Inclusion Criteria:

          -  Pathologically (histologically or cytologically) confirmed diagnosis of locoregional
             recurrent or any new primary squamous cell carcinoma of the head and neck (including
             of the nasopharynx or paranasal sinus) that is not amenable to curative resection.

               -  Patients for whom curative resection would be medically contraindicated and/or
                  would impose excessive surgical risk are eligible.

               -  Patients who are medically and surgically resectable but for whom surgery would
                  be associated with undue surgical morbidity are eligible.

               -  For purposes of this protocol, undue surgical morbidity would include total
                  glossectomy; carotid artery resection; laryngectomy or pharyngolaryngectomy; and
                  major ablative resection requiring free flap reconstruction. Patients with
                  primary tumors that can be resected without the forgoing are ineligible.

               -  The principal investigators are available to review these criteria on a case by
                  case basis if helpful to the enrolling institution.

        A new primary HNSCC is defined where any one of the following criteria are met:

          -  Metachronous invasive SCC developing ≥ 6 months after an index HNSCC, more than 3 cm
             from the index lesion;

          -  SCC developing in the same region as the index SCC if ≥ 36 months after the index
             diagnosis and if within 3 cm of a site where disease was completely resected or
             complete response was documented;

          -  New SCC that is cytologically or molecularly distinct from index SCC (e.g. new HPV
             negative SCC with prior index SCC that was HPV positive).

               -  Tumor tissue testing for p16 status is required for base of tongue, soft palate,
                  and tonsil cancer. If a p16 testing has been previously performed on an
                  oropharynx cancer that has recurred, then repeat testing for p16 status is not
                  required. Participants whose first cancer was an unknown primary must have p16
                  testing from either the new primary tumor or the recurrent cancer.

               -  Prior radiotherapy (RT) to the head and neck (30 Gy minimum)

               -  Disease must be limited to a single site or adjacent sites that can be treated in
                  a single contiguous target volume for which the maximum total tumor dimension
                  (GTV) must be <7.5cm. Examples of eligible patients include:

                    1. A primary site recurrence in the oropharynx with a concurrent level 2 nodal
                       mass, or a laryngeal recurrence with a level 3 nodal mass

                    2. Multiple nodes in the same (level 2) or adjacent nodal levels (levels 2 and
                       3)

                    3. Skull base recurrence with a lateral pharyngeal or high level 2 node

        Note: These cases will be eligible provided that the maximum total tumor dimension is
        <7.5cm. For cases in which a tumor biopsy was performed and there is a biopsy/tumor
        debulking bed adjacent to the gross residual disease, all of the preoperative radiographic
        abnormalities must be included in the GTV and meet the <7.5cm maximal dimension criteria to
        meet eligibility.

        Note: Patients who meet these criteria only after surgical removal of a portion of the
        patient's disease (e.g. removal of level 4 nodal mass in a patient with a tongue base
        primary; or of a contralateral nodal mass in an N2c patient) are ineligible.

          -  Patients who have undergone a recent biopsy (e.g. incisional) are eligible. Any
             preceding surgical procedure beyond a biopsy (e.g. debulking) must be reviewed as
             follows:

               -  Patients rendered free of gross disease are not eligible.

               -  Patients with gross residual disease postoperatively, must be reviewed by the
                  Surgical Co-PI for determination of eligibility.

               -  Patients eligible for study must have cutaneous wounds healed for 4-6 weeks prior
                  to the initiation of SBRT.

          -  History/physical examination within 56 days prior to entry

          -  Examination by a Radiation Oncologist and Medical Oncologist within 56 days prior to
             entry; [Note: Baseline dental assessment is strongly recommended prior to start of
             therapy but is not required for eligibility]

          -  Contrast enhanced CT or MRI, of the tumor and neck within 56 days prior to entry.

          -  Chest CT scan or full body PET/CT within 56 days prior to entry; patients with
             equivocal pulmonary nodules that are < 1.5 cm, that cannot be safely biopsied, or that
             are negative on PET/CT imaging are eligible.

          -  Zubrod Performance Status of 0-1 within 28 days prior to entry.

