PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No concurrent leucovorin or other chemotherapy agents, except 5-FU with or without levamisole Endocrine therapy: Not specified Radiotherapy: See Disease Characteristics No prior pelvic radiotherapy No brachytherapy prior to completion of all external beam radiotherapy Surgery: No abdominal-perineal resection, Hartmann procedure, or other surgical procedure that has left patient without a functioning rectum