Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed adenocarcinoma of the rectum

- MRI-defined locally advanced disease, as defined by 1 of the following:

- Mesorectal fascia involvement

- Mesorectal fascia threatened (tumor ≤ 1 mm from mesorectal fascia)

- Any T3 tumor < 5 cm from anal verge

- No evidence of metastatic disease

PATIENT CHARACTERISTICS:

- ECOG or WHO performance status 0-1

- ANC ≥ 1.5 x 10^9/L

- Platelet count ≥ 100 x 10^9/L

- Serum bilirubin < 1.25 times upper limit of normal (ULN)

- Serum transaminase(s) < 3 times ULN

- Serum alkaline phosphatase < 5 times ULN

- Estimated glomerular filtration rate > 50 mL/min

- Not pregnant or nursing

- Fertile patients must use effective contraception

- Fit to receive all study treatments

- Able to comply with oral medication

- No comorbidity or coagulation problem that would deem the patient unsuitable for
surgery

- No pre-existing condition that would preclude radiotherapy (e.g., fistulas, severe
ulcerative colitis [particularly patients currently taking sulfasalazine], Crohn's
disease, prior adhesions)

- No current or impending rectal obstruction (unless a defunctioning stoma is present)
or metallic colonic rectal stent in situ

- No significant small bowel delineated within the radiotherapy fields

- No pelvic sepsis

- No gastrointestinal disorder that would interfere with oral therapy or oral
bioavailability

- No uncontrolled cardiac, respiratory, or other disease that would preclude study
therapy or informed consent

- No serious medical or psychiatric disorder that would preclude study therapy or
informed consent

- No known dihydropyrimidine dehydrogenase deficiency

PRIOR CONCURRENT THERAPY:

- No prior chemotherapy

- No prior radiotherapy to the pelvis

- No concurrent participation in other studies, except genetic studies (e.g.,
NSCCG-National Study of Colorectal Cancer Genetics)

- No concurrent St. John wort

- No other concurrent cytotoxic treatment or radiotherapy