Radiation Therapy With or Without Chemotherapy Before Surgery in Treating Patients With Stage II or Stage III Rectal Cancer

OBJECTIVES: * Compare the recurrence-free and overall survival of patients with resectable stage II or III adenocarcinoma of the rectum treated with neoadjuvant radiotherapy with or without concurrent chemotherapy comprising fluorouracil and leucovorin calcium followed by surgery and adjuvant fluorouracil and leucovorin calcium. OUTLINE: This is a randomized, controlled, multicenter study. Patients are stratified according to participating center, gender, location of the tumor relative to the anal margin (0-5 cm vs \> 5 cm), and tumor stage (T3 vs T4). Patients are randomized to 1 of 2 treatment arms. * Neoadjuvant therapy: Patients are randomized to 1 of 2 neoadjuvant therapy arms. * Arm I: Patients receive leucovorin calcium IV and fluorouracil IV over 15 minutes on days 1-5. Treatment repeats every 28 days for 2 courses in the absence of disease progression or unacceptable toxicity. Patients also undergo concurrent radiotherapy 5 days a week for 5 weeks. * Arm II: Patients undergo radiotherapy as in arm I. * Surgery: In both arms, patients undergo surgical resection within 3-10 weeks after completing neoadjuvant therapy. * Adjuvant therapy: Within 3-10 weeks after surgery, all patients receive adjuvant chemotherapy comprising leucovorin calcium IV and fluorouracil IV over 15 minutes on days 1-5. Treatment repeats every 21 days for 4 courses in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed every 6 months for at least 5 years. PROJECTED ACCRUAL: A total of 762 patients will be accrued for this study.