The purpose of this study is to see whether three new types of MRI techniques used during magnetic resonance imaging (MRI) of the pelvis to look at rectal cancer can help doctors to tell if the tumor is getting better in response to the radiation and/or chemotherapy treatments.

This is a pilot study of dynamic contrast enhanced magnetic resonance imaging (DCE-MRI, aka perfusion MRI) and diffusion-weighted MRI (DWI-MRI), herein referred to in combination as advanced MRI (aMRI) in the investigation of early tumor response to standard multi-dose, fractionated external beam radiotherapy (EBRT) of the pelvis given in the neoadjuvant setting concurrent with chemotherapy as well as induction chemotherapy prior to chemoradiotherapy or consolidation chemotherapy after chemoradiotherapy for primary rectal adenocarcinoma. This protocol aims to expand upon the growing body of knowledge concerning early changes in tumor neovascularity and cellular density as a potential biomarker of therapy efficacy. It further aims to address the trend towards more refined treatment stratification for lower risk tumors to avoid morbidity from potentially unnecessary radiation, chemotherapy or even radical surgery, by assessing the earliest changes that occur in microvasculature and perfusion and diffusion of water during this treatment to see if these can be predictive of long-term efficacy of therapy.

Inclusion Criteria:

• Patients with primary locally advanced rectal adenocarcinoma (0-18cm from the anal verge) confirmed by MSKCC pathologist and eligible to undergo chemoradiation and surgical resection at MSKCC.
• Written informed consent
• Age equal to or greater than 21 years
• Willing and able to undergo all study procedures
• Patients must have a planned surgical resection of the rectum

Exclusion Criteria:

• Patients younger than 21 years
• Pregnant and nursing women
• Contraindications for MRI (such as claustrophobia, pacemaker, non MR-compatible artificial heart valves, cochlear implants, surgical clips in the brain, metal fragments in eye)
• Estimated GFR (using Cockcroft formula Appendix 2) less than 30 ml/min/1.73m2 (FDA advises caution in using gadolinium-based contrast agents in patients with severe renal impairment).
• History of allergic reaction to MR contrast media
• Inability to give informed consent in person