“Volume 6) Effects of ionizing radiation Number + tion of elements of the radium and thorium series, of potassium-+0 and of carbon-14, provides radiation exposure from internal sources in dose rates estimated to be 126 mrem per year to the gonads, 130 mrem peryearto the cells lining bone surfaces, and 122 mrem per year to the hematopoietic tissues. Medical diagnostic radiology constitutes another source of inevitable low-level irradiation, the magnitude of which has been the subject of recent inquiry.°% ° 9? The annual genetically significant dose received by an individual in the United States from diagnostic roentgenologic procedures has been estimated to be 50 + 25 mrem minimum and 140 + 100 mrem probable.'* Although estimates of bone marrow dose are based on sparse data and assumptions, the UNSCEAR in two reports has suggested that the estimate of the population per capita dose “might be of the order of 50 to 100 mrem/ y.’"'§ The radiation exposure from radio- isotopes in pediatric patients for a numberof diagnostic tests has also been calculated.*° Adequate information on effects of low doses in both man and experimental animals is lacking. In 1959, Brues®** commented that the subject of effects of low-level irradiation concerned “hazards which, if they exist, cannot possibly be demonstrated to exist because they are relatively so small.’’ Upton,®® in re- viewing radiation carcinogenesis, stated that 663 was significantly increased by pelvic irradiation of the mother during the child’s intrauterine phase.’ These findings were supported by data from several other studies. 3, 33, 97 MacMahon? has reported the results of a study of 734,243 children born in and discharged alive from 37 large maternity hospitals in the northeastern part of the United States from 1947 through 1954. For each of three categories, leukemia, neoplasms of the central nervous system, and other neoplasms, the cancer rate was found to be “about 40 percent higher in the X-rayed than in the unX-rayed members of the study population. The excess cancer mortality in the X-rayed group was most marked at ages 5 through 7 years, at which time the relative risk was 2.0. The excess risk apparently was exhausted by age 8.” MacMahon hasestimated that the probability of death from leukemia for white children in the United States up to the age of 10 years will be increased from 46 per 100,000 children to 62 per 100,000 children by prenatal irradiation.*” In a_ prospective approach,®* 43,742 women who between 1945 and 1956 re- ceived pelvic irradiation during pregnancy were identified from the records of selected hospitals (Edinburgh and London) and the subsequent deaths from leukemia of the chil- “existing data... are not adequate to permit confident estimation of the risks of small increases in background radiation.” Somatic effects. The report by Stewart dren of these pregnancies were then investi- diation of the pregnant mother wasassociated with subsequent development of leukemia and other malignant neoplasms in the child who was exposed in utero triggered a number of similar epidemiologic studies. In sub- indicated that ‘there was no evidence of any disproportionate occurrence of leukaemia among the children who had been most and associates*® that diagnostic pelvic irra- sequent communications, the original con- clusions were confirmed and amplified.? ? The study attempted to trace all children in England and Wales who had died of leukemia (792 cases) or other cancer (902 cases) before the tenth birthday during 1953-1955. It was concluded that the risk of subsequent malignant changes in the child gated. Court-Brown, Doll, and Hill,°* in this study, found 9 instances of leukemia among 39,166 liveborn children when 10.5 was the estimated expected number. The study also heavily irradiated nor among the children whohad beenirradiated early in intrauterine life.” Data not in accord with those of Stewart and of MacMahon have also been reported from several other studies.** *° Although the exposure dose cannot be precisely determined in these studies,‘” the reported association between prenatal pelvic irradiation at diagnostic dose levels and increased leukemogenesis (and carcinogenesis)