Adaptive Radiation for Locally Advanced Rectal Adenocarcinoma

Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) Performance status of 0, 1 * Histologically confirmed diagnosis of adenocarcinoma of the rectum. * Clinical Stage II or III as determined on rectal MRI with a distal tumor extent/margin no more than 1 cm proximal to the anterior peritoneal reflection. * Must have the ability to swallow and retain oral medications * Adequate hematologic function within 28 days before registration as defined in protocol. * Adequate hepatic function within 45 days before registration, as defined in protocol. * Adequate renal function within 28 days before registration, as defined in protocol. * International normalized ratio of prothrombin time (INR) and prothrombin time (PT) within 28 days before registration must be within normal limits for the lab. Patients who are therapeutically treated with an agent such as warfarin may participate if they are on a stable dose and no underlying abnormality in coagulation parameters exists per medical history * Pregnancy test done within 14 days before registration must be negative (for women of childbearing potential only). Pregnancy testing should be performed according to institutional standards. Exclusion Criteria: * Patients with one or more of the following conditions are NOT eligible for this study: * Rectal cancer histology other than adenocarcinoma (i.e., sarcoma, lymphoma, squamous cell carcinoma, mucosal melanoma, mixed adenoneuroendocrine, anal adenocarcinoma, etc.). * History of…