RATIONALE: Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Radiation therapy uses x-rays to damage tumor cells. Drugs such as capecitabine may make the tumor cells more sensitive to radiation therapy.

PURPOSE: Phase I trial to study the effectiveness of capecitabine in combination with radiation therapy in treating patients who have unresectable, residual, or recurrent colorectal cancer located in the pelvis.

OBJECTIVES: I. Determine the maximum tolerated dose of oral capecitabine administered in combination with pelvic radiotherapy in patients with unresectable, residual, or locally recurrent cancer localized in the pelvis. II. Determine the tolerance of this regimen in these patients. III. Determine the preliminary evidence of therapeutic acticivity of this regimen in these patients.

OUTLINE: This is a dose-escalation study of capecitabine. Patients receive oral capecitabine twice a day 7 days a week for 6 weeks with concurrent radiotherapy. Radiotherapy is initiated on the same day as the initiation of capecitabine and is administered 5 days a week for 5.5-6 weeks. Cohorts of 3-6 patients receive escalating doses of capecitabine until the maximum tolerated dose (MTD) is reached. The MTD is defined as the dose at which 2 of 3 or 6 patients experience dose-limiting toxicity. Patients are followed every 3 months for 2 years and then every 6 months for 1 year.

PROJECTED ACCRUAL: A maximum of 51 patients will be accrued for this study.

DISEASE CHARACTERISTICS: Histologically confirmed carcinoma located in the pelvis that is unresectable, incompletely resected, or locally recurrent Must be biopsy-proven or have radiological progression All disease must be encompassable within standard pelvic radiotherapy fields No evidence of metastatic disease outside of the planned radiotherapy field

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-2 Life expectancy: Not specified Hematopoietic: WBC at least 3,500/mm3 Platelet count at least 100,000/mm3 Hepatic: Not specified Renal: Creatinine clearance at least 30 mL/min Other: Must maintain adequate oral nutrition (i.e., at least 1200 calories estimated intake per day) No significant infection or other concurrent medical condition that would preclude study No significant nausea or vomiting No malabsorption secondary to short-gut syndrome, Crohn's disease, or sprue No patients with more than 6 bowel movements per day or baseline no greater than grade 3 diarrhea for patients with colostomy Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior chemotherapy for metastatic cancer At least 6 months since prior adjuvant chemotherapy Endocrine therapy: Not specified Radiotherapy: See Disease Characteristics No prior radiotherapy Surgery: At least 21 days since prior laparotomy or laparoscopic procedure with or without resection Other: No concurrent sorivudine or chemically related analogues (e.g., brivudine)