Inclusion Criteria:  History of biopsy-proven adenocarcinoma of the rectum, anus or rectosigmoid junction of any stage now with recurrent disease in the pelvis One prior course of radiation therapy to the pelvis for rectal cancer ECOG performance status 0-2 At least 18 years of age 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:  Patients with pre-existing radiosensitizing conditions, such as connective tissue disorders (i.e. lupus, scleroderma) and genetic mutations (i.e. ataxia-telangiectasia) A history of other malignancy with the exception of malignancies for which all treatment was completed at least 2 years before registration and the patient has no evidence of disease, basal cell or squamous cell carcinoma of the skin that 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 More than one prior course of radiation to the pelvis for rectal cancer Prior radiation to the pelvis for disease other than rectal cancer Current treatment with any investigational agents. Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, or unstable angina pectoris Pregnant and/or breastfeeding. Women of childbearing potential must have a negative urine or serum pregnancy test within 14 days of study entry.