Incidence of Leukemia in Atomic Bomb Survivors Belonging to a Fixed Cohort in Hiroshima and Nagasaki, 1950-71 Radiation dose, years after exposure, age at exposure, and type of leukemia
The leukemogenic effect of atomic radiation was examined in relation to age at the time of the bomb (ATB), calendar time, and type of leukemia over the period 1950-71. Confirmed cases of leukemia in the Leukemia Registry, a fixed cohort of 109, 000 subjects and the T65 dose calculations provided the basis for the analysis. Calendar time was divided into three periods, 5-10, 10-15, and 15-26 years after the bombs. The larger the exposure dose and the younger the age ATB, the greater was the effect in the early period and the more rapid was the decline in risk in subsequent years. In the oldest group, aged 45 or over ATB, the increase in risk appeared later and was sustained in the period 1960-71. Chronic granulocytic leukemia contributed substantially to the total leukemogenic effect initially but made little contribution after 1955. Sensitivity to the leukemogenic effect of atomic radiation not only depended on age ATB but its expression varied by type of leukemia and with time after exposure. Although the effect of atomic radiation on the incidence of leukemia in the atomic survivors is now greatly reduced and apparently on the wane, in the period 1966-71 the incidence was still greater than expected, especially in Hiroshima. In the Nagasaki sample, no case of leukemia was observed among the high-dose subjects from July 1966 to the end of 1971.