RecruitingProcedureTumor-Informed ctDNA Testing for MRD Following Treatment of Squamous Cell CarcinomaThis is a single-center, non-interventional, observational study that evaluates the correlation of circulating tumor DNA (ctDNA) testing to cancer relapse for participants with squamous cell carcinomas (HNC) of the head and neck mucosa and skin after curative-intent primary radiation or surgery.