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.