- Radiotherapy: No prior radiotherapy within four weeks of study entry. No prior external beam irradiation to a field that includes more than 30% of the red marrow. No prior radiation to maximal tolerable levels for any critical organ (e.g., 3,000 cGy for the liver, and 2,000 cGy for the lungs and kidneys). Patients who have had standard pelvic field radiation as adjuvant therapy for rectal carcinoma will be eligible only after the MTD is established.