APPENDIX A 



Interim Statement of the Subcommittee on Acute and 

 Long-Term Hematological Effects 



The Subcommittee met in St. Louis, Missouri, November 20-21, 1959. The problems dis- 

 cussed were ( 1 ) detection of the effects of low-doses of radiation by hematologic means, and 

 (2) the quantitative relationship between exposure to radiation and the increased risk of 

 leukemia. The present acceptability of the 1956 Subcommittee Report* was also discussed; 

 it was the unanimous opinion that, rather than undertake an extensive revision, a new report 

 should be prepared summarizing the current status of the problems cited, and incorporating 

 any new information that is available. 



The Subcommittee agreed unanimously that chronic exposure to radiation is a major 

 problem. Serious efforts should be made to determine the total morbidity and mortality asso- 

 ciated with continuous or repeated exposure to small amounts of ionizing radiation. Estimates 

 of the risk of leukemia (and other hematologic diseases) should be based on the best data 

 available at the time in question. Hopefully, successive approximations, based on data which 

 are more and more nearly complete, will make it possible to estimate risks after permissible 

 doses with greater assurance than is presently possible. Leukemia is an appropriate subject 

 on which to base estimates of maximum permissible dose because some quantitative human 

 data are available. However, the data are limited to ( 1 ) the effects of acute single doses in 

 the Japanese, and (2) the effects of therapeutic irradiation for spondylitis. The presently 

 available data on radiologists and on children in whom the thymus was irradiated confirm 

 the fact that radiation is leukemogenic, but contributes little to an understanding of the dose- 

 effect relationship. The leukemia which has occurred in a small fraction of patients treated 

 with I"' needs to be evaluated critically. The cardinal advantage of the human leukemia data 

 is that some quantitative data, incomplete though they may be, are available on which to base 

 predictions. It is possible to estimate the increased risk of leukemia following single doses in 

 the range of approximately 50-100 rads to the highest dose after which there is survival. There 

 is uncertainty as to the absolute value of estimates of doses at Hiroshima and Nagasaki. The 

 50-100 rads mentioned may be significantly revised in the future with the acquisition of new 

 data. Predictions of this sort are not at the present time as practical with respect to life short- 

 ening, other cancer, and cataracts. 



Prediction requires evaluation of the possibility that there is a threshold dose below 

 which there is no probability of inducing leukemia, a concept which implies a factor of safety 

 that would be most reassuring to those who are exposed to radiation in excess of the natural 

 background as well as to those who must make policy decisions. Some members of the Sub- 

 committee believe, on the basis of analogy to radium data and chemical poisons, that there 

 must be a threshold; however, no member of the Subcommittee feels that he can estimate the 



NAS — NRC Publication 452, Pathologic Effects of Atomic Radiation, 1956. 



34 



