Follow-up studies of persons exposed to medical radiation have long shown that radiation induces cancer in man. This, coupled with increasing exposure from other sources including occupational and environmental radiations, has resulted in greater recognition of the importance of research on radiation-induced carcinogenesis and risk assessment with a view to radiation protection. One of the well-known late effects of radiation is the increased incidence of leukemia that occurred among atomic bomb survivors beginning two or three years after expo- sure. A remarkable increase of solid tumors...