active medical conditions that are contraindications to study radiotherapy (i.e.
             scleroderma), as determined by the treating radiation oncologist.

          -  Pregnant or lactating women are excluded from this study because radiotherapy is
             contraindicated in pregnancy and because there is an unknown but potential risk for
             adverse events in nursing infants secondary to treatment of the mother with
             immunotherapy.

          -  Participation in another clinical study with an investigational product during the
             last 3 months.

          -  Any previous treatment with a PD1 or PD-L1 inhibitor.

          -  Any anti-cancer therapy (chemotherapy, immunotherapy, endocrine therapy, targeted
             therapy, biologic therapy, tumor embolization, monoclonal antibodies, other
             investigational agent) within the last 30 days.

          -  Mean QT interval corrected for heart rate (QTc) ≥470 ms except for patients with
             pacemaker who have a paced ventricular rhythm.

          -  Current or prior use of immunosuppressive medication within 30 days, with exceptions
             of intranasal and inhaled corticosteroids or systemic corticosteroids at physiological
             doses, which are not to exceed 10 mg/day of prednisone, or an equivalent
             corticosteroid.

          -  Any unresolved toxicity (>CTCAE grade > 2) from previous anti-cancer therapy. Subjects
             with irreversible toxicity that is not reasonably expected to be exacerbated by the
             investigational product may be included (e.g., hearing loss, peripherally neuropathy).

          -  Any prior Grade ≥3 immune-related adverse event (irAE) while receiving any previous
             immunotherapy agent, or any unresolved irAE >Grade 1.

          -  Active or prior documented autoimmune disease within the past 2 years, NOTE: Subjects
             with vitiligo, Grave's disease, or psoriasis not requiring systemic treatment (within
             the past 2 years) are not excluded.

          -  Active or prior documented inflammatory bowel disease (e.g., Crohn's disease,
             ulcerative colitis).

          -  History of primary immunodeficiency.

          -  History of allogeneic organ transplant.

          -  History of hypersensitivity to any excipient in pembrolizumab.

          -  History of pneumonitis or interstitial lung disease.

          -  Subjects with uncontrolled seizures.

          -  Uncontrolled intercurrent illness including, but not limited to, ongoing or active
             infection, symptomatic congestive heart failure, uncontrolled hypertension, unstable
             angina pectoris, uncontrolled cardiac arrhythmia, active peptic ulcer disease or
             gastritis, active bleeding diatheses, evidence of acute or chronic hepatitis B,
             hepatitis C or human immunodeficiency virus (HIV), or psychiatric illness/social
             situations that would limit compliance with study requirements or compromise the
             ability of the subject to give written informed consent.

          -  Known history of active tuberculosis.

          -  Receipt of live attenuated vaccination within 30 days prior to study entry or within
             30 days of receiving pembrolizumab.

          -  Any condition that, in the opinion of the investigator, would interfere with
             evaluation of study treatment or interpretation of patient safety or study results.