A Study of Response to Standard Treatment Before Surgery in People With Rectal Cancer

Inclusion Criteria: * 18 years of age or older on day of signing informed consent. * Have a histologically confirmed diagnosis of invasive adenocarcinoma of the rectum with no known mismatch repair deficiency or Her2 amplification. * Eligible for and plan to initiate standard-of-care therapy with any of the following regimens: * Induction fluoropyrimidine (capecitabine \[preferred\] or 5-FU) based chemoradiation * With plan for consolidative CAPEOX, FOLFOX, or FOLIRINOX * Woman with childbearing potential who are negative for pregnancy test (urine or blood) and who agree to use an effective contraceptive method. A woman of childbearing potential is defined as one who is biologically capable of becoming pregnant. Reliable contraception should be used from the time of screening and must be continued throughout the duration of treatment as per standard of care. Exclusion Criteria: * Patients with a prior or concurrent malignancy whose natural history or treatment has the potential to interfere with the safety or efficacy assessment of the investigational regimen. * Patients with prior external beam radiation therapy to the pelvis or brachytherapy seed implantation of the prostate. * Patients who are pregnant or breastfeeding. * Men or women not using effective contraception. * Patients with a contraindication to MR imaging. * Patients on blood thinners prohibiting endoluminal tumor biopsies.