Inclusion Criteria:

          -  Histologically confirmed diagnosis of rectal adenocarcinoma

          -  Eligible for curative resection of rectal adenocarcinoma

          -  Rectal tumor location =< 12 cm from the anal verge as determined by endoscopy or
             magnetic resonance imaging (MRI) (if endoscopy report is not available or deemed
             inadequate my treating oncologist)

          -  Nodal involvement confined to the radiation field

          -  Radiologically measurable or clinically evaluable disease as defined in the protocol

          -  Eastern Cooperative Oncology Group (ECOG) performance status (PS): 0, 1 or 2

          -  Clinical Stage: Stage II and III. N2 disease is to be estimated as four or more lymph
             nodes that are >= 10 mm. Clinical staging should be estimated based on the combination
             of the following assessments: physical exam by the primary surgeon including digital
             rectal exam (DRE), computed tomography (CT) or positron emission tomography (PET)/CT
             scan of the chest/abdomen/pelvis and a pelvic MRI. If a pelvic MRI is performed, it is
             acceptable to perform CT of the chest/abdomen, omitting CT imaging of the pelvis.
             PET/CT is optional.

          -  No known contraindication to standard (fluoropyrimidine-based) pelvic chemoradiation
             (e.g. dihydropyrimidine dehydrogenase [DPD] deficiency)

          -  Patient of child-bearing potential is willing to employ adequate contraception during
             treatment and after treatment, as directed by treating clinical team

          -  Willing to provide written informed consent

          -  Willing to return to enrolling medical site for all study assessments

        Exclusion Criteria:

          -  Diagnosis of inflammatory bowel disease (IBD)

          -  Diagnosis of MSI-H colorectal cancer at time of consent

          -  Recurrent rectal cancer

          -  Tumor is causing symptomatic bowel obstruction (patients who have diverting ostomy are
             eligible)

          -  Any prior pelvic radiation

          -  Other invasive malignancy undergoing active treatment. Patients receiving prior
             treatment that precludes standard chemoradiation or ability to receive
             consolidation/adjuvant chemotherapy will be excluded from survival analyses

          -  Patients unwilling or unable to undergo pelvic MRI