OBJECTIVES: Primary * Determine the maximum tolerated dose of neoadjuvant boost intensity-modulated radiotherapy when combined with capecitabine before surgery in patients with locally advanced rectal cancer. Secondary * Determine the pathologic tumor response in patients treated with this regimen. * Determine the quality of life of patients treated with this regimen. OUTLINE: This is a dose-escalation study of boost intensity-modulated radiotherapy (IMRT). Patients undergo neoadjuvant IMRT with incorporated boost once daily 5 days a week for 5 weeks. Beginning on the first day of radiotherapy, patients receive oral capecitabine twice daily 7 days a week for 5 weeks. Patients undergo surgical resection 4-8 weeks after completion of chemoradiotherapy. Cohorts of 3-6 patients undergo escalating doses of boost IMRT until the maximum tolerated dose (MTD) is determined. The MTD is defined as the dose preceding that at which 2 of 3 or 3 of 6 patients experience dose-limiting toxicity. Quality of life is assessed at baseline, at week 5 of chemoradiotherapy, before surgery, and then at 1, 3, and 12 months after surgery. Patients are followed at 1, 3, and 12 months after surgery. PROJECTED ACCRUAL: Approximately 3-15 patients will be accrued for this study.
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