Leukemia in Mice Following Exposure to X-Rays

The possible causal relation between exposure to x-rays and the occurrence of leukemia has been studied in only a few instances. For this report we shall follow the procedure of Richter and MacDowell (1) and use the term leukemia in the broader sense to cover not only the myeloid, lymphoid, and monocytic forms, but also lymphomatosis, lymphosarcoma, lymphadenosis, and pseudoleukemia. Krebs, Rask-Nielsen, and Wagner (2) appear to have been the first to carry out investigations on the induction of leukemia by x-rays. With single whole body exposures of 400 r, given to large numbers of mice, the incidence was raised from 0.06 in the controls to 0.35 per cent. Thus, while the actual incidence was still low, the increase amounted to nearly six-fold. Furth (3) gave single whole body treatments of approximately 400 r and found a slight but nevertheless definite increase in leukemia of both myeloid and lymphoid types. Hueper (4), using hard x-rays, gave repeated general body treatments to mice, doses up to 480 r being administered in a six-week interval. He observed an incidence as high as 74 per cent in animals in which, he states, the normal incidence was 19 per cent. Furth and Furth (5) irradiated the whole bodies of mice with single or repeated doses of 200 to 400 r and observed an increase in myelosis as high as eight-fold, and of mediastinal lymphomatosis as high as seven-fold. The actual incidence of lymphomatosis and myelosis, however, did not exceed 13.9 and 7.6 per cent, respectively. Descriptions have been given and summaries made (6) of the occurrence of leukemia in persons occupationally exposed to high-energy radiations. Similarities have been pointed out between radiation-induced lesions of the skin and those in hematopoietic organs which later become neoplastic. The plausibility of the idea that radiation may induce leukemia has been openly discussed, but as yet adequate statistical evidence showing a greater occurrence of leukemia among exposed persons is lacking. Henshaw and Hawkins (7) have found the incidence of leukemia in physicians to be approximately 1.7 times greater than in white males of the general population. On the assumption that physicians as a group are more exposed to high-energy radiations than white males of the general population, this observation may be said to be in accord with the findings with experimental animals, but since the higher incidence of leukemia in physicians may be accounted for by other factors, radiation is not fully indicted as the causal agent. In the studies with animals previously reported, doses of 200 to 400 r were used, and not much attention was given to the number or frequency of treatments.