Gene Expression and FDG PET/MRI in Evaluating Treatment Response in Patients With Locally Advanced Rectal Cancer Undergoing Chemotherapy and Radiation Therapy Before Surgery

Inclusion Criteria:

* Biopsy-proven adenocarcinoma of the rectum.
* Surgical candidate.
* Candidate for neoadjuvant chemotherapy.
* Clinical evidence of T1-3N1 or T3N0 disease. This can be either by imaging studies or by physical exam findings.
* At least 18 years of age.
* Zubrod performance status of 0-2.
* Able to understand and willing to sign a written informed consent document.
* Patients with distant metastatic disease will be eligible if they satisfy all other conditions.

Exclusion Criteria:

* Pregnant.
* Past history of pelvic radiotherapy.
* Received prior chemotherapy for colorectal cancer.
* Uncontrolled diabetes with a fasting glucose ≥ 200 mg/dl at the time of PET/MRI imaging.
* Claustrophobia or any other contraindication to MRI imaging evaluated by a standardized safety questionnaire.
* Renal insufficiency (defined as glomerular filtration rate (GFR \< 30 mL/min/1.73 m2) measured no more than 60 days before the first research scan) which precludes safe administration of the contrast agent.
* Currently on dialysis.
* Known prior allergic reaction to gadolinium-based MR contrast agents.