Neoadjuvant Celecoxib and Capecitabine Combined With Pelvic Irradiation in Treating Patients With Stage II or Stage III Adenocarcinoma (Cancer) of the Rectum

DISEASE CHARACTERISTICS: Histologically confirmed rectal adenocarcinoma Clinical stage T3, N0, M0 OR any T, N1-3, M0 disease Treatment with neoadjuvant chemotherapy and pelvic radiotherapy is indicated All disease must be encompassable within standard pelvic radiotherapy fields Distal border of the tumor must be at or below* the peritoneal reflection, defined as within 12 cm of the anal verge by endoscopy NOTE: *If a portion of the tumor is below the peritoneal reflection at the time of surgery, patients are eligible regardless of the distance of the tumor determined at endoscopy Tumor must be determined to be clinically resectable Tumor may not be clinically fixed Negative margins by routine examination of an unanesthetized patient Transmural penetration of tumor through the muscularis propria by CT scan, endorectal ultrasound, or MRI No distant metastatic disease No evidence of tumor outside the pelvis, including any of the following: Metastatic inguinal lymphadenopathy Peritoneal seeding Liver metastases PATIENT CHARACTERISTICS: Age 18 and over Performance status ECOG 0-1 Life expectancy At least 6 months Hematopoietic Absolute neutrophil count ≥ 1,500/mm^3 Platelet count ≥ 100,000/mm^3 Hepatic Bilirubin ≤ upper limit of normal (ULN) AST ≤ 3 times ULN Alkaline phosphatase ≤ 4 times ULN if AST < ULN Renal Creatinine clearance ≥ 30 mL/min No renal impairment Cardiovascular No congestive heart failure No symptomatic coronary artery disease No uncontrolled cardiac arrhythmias No myocardial infarction No history of transient ischemic attacks or stroke No other clinically significant cardiac disease Gastrointestinal No bleeding peptic ulcer disease within the past 12 months No lack of physical integrity of the upper gastrointestinal tract No malabsorption syndrome No active inflammatory bowel disease Must be able to swallow study drugs Other No dihydropyrimidine dehydrogenase deficiency No history of uncontrolled seizures No CNS disorders No clinically significant psychiatric illness that would preclude study compliance or giving informed consent No other malignancy within the past 5 years except basal cell or squamous cell skin cancer or carcinoma in situ of the cervix No known sensitivity to NSAIDs, sulfonamides, or aspirin No other serious medical illness that would preclude study treatment No other conditions that would preclude study participation Must be able to tolerate major surgery that may include abdominal-perineal resection Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception during and for 30 days after study treatment PRIOR CONCURRENT THERAPY: Biologic therapy Not specified Chemotherapy See Disease Characteristics No prior systemic anticancer chemotherapy Endocrine therapy Not specified Radiotherapy See Disease Characteristics No prior radiotherapy to the pelvis Surgery See Disease Characteristics More than 3 weeks since prior major surgery and recovered Other At least 7 days since prior nonsteroidal anti-inflammatory drugs (NSAIDs), including aspirin No other concurrent investigational drugs No other concurrent anticancer treatment No concurrent NSAIDs No concurrent primary prophylactic therapy for hand-foot syndrome No concurrent loperamide prophylaxis for diarrhea No concurrent sorivudine or brivudine