Organ Preservation First Strategy and Intentional Watch and Wait for MRI Defined Low-risk Rectal Cancer

Inclusion Criteria: * Age ≥ 18 years and ≤85 years * ECOG Performance status 0-1 * Histologically confirmed diagnosis of adenocarcinoma of the rectum, with tumor differentiation Grade 1-3 * The distance from down verge of tumor to anal-rectal junction (ARJ) ≤4cm based on MRI, or ≤8 cm based on sigmoidoscopy * Clinical Stage T2 or T3a or T3b and EMVI (-) and MRF (-) and extra-mesorectal metastatic lymph node (-) based on MRI * The maximum diameter of the tumor is ≤4cm or the circumferential invasion range is less than 1/3 of the intestinal circumference * No evidence of distant metastases * No prior pelvic radiation therapy * No prior chemotherapy or surgery for rectal cancer * No active infections requiring systemic antibiotic treatment * ANC \> 1.5 cells/mm3, HGB \> 10.0 g/dL, PLT \> 100,000/mm3, total bilirubin ≤ 1.5 x ULN, AST≤ 3 x ULN, ALT ≤ 4 x ULN * Patients must read, agree to, and sign a statement of Informed Consent prior to participation in this study. Exclusion Criteria: * Recurrent rectal cancer * Primary unresectable rectal cancer. A tumor is considered unresectable when invading adjacent organs and an en-bloc resection will not achieve negative margins * Creatinine level greater than 1.5 times the upper limit of normal * Patients who have received prior pelvic radiotherapy * Patients who are unable to undergo an MRI * Patients with a history of a prior malignancy within the past 5 years, except for well treated basal cell cancer, squamous cell skin cancer, breast cancer, thyroid cancer or small renal cancer, and with DFS \>5 years * Patients with a history of any arterial thrombotic event within the past 6 months. This includes angina (stable or unstable), MI, TIA, or CVA * Other anticancer or experimental therapy * Women who are pregnant or breast-feeding * Patients with any other concurrent medical or psychiatric condition or disease which would make them inappropriate candidates for entry into this study.