          -  Age ≥ 18

          -  Trial is open to all genders

          -  Hematologic: Absolute neutrophil count (ANC) ≥ 1,500 cells/mm3, Platelets ≥ 100,000
             cells/mm3, Hemoglobin ≥ 9 g/dL

          -  Hepatic: Total bilirubin ≤ 1.5 x ULN OR Direct bilirubin ≤ ULN for subjects with total
             bilirubin levels > 1.5 x ULN, AST (SGOT) and ALT (SGPT) ≤ 2.5 x ULN

          -  Creatinine ≤ 1.5 x ULN, OR measured or calculated creatinine clearance > 60 mL/min for
             subject with creatinine levels > 1.5 x institutional ULN [NOTE: Calculated creatinine
             per institutional standard; GFR may be used in place of creatinine or CrCl]

          -  Coagulation: International normalized ratio (INR), OR prothrombin time (PT), and
             Activated partial thromboplastin time (aPTT): ≤1.5 × ULN unless participant is
             receiving anticoagulant therapy as long as PT or aPTT is within therapeutic range of
             intended use of anticoagulants

          -  Negative serum pregnancy test within 14 days prior to entry for women of childbearing
             potential. (Note: A pregnancy test must be repeated within 3 days prior to the
             administration of the first dose of pembrolizumab)

          -  The patient or legally authorized representative must provide study-specific informed
             consent prior to study entry.

        Exclusion Criteria:

          -  Distant metastases.

          -  Tumors that involve more than 180 degrees of the carotid artery on diagnostic CT or
             MRI of the neck within 56 days prior to entry. Investigators are encouraged to review
             the CT simulation imaging and ensure that tumor progression has not occurred whereby
             patients who were initially eligible based on diagnostic imaging, would be rendered
             ineligible based on CT simulation imaging (e.g. tumor size >7.5cm, skin involvement,
             >180 degrees of carotid encasement by tumor). If this does occur, the patient should
             be removed from the study and the Radiation Oncology Co-PIs should be notified via
             email. Note: It is strongly recommended that CT simulation be performed prior to
             entry.

          -  Patients with gross skin involvement (i.e. tumor ulceration through the skin) are
             excluded. Patients with tumor approaching the skin but in which the overlying skin
             remains intact are eligible, providing that planning constraints can be achieved
             without the use of bolus.

          -  Disease that requires two or more discontiguous target volumes will be ineligible.
             Examples of such cases include:

               -  Bilateral nodal targets

               -  Level 2 and level 4 nodes

               -  An oropharyngeal recurrence with a low level 4 node;

          -  Patients for whom the maximal total tumor dimension (GTV) is >7.5cm

          -  Prior radiation to primary tumor within 6 months of entry

          -  Prior systemic therapy, investigational agent or investigational device within 28 days
             of start of study treatment.

          -  Surgical resection of the qualifying cancer is not permitted. (Patients who have
             undergone biopsies are eligible). Patients without radiographically apparent gross
             tumor are ineligible. For cases where an operation more extensive than a biopsy was
             performed but radiographically apparent gross residual tumor remains, will be reviewed
             by the Surgical Co-PI for determination of eligibility.

          -  No concurrent treatment with other investigational agent or investigational device.

          -  Prior therapy with a checkpoint inhibitor (eg anti-CTLA-4, anti-PD-1 or anti-PD-L1
             therapy).

          -  Patients with immunodeficiency, or receiving systemic steroid, or any form of
             immunosuppressive therapy at the time of registration (e.g. history of human
             immunodeficiency virus - HIV). Use of physiologic doses corticosteroids may be
             approved with consultation with study chairs.

          -  Active autoimmune disease that has required systemic treatment in the past 2 years
             (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive
             drugs). Replacement therapy (e.g. thyroxin, insulin, or physiologic corticosteroid
             replacement therapy for adrenal or pituitary insufficiency etc.) is not considered a
             form of systemic therapy

          -  Known active hepatitis B (positive test for virus surface antigen - HBsAg) or
             hepatitis C virus (e.g. positive HCV RNA qualitative test).

          -  History of (non-infectious) pneumonitis that required steroids or current pneumonitis.

          -  Treatment with a live vaccine within 30 days of entry. Note: Seasonal influenza
             vaccines for injection are generally inactivated flu vaccines and are allowed; however
             intranasal influenza vaccines (e.g., Flu-Mist®) are live attenuated vaccines and are
             not allowed

          -  Unstable angina and or congestive heart failure requiring hospitalization in the last
             6 months.

          -  Transmural myocardial infarction within the last 6 months.

          -  Active bacterial or fungal infection requiring intravenous antibiotic at the time of
             registration; Note: If the infection resolves and the patient is on p.o. and still
             within, the required registration timeframe, then the patient is eligible

          -  Chronic Obstructive Pulmonary Disease exacerbation or other respiratory illness
             requiring hospitalization or precluding study therapy within 30 days of entry.

          -  Other significant medical, surgical or psychiatric conditions or requirements for any
             medication or treatment that in the opinion of the investigator may interfere with
             compliance, make administration of anti-PD-L1 therapy hazardous, or obscure
             interpretation of adverse events (AEs), such as a condition associated with frequent
             diarrhea.

          -  Pregnancy, nursing females, or women of childbearing potential and men who are
             sexually active and not willing/able to use medically acceptable forms of
             contraception; this exclusion is necessary because the treatment involved in this
             study may be significantly teratogenic.