PATHOLOGIC EFFECTS 35 



size of the threshold or, for that matter, even prove its existence. Accordingly, the Subcom- 

 mittee believes it is prudent to assume that there is no threshold. 



The Japanese single-dose whole-body data are sufficiently extensive with respect to size 

 of the population at risk and the range of doses to be useful for predicting the incidence of 

 leukemia after single doses. In this range, the incidence increased with dose and the relation- 

 ship is approximately linear. Among the Japanese exposed to doses believed to be less than 

 50-100 rads, the occurrence of leukemia since 1945 is not significantly greater than in the 

 general population. Since this group of survivors (i.e., less than 100 rads) is the largest group 

 available for study, the failure to observe an increase in the rate of leukemia is of interest. In 

 the range of exposure dose where leukemia was increased, the relationship to dose can be 

 expressed as equivalent to 1 case per 10" people at risk per rad per year. The Subcommittee 

 feels that, in this dose range, fairly accurate predictions can be made after single doses of 

 radiation, and for the time interval (about 15 years) corresponding to the interval since ex- 

 posure in Japan. The Subcommittee is not willing to accept the assumptions of E. B. Lewis 

 and others that: (1 ) there will be a continued constant incidence of leukemia per rad ex- 

 posure for the duration of life, (2) the incidence will be identical for acute and chronic expo- 

 sure (i.e., no dose-rate dependence), and (3) it is possible to predict the incidence of leu- 

 kemia on the basis of estimates of the absorbed dose due to radioisotopes such as SR*"""" or 

 radium deposited in the bones. 



Available studies of man and of experimental animals, following chronic exposure to 

 external sources of radiation or after the internal deposition of radioactive materials, provide 

 no basis for predictions of the incidence of radiation-induced leukemia. In fact, the data of 

 Mole on the induction of leukemia in mice suggest that chronic exposure at low dose rate is 

 less efficient than acute exposure in this respect. 



The majority of the cases of leukemia attributed to radiation in the Japanese survivors 

 and in the British spondylitics have been of the myelocytic variety. Estimates of the radiation 

 dose to the bone marrow carry the implication that it is the vulnerable tissue. It is of interest 

 that no cases of chronic lymphocytic leukemia in man have been attributed to prior radiation 

 exposure. It is possible that some of the cases of leukemia in children subjected to irradiation 

 of the thymus in infancy may have been of the acute lymphocytic type. Myelocytic leukemias 

 are presumably induced only by irradiation of large segments of the bone marrow. 



At the present time it is not possible to attribute to radiation exposure the increased in- 

 cidence in leukemia which has been reported in several countries including the United States. 

 One reason is the fact that a substantial portion of the increased incidence is due to chronic 

 lymphocytic leukemia, the occurrence of which has not yet been shown to be influenced by 

 ionizing radiations. Another reason is the fact that there are many toxic substances in the 

 modern environment which have not been evaluated adequately with respect to their influence 

 on leukemogenesis even though some of them (e.g., benzpyrene, arsenic, etc.) are carcino- 

 genic in laboratory animals and in man. 



Another problem of interest to the Subcommittee is the possibility of detecting low doses 

 of radiation by hematologic means. This is a very important, unsolved problem which the 

 Subcommittee believes to have a high priority. To date the enumeration of binucleated lym- 

 phocytes appears to be the best available indicator. However, at the present stage of develop- 

 ment it is not very practical for widespread application because of the large number of cells 

 that must be scored visually. There are some indications that electronic methods of scanning 



