Intensity-Modulated Radiation Therapy With Incorporated Boost and Capecitabine Before Surgery in Treating Patients With Locally Advanced Rectal Cancer

DISEASE CHARACTERISTICS: Histologically confirmed primary adenocarcinoma of the rectum Distal border of the tumor within 12 cm of the anal verge by proctoscopic exam Clinical stage T3-4, N1-2 (stage II or III) disease by 2 of the following tests: Physical exam Transrectal ultrasound Pelvic CT scan Pelvic MRI No clinical evidence of metastatic disease PATIENT CHARACTERISTICS: Age 18 and over Performance status ECOG 0-1 Life expectancy More than 3 months Hematopoietic Absolute neutrophil count ≥ 1,500/mm^3 Platelet count ≥ 100,000/mm^3 No known, uncontrolled coagulopathy Hepatic Bilirubin ≤ 1.5 times upper limit of normal (ULN) SGOT and SGPT ≤ 1.5 times ULN Alkaline phosphatase ≤ 2.5 times ULN Renal Creatinine ≤ 1.5 times normal Creatinine clearance > 50 mL/min Cardiovascular No clinically significant cardiac disease No congestive heart failure No symptomatic coronary artery disease No poorly controlled cardiac arrhythmias No myocardial infarction within the past year Gastrointestinal No active inflammatory bowel disease No lack of physical integrity of the upper gastrointestinal tract No malabsorption syndrome Other No other prior or concurrent malignancy except inactive, non-invasive carcinoma of the cervix or non-melanoma skin cancer No concurrent serious, uncontrolled infection(s) No prior unanticipated severe reaction to fluoropyrimidine therapy No known sensitivity to fluorouracil No prior uncontrolled seizures No CNS disorders that would preclude study participation No other medical or psychiatric condition that would preclude study participation Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception during and for 3 months after study participation PRIOR CONCURRENT THERAPY: Biologic therapy No prior immunotherapy for rectal cancer Chemotherapy No prior chemotherapy for rectal cancer Endocrine therapy Not specified Radiotherapy No prior radiotherapy for rectal cancer No prior pelvic radiotherapy Surgery More than 4 weeks since prior major surgery and recovered No prior surgery for rectal cancer Other More than 4 weeks since prior participation in another investigational drug study No concurrent celecoxib