Inclusion Criteria

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