S0304 Induct Chemo Then Chemo-RT in Pts w/Locally Advanced Adenocarcinoma of the Rectum

DISEASE CHARACTERISTICS: Histologically confirmed primary adenocarcinoma of the rectum Locally advanced disease (clinical T3-4, N0-2, M0) based on at least 1 of the following criteria: Clinically fixed tumors on rectal examination with tumor adherent to the pelvic sidewall and/or sacrum Sciatica attributed to sacral root invasion with CT scan/MRI evidence of the lack of clear tissue plane is considered evidence of fixation Hydronephrosis on CT scan or intravenous pyelogram of ureteric or bladder invasion by cystoscopy and cytology or biopsy Invasion into the prostate, vagina, or uterus Transmural penetration of tumor through the muscularis propria as evidenced by CT scan or MRI and endorectal ultrasound Distal border of the tumor must be at or below the peritoneal reflection (within 12 cm of the anal verge) by proctoscopic examination Measurable disease by x-ray, scans, or physical examination Available tumor tissue to determine molecular profile of the tumor before study treatment No clinical evidence of high-grade (lumen diameter < 1 cm) large bowel obstruction unless a diverting colostomy has been performed PATIENT CHARACTERISTICS: Age 18 and over Performance status Zubrod 0-2 Life expectancy Not specified Hematopoietic WBC ≥ 3,500/mm^3 Absolute neutrophil count ≥ 1,500/mm^3 Platelet count ≥100,000/mm^3 Hepatic Bilirubin ≤ 1.5 times upper limit of normal (ULN) SGOT or SGPT ≤ 2.5 times ULN Alkaline phosphatase ≤ 2.5 times ULN Renal See Disease Characteristics Creatinine ≤ 1.5 times ULN OR Estimated creatinine clearance > 50 mL/min Cardiovascular No significant cardiac disease No recent myocardial infarction Gastrointestinal See Disease Characteristics Able to swallow oral medication No active inflammatory bowel disease Other Not pregnant or nursing Fertile patients must use effective contraception No other malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer or carcinoma in situ of the cervix No prior unanticipated severe reaction to study drugs No known dihydropyrimidine dehydrogenase deficiency No serious uncontrolled infection No other serious medical illness that would preclude study treatment PRIOR CONCURRENT THERAPY: Biologic therapy Not specified Chemotherapy No prior chemotherapy for colon or rectal cancer Endocrine therapy Not specified Radiotherapy No prior pelvic radiotherapy No prior intra-operative radiotherapy or brachytherapy No concurrent intra-operative radiotherapy or brachytherapy No concurrent intensity-modulated radiotherapy Surgery See Disease Characteristics See Radiotherapy