Study of De_Intensified Postoperative Radiation Therapy for HPV Associated Oropharyngeal Squamous Cell Carcinoma

Inclusion Criteria: * Histologically or cytologically confirmed squamous cell carcinoma of the tonsil or base of tongue (oropharynx) or unknown primary with p16-positive cervical lymph node metastases * HPV-associated tumor as defined by: positive p16 immunohistochemistry (\>70%) OR in situ hybridization OR PCR-based methods * Eligible for curative-intent surgery with anticipated negative margins * Surgery performed at Brigham \& Women's Hospital * Age 18 or older years. * ECOG performance status 0-1 (Karnofsky ≥70%, see Appendix A) * Normal organ and marrow function as defined below: * leukocytes ≥3,000/mcL * absolute neutrophil count ≥1,000/mcL * platelets ≥100,000/mcL * total bilirubin within normal institutional limits * AST(SGOT)/ALT(SGPT) ≤2.5 × institutional upper limit of normal * creatinine ≤ 1.5 times the institutional upper limit of normal OR * creatinine clearance ≥60 mL/min/1.73 m2 for participants with creatinine levels above institutional normal. * Patients with ≤20 pack-years of cumulative cigarette smoking. Pack-years are calculated by multiplying the number of years smoked with the pack of cigarettes smoked per day. One pack is considered to contain 20 cigarettes. * Tumor clinical stage (AJCC 8th edition): T0 or T1 or T2 * Nodal clinical stage (AJCC 8th edition): N0 or N1 * No distant metastases. Clinical M-stage must be M0 (AJCC 8th edition). * Ability to understand and the willingness to sign a written informed consent document. Exclusion …