Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed rectal adenocarcinoma that is involving the distal 12 cm of
the rectum (above the anal verge)

- Staged within 8 weeks prior to initiation of study by endoscopic ultrasound OR
MRI or CT scan if endorectal ultrasound is non-conclusive or non-tolerable

- T3-T4 tumor or evidence of lymph node involvement defined by the presence of at
least 1 enlarged peri-rectal lymph node

- No evidence of distant or known metastases

PATIENT CHARACTERISTICS:

- ECOG performance status (PS) 0-1 OR Karnofsky PS 70-100%

- Life expectancy > 1 year

- Leukocytes ≥ 3,000/µL

- Absolute neutrophil count ≥ 1,500/µL

- Platelet count ≥ 100,000/µL

- Total bilirubin ≤ upper limit of normal (ULN)

- AST/ALT ≤ 2.5 times ULN

- Creatinine ≤ ULN OR creatinine clearance ≥ 60 mL/min

- Able to receive oral medication

- Not pregnant or nursing

- Negative pregnancy test

- Fertile patients must use effective contraception

- No other concurrent or previous malignancies unless disease free for > 5 years
(excluding nonmelanoma skin cancer)

- No neuropathy ≥ grade 2

- No history of allergic reaction attributed to compounds of similar chemical or
biologic composition to oxaliplatin, capecitabine, or selenomethionine

- No uncontrolled intercurrent illness including, but not limited to, any of the
following:

- Ongoing or active infection

- Symptomatic congestive heart failure

- Unstable angina pectoris

- Cardiac arrhythmia

- Psychiatric illness/social situations that would limit compliance with study
requirements

PRIOR CONCURRENT THERAPY:

- No prior radiotherapy to the pelvis

- No prior chemotherapy

- No other concurrent investigational or anticancer agents or therapies

- No concurrent vitamin B6 supplementation (except as part of a standard, multivitamin
supplement)