Non-Operative Radiation Management of Adenocarcinoma of the Lower Rectum

Inclusion Criteria: * Diagnosis of biopsy proven stage I-IIIB (cT1-3, N0-1, M0) adenocarcinoma of the rectum; staging must also be based on multidisciplinary evaluation including MRI and/or endorectal ultrasound * Tumor ≤ 12 cm from anal verge as determined by MRI or endoscopy a * ECOG performance status 0-2 * At least 18 years of age * Adequate bone marrow function defined as: * ANC \> 1,500 cells/mm3 * Hgb \> 8 g/dl * platelets \>100,000 cells/mm3 * Women of childbearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control, abstinence) prior to study entry and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while participating in this study, she must inform her treating physician immediately. * Able to understand and willing to sign an IRB-approved written informed consent document. Exclusion Criteria: * No clinically detectable (MR, endoscopy or DRE) tumor present * Prior radiation therapy, chemotherapy or extirpative surgery for rectal cancer. * Any evidence of disease from another malignancy or history of other malignancy ≤ 3 years previous with the exception of basal cell or squamous cell carcinoma of the skin which were treated with local resection only or carcinoma in situ of the cervix). Patients with history of prostate cancer treated without radiotherapy and no evidence of disease are eligible. * Currently receiving any investigational agents. * A history of allergic reaction attributed to compounds of similar chemical or biologic composition to 5FU, oxaliplatin, or leucovorin. * Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements. * Pregnant and/or breastfeeding. Women of childbearing potential must have a negative serum pregnancy test within 14 days of study entry. * Known HIV-positivity and on combination antiretroviral therapy because of the potential for pharmacokinetic interactions with the study drugs. In addition, these patients are at increased risk of lethal infections when treated with marrow-suppressive therapy. Appropriate studies will be undertaken in patients receiving combination antiretroviral therapy when indicated.