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Among the Japanese radiological technicans, four deaths from aplastic anemia have been reported after 1930. On the other hand, during the period from 1930 to 1960, the population of radiological technicians is estimated to be 74,400 man-years, in which 0.5 aplastic anemias are expected. However, actually three died from aplastic anemia. This difference is statistically significant at the 1% level. While, in the period from 1961 to 1973, the observed value is 1 against 0.7 expected. It is concluded that aplastic anemia had been induced frequently among the Japanese radiological technicians in the era when there was much exposure to occupational radiation.

A model of tumor regression during the course of radiotherapy is presented, which considered the lethal effect of radiation, the elimination of dying cells and the reproduction of surviving cells. The model was applied to multiple lung metastases in a woman, which were treated with different doses per fraction. The regression of tumor volume was measured from chest radiographs which were taken once a week during the treatment. Radiosensitivity of tumor cells estimated by the model was of D0=150 R and n=5. The half life of dying cells in the tumor was 15 days.