Evaluation of Rectal Cancer Treatment Response Using PET/MRI

Inclusion Criteria: * A diagnosis of histopathologically confirmed rectal adenocarcinoma. * Locally advanced disease as determined by ERUS or pelvic MRI. Endoscopy reports should clearly state both the T and N stage. * Patients with cT2N1-3M0 or cT2-4NxM0 will be considered eligible for participation. * Age ≥18. * Ability to understand a written informed consent document and the willingness to sign it. * Tumor must be determined to be surgically resectable. Surgical resection is planned to take place at Univeristy of California, San Francisco (UCSF). * Neoadjuvant chemoradiation prior to resection is planned.. * Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2. Exclusion Criteria: * ERUS tumor state of T1. * Radiographic evidence of metastatic disease * Weight in excess of limitations of the scanner or girth that prohibits entry into the bore of the PET/CT scanner due to size. * Any medical condition which impairs the ability to lie flat and without movement for 15 minutes (e.g. cough, severe arthritis, etc.) * Prior receipt of any therapy, including local excision, radiation or chemotherapy, for the diagnosed rectal adenocarcinoma. * Prior history of pelvic radiation. * Uncontrolled hyperglycemia (defined as inability to achieve a glucose of \<250 mg/dL at time of fluorodeoxyglucose (FDG) injection). * Impaired renal function (CKD 4 or 5: estimated glomerular filtration rate (eGFR) \< 30 mLs/min), which is a contraindication to gadolinium containing contrast. * Known allergy to gadolinium containing contrast agents. * Contraindication to use of fluoropyrimidines as a radiosensitizing agent. * Pregnancy or nursing. Women of child bearing potential must have a negative serum or urine pregnancy test (minimum sensitivity 25 IU/L or equivalent units of HCG) within 72 hours prior to randomization. This is because of the potential teratogenic effects of the involved imaging modalities, radiation, and chemotherapy.