DISEASE CHARACTERISTICS: Histologically confirmed unresectable, incompletely resected with gross residual disease, or locally recurrent large bowel carcinoma confined to a site within the abdomen or pelvis All disease must be encompassable within a single radiotherapy port No evidence of uncontrolled metastatic disease outside of the planned radiotherapy port

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: CALGB 0-2 Life expectancy: Not specified

Hematopoietic: Absolute neutrophil count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hepatic:

Bilirubin no greater than 2.0 mg/dL Renal: Not specified Cardiovascular: No history of significant myocardial disease No New York Heart Association class III or IV disease No unstable angina No myocardial infarction in the past 4 months Other: No significant infection or other coexistent medical condition that would preclude protocol therapy Maintain an adequate oral nutrition intake (at least 1,200 calories estimated per day) No greater than 6 bowel movements per day prior to treatment No significant nausea or emesis on optimal antiemetic therapy Not pregnant or nursing Effective contraception required of all fertile patients

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior irinotecan Endocrine therapy:

Not specified Radiotherapy: No prior abdominal or pelvic radiotherapy Surgery: At least 3 weeks since laparotomy or laparoscopic procedure with or without resection