Epidemiological studies of leukemia in persons exposed to ionizing radiation.

After reviewing the epidemiological studies of persons exposed to ionizing radiation, the author concludes that there is no question that radiation exposure in man is asso ciated with an increased incidence of leukemia and that a cause-and-effect relationship must be accepted. Such an association holds in the higher dose range regardless of (a) sex and age, (6) the amount of body exposed (provided a substantial amount of bloodforming tissue is irradiated), and (c) whether or not the radiation was administered as a single dose or intermittently over a long period of time. In two studies which have an adequate sample size and a sufficient number of cases in each of several exposure groups to permit analysis, there is a proportionality between leukemia incidence and dosage in the middle dose range. Probably because of the small number of cases studied, such a relationship was not observed in children treated for thymic enlargement in infancy. The author believes that the available data are insufficient to permit conclusions to be drawn concerning the relationship between leukemia incidence and exposure to low doses of radiation. Thus, in the British series of spondylitics, only one case of leukemia (excluding a second case with extraspinal radiation) occurred in the relatively few pa tients receiving less than 500 roentgens to the spinal marrow. In the Japanese, uncer tainties regarding actual dose values as well as the small number of cases in the ex posure zones between 1,600 and 2,000 meters preclude extrapolation of the curve of dose-versus-incidence to the low dose range. The other studies reviewed in this paper do not lend themselves to analysis in the low dose range either because of the small number of cases or dose uncertainties or, in the case of prenatal exposures, because the role of radiation in the development of the disease has not been established. Although there is some suggestion that children are more sensitive to the leukemogenic effects of radiation than adults, the author concludes that the present retrospec tive studies of prenatal exposure of children dying of leukemia or cancer do not estab lish exposure to diagnostic doses of radiation as an etiologic factor. The difficulties of carrying out epidemiological studies in man to show an increase in a rare event such as leukemia have been pointed out. The size of the group needed for study, the long latent period between exposure and study and the choice of proper con trol groups present serious problems, especially in the low dose range. In view of our need for information about radiation hazards in the low dose range, it is emphasized that restraint must be exercised to avoid reading more significance into the available data than is justifiable. This does not mean that the possibility of radiation hazards existing at low doses should not be recognized. Clearly, unnecessary exposure of large groups of people should be reduced to a minimum.