ctDNA-Informed Management of Early-Stage Rectal Cancer

Inclusion Criteria: * Tumor tissue histologically confirming rectal adenocarcinoma that is available for Natera ctDNA assay. * Cohort A only: Patients appropriate for receiving TNT including chemotherapy and chemoradiation. * Cohort B only: TNT must have included at least 4 cycles of CAPEOX or 6 cycles of FOLFOX and at least 45 Gy in 25 fractions to the pelvis during chemoradiation. * Patients ≥18 years of age at time of consent. * Ability to understand and willingness to sign the informed consent form (ICF). * Ability and stated willingness to adhere to the study visit schedule and protocol procedures/requirements. Exclusion Criteria: * Prior treatment for rectal cancer, except for cohort B. * Evidence of distant metastatic disease on staging imaging (CT chest with abdominopelvic imaging by CT or MRI) within 8 weeks of enrollment. * Patients on hemodialysis. * Any condition that in the opinion of the investigator would interfere with the participant's safety or compliance while on trial