SWOG-9304 Chemotherapy Plus Radiation Therapy in Treating Patients With Rectal Cancer That Has Been Surgically Removed

DISEASE CHARACTERISTICS: Histologically proven stage II or III adenocarcinoma of the rectum Tumor extends through the bowel wall and into perirectal fat or soft tissue (TNM T3-4, N0, M0) Nodes are involved with tumor (TNM T1-4, N1-3, M0) Tumor completely resected en bloc with no gross or microscopic evidence of residual disease Circumferential (radial) margins of resected adherent tumors must be specifically documented free of disease (with the sole exception of extraperitoneal serosal margins) No evidence of metastasis No regional nodal metastases (metastases outside of the pelvis) that cannot be resected en bloc with the primary lesion No distant peritoneal metastases (metastases that are not a direct extension from the primary tumor) even if grossly resected (direct extension into another structure permitted) Abdominopelvic CT required unless: Bilirubin, SGOT, and alkaline phosphatase are within normal limits, AND Operative report describes liver as normal on exploration No tumors of colonic origin, i.e.: Lower edge of the tumor is below the peritoneal reflection or a portion of the tumor is retroperitoneally located (usually posteriorly) as defined by the surgeon at laparotomy OR Lower margin of the tumor is 12 cm or less from the anal verge by proctoscopic exam No prior history of rectal cancer No stage II or III cancers of the extrapelvic colon within the past 5 years Complete surgical resection at least 5 years prior to protocol registration allowed provided no other therapy was administered Synchronous modified stage I or IIa colorectal cancer (no nodal involvement or penetration through the muscularis propria) that has been completely resected allowed Registration between 20 and 70 days after the definitive surgical procedure required Chemotherapy must begin no later than day 70 following surgery Concurrent registration on protocol SWOG-9419 allowed for patients with adequate tissue samples PATIENT CHARACTERISTICS: Age: Over 18 Performance status: SWOG 0-2 Hematopoietic: WBC at least 4,000/mm3 Platelet count normal Hepatic: Bilirubin no greater than 2 times upper limit of normal (ULN) SGOT no greater than 2 times ULN Alkaline phosphatase no greater than 2 times ULN Renal: Not specified Other: No chronic ulcerative colitis No other serious medical illness that would preclude protocol therapy No psychiatric condition that would preclude informed consent No noncolorectal malignancy within 5 years except: Adequately treated nonmelanomatous skin cancer Adequately treated carcinoma in situ of the cervix Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy: No prior immunotherapy Chemotherapy: No prior chemotherapy Endocrine therapy: Not specified Radiotherapy: No prior radiotherapy Surgery: See Disease Characteristics Other: No other concurrent antineoplastic therapy