~ ~ soe =e - vt eh ee tea ee ar oe . + ghee a ecm © vee ge ~6- From this and other evidence, it appears that iodine-131 is consid- erably less effective than comparable doses of externally applied X-rays in producing thyroid cancer. The magnitude of this difference cannot be stated since not-a single confirmed case ascribable to iodine-131 is known. In making a comparison it must be kept in mind that X-rays are ordinarily delivered at a high dose rate in a single sitting or in a few divided doses; and the thyroid as well as the blood vessels supplying it and the adjoining tissues are uniformly exposed. In addition the age of the patients treated and their clinical conditions are rarely the same in the two types of treat- ment, If the radiation were administered at the same low dose rate for both iodine-131 and X-ray, the ostensible difference might not be as great. 6. Reactor Accident In a reactor accident, radioactive iodine was released to the environmentt, and control measures were instituted to prevent human thyroid doses from exceeding 20 rads. No increase in the incidence of thyroid neoplasia in this area has been reported, but it must be remembered that the complete effects of this episode will not be kmown for many years. 7. Total Radiation Doses in Childhood Cancer of the Thyroid Although thyroid cancer occurs in children who are not known to have been exposed to artificially produced radiation, some history of X-ray exposure has been obtained by several workers in well over half the cases in which careful inquiry has been made, The X-ray doses received by most of these subjects are difficult to determine, but by tracing the records of radiologists it appears that the smallest doses associated with subse- quent cancer range from about 100 r measured in air upwards. Only a very small number at these low levels have been reported. 8. / Thyroid Abnormalities in a Marshallese Population Exposed to Radioactive Fallout The development of thyroid nodules and hypothyroidism has been noted in a number of Marshallese people of Rongelap Island in the Pacific who had been exposed to radioactive fallout in 1954.9 This development is believed to be a late effect of irradiation of the thyroid gland from internal absorption of radioiodines and from external gamma radiation at the time of the fallout. A total of 64 Rongelap people (54 living in 1966) received a whole body dose of gamma radiation of 175 rads, extensive direct irradiation of the skin from deposition of fallout thereon and internal absorption of radionuclides in the fallout. Eighteen other Rongelap people (14 living in 1966) received a lesser exposure of about 70 rads of gamma radiation along with a smaller dose to the skin and internal absorption of less amounts of radioelements. A total of 157 individuals on the island of Utirik received a whole body gamma dose of approximately 14 rads, %conard, R. A., Rall, J. E., and Sutow, W. W.: Thyroid Nodules as a Late Sequela of Radioactive Fallout. New England J. Med. 274:1392-1399, 6/23/66. DOE ARCHIVES