ctDNA and Organ Preservation/Pathologic CR in Rectal Cancer

Inclusion Criteria: * Participants with T3, T4, or node-positive non-metastatic rectal cancer. * Participants must have original tumor tissue (formalin-fixed, paraffin embedded specimens) available for analysis or be willing to undergo a baseline research biopsy. * Participants must be 18 years of age or older. * ECOG 0-2. * Participants must be eligible for at least 3 months of FOLFOX, FOLFIRINOX/FOLFOXIRI, or CAPOX * Participants must be eligible for long course chemoradiation to 40-54 Gy. * Participants must be able to understand and willing to sign a written informed consent document. Exclusion Criteria: * Participants must not have any other organ cancer evident at the time of enrollment. * Participants may not have any other concurrent serious illness that makes participation on this study impractical or clinically inappropriate. * Participants must not be actively or planning to be pregnant or breastfeeding