Inclusion Criteria:

          -  Histologically or cytologically newly confirmed diagnosis of squamous cell carcinoma
             (including the histological variants of papillary or basaloid) of the oropharynx
             (tonsil, base of tongue, soft palate, or oropharyngeal walls)

          -  Clinical American Joint Committee on Cancer (AJCC) 7th edition stage T1N2a-N2CM0,
             T2N1-N2CM0, T3N0-N2CM0, equivalent to AJCC 8th edition stage 1 and 2 (T1 N2, T2 N1-N2,
             T3 N0-N2) excluding T1N0-N1 and T2N0 (Brian O'Sullivan et al. 2016)

          -  Tumor positive for p16 immunohistochemistry (IHC) (defined as greater than 70% strong
             nuclear or nuclear and cytoplasmic staining of tumor cells) and HPV DNA in situ
             hybridization or HPV messenger ribonucleic acid (mRNA) RNAScope. Repeat samples may be
             required if adequate diagnostic tissue is unavailable for testing

          -  Zubrod Performance Status of 0-1

          -  Patients must have radiographically evident measurable disease at the primary site or
             at nodal stations per response evaluation criteria in solid tumors (Response
             Evaluation Criteria in Solid Tumors [RECIST]) 1.1 documented by diagnostic quality CT
             or magnetic resonance imaging (MRI) of the neck with contrast within 28 days prior to
             registration; a FDG-PET/CT of the neck performed for the purposes of radiation
             planning is acceptable as a substitute if the CT is of diagnostic quality

          -  Diagnostic quality cross sectional imaging of the thorax within 28 days prior to
             registration. A 18-FDG-PET/CT or conventional CT are acceptable

          -  FDG-PET/CT of the neck is required within 28 days prior to registration for comparison
             to post treatment FDG-PET/CT. Note: Repeat imaging for variability within 96 hours of
             this time frame should be allowed to avoid unnecessary re-imaging and its financial
             and potential physical consequences for patients

          -  History and physical exam within 1 month prior to registration

          -  Fiberoptic exam with laryngopharyngoscopy (mirror and/or fiberoptic and/or direct
             procedure) by radiation oncologist, medical oncologist or ear, nose, throat (ENT)/head
             and neck surgeons within 28 days prior to registration

          -  Absolute neutrophil count (ANC) >= 1,500 cells/mm^3 (within 2 weeks prior to
             registration)

          -  Platelets >= 100,000 cells/mm^3 (within 2 weeks prior to registration)

          -  Hemoglobin >= 8.0 g/dl (within 2 weeks prior to registration); Note: The use of
             transfusion or other intervention to achieve Hgb >= 8.0 g/dl is acceptable

          -  Serum creatinine < 1.5 mg/dl or creatinine clearance (CC) >= 50 ml/min determined by
             24-hour collection or estimated by Cockcroft-Gault formula (within 2 weeks prior to
             registration)

          -  Bilirubin < 2 mg/dl (within 2 weeks prior to registration)

          -  Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) < 3 x the upper
             limit of normal (within 2 weeks prior to registration)

          -  Sodium (Na), potassium (K), chloride (Cl), glucose, calcium (Ca), magnesium (Mg),
             albumin, amylase, lipase, thyroid stimulating hormone (TSH) within 2 weeks prior to
             registration

          -  Negative serum pregnancy test within 2 weeks prior to registration for women of
             childbearing potential

          -  Seronegative for active hepatitis-B or hepatitis-C infection and seronegative for
             human immunodeficiency virus (HIV)

          -  Mandatory submission of hematoxylin and eosin (H&E) and paraffin-embedded tumor block
             or unstained slides

        Exclusion Criteria:

          -  Cancers of the oral cavity (oral tongue, floor mouth, alveolar ridge, buccal or lip),
             or the nasopharynx, hypopharynx, or larynx, even if p16 positive

          -  Carcinoma of the neck of unknown primary site origin (even if p16 positive)

          -  Definitive clinical or radiologic evidence of metastatic disease or adenopathy below
             the clavicles

          -  Gross total excision of both primary and nodal disease with curative intent; this
             includes tonsillectomy, local excision of primary site, and nodal excision that
             removes all clinically and radiographically evident disease

          -  Simultaneous primary cancers or separate bilateral primary tumor sites

          -  Prior invasive malignancy (except non-melanomatous skin cancer) unless disease free
             for a minimum of 1095 days (3 years) (for example, carcinoma in situ of the breast,
             oral cavity, or cervix are all permissible)

          -  Prior systemic chemotherapy for the study cancer; note that prior chemotherapy for a
             different cancer is allowable; patients who have received PD-1/PD-L1 or CTLA4 therapy
             for a previous malignancy are not eligible

          -  Prior RT to the region of the study cancer that would result in overlap of radiation
             therapy fields

          -  Severe, active co-morbidity defined as any of the following: Unstable angina and/or
             congestive heart failure requiring hospitalization within the last 6 months; acute
             bacterial or fungal infection requiring intravenous antibiotics at the time of
             registration; chronic obstructive pulmonary disease exacerbation or other respiratory
             illness requiring hospitalization or precluding study therapy within 30 days of
             registration; hepatic insufficiency resulting in clinical jaundice and/or coagulation
             defects; a diagnosis of immunodeficiency or use of any form of systemic
             immunosuppressive therapy. The use of physiologic doses of corticosteroids may be
             approved after consultation with the sponsor

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

          -  Has evidence of active, non-infectious pneumonitis

          -  Has received a live vaccine within 30 days of planned start of study therapy

          -  Pregnancy or breast-feeding; this exclusion is necessary because the treatment
             involved in this study may be significantly teratogenic

          -  History of severe hypersensitivity or contraindication to CT or PET contrast material
             uncontrolled with pre-medications (steroids, antihistamines)