Radiation Therapy Plus Fluorouracil With or Without Additional Chemotherapy in Treating Patients With Primary Anal Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed primary epidermoid anal cancer * Squamous cell * Basaloid * Cloacogenic * No adenocarcinoma, malignant melanoma, mucoepidermoid carcinoma, lymphoma, or microinvasive anal intraepithelial neoplasia (without evidence of invasive disease) in the anal canal or margin * No metastatic disease PATIENT CHARACTERISTICS: Age: * Not specified Performance status: * 0-2 Life expectancy: * Not specified Hematopoietic: * WBC greater than 3,000/mm\^3 * Platelet count greater than 100,000/mm\^3 * Hemoglobin greater than 10 g/dL Hepatic: * Liver function tests no greater than 2 times normal Renal: * Glomerular filtration rate at least 50 mL/min Cardiovascular: * No cardiovascular disease * No uncontrolled angina pectoris * No heart failure * No clinically significant cardiac arrhythmias Other: * HIV negative * No other significant concurrent illness * Not predominately bed-bound or frail * No severe sepsis * No other prior or concurrent cancer or illness that would preclude study participation PRIOR CONCURRENT THERAPY: Biologic therapy: * Not specified Chemotherapy: * Not specified Endocrine therapy: * Not specified Radiotherapy: * No prior radiotherapy to pelvis Surgery: * Not specified Other: * No prior therapy for anal cancer