Post-treatment Surveillance in HPV+ Oropharyngeal SCC

Inclusion Criteria: * Participants must have histologically or cytologically confirmed incident or recurrent squamous cell carcinoma of the tonsil or base of tongue (oropharynx) - includes biopsy of any lymph node with clinical evidence of an oropharynx tumor (unknown primary carcinoma in an involved cervical neck lymph node is permitted if high-risk HPV testing is positive on the involved node) * HPV-associated tumor as defined by one or more : positive p16 immunohistochemistry (\>70%) OR in situ hybridization OR PCR-based methods * Age 18 years or older * Will undergo oropharyngeal cancer treatment with curative intent * Ability to understand and the willingness to sign a written informed consent document. * Baseline positive or detectable ctHPV DNA result in plasma OR detectable corresponding HPV DNA in tumor tissue prior to treatment Exclusion Criteria: * Distant metastatic disease (M1, AJCC 8th edition)