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.