eit eR ae ak neoa a ae rt ee oenee a” Se to -3preceding few days or weeks. Iodine not absorbed by the thyroid will be rapidly excreted, primarily in the urine. Thus, in a short time after a single intake, the gland will contain the highest concentration of the iodine remaining in the body. Because of its great avidity for radio- iodine the thyroid will receive the largest radiation dose, particularly if the form is iodine-131: The radiation dose delivered to other tissues will be, at most, a few percent of that received by the thyroid gland. A concentration of 1 pe of iodine-131 per gram of thyroid tissue will produce_a time-integrated dose of about 113 rads ,° ranging from 90 to 130 rads,’ About 10 percent of this dose will be received in the first 24 hours, probably over 25 percent in the rirst 2 days. When the thyroid is included in the beam from an external radiation source, such as an X-ray tube, a considerable amowmt of surrounding tissue will necessarily receive doses equal to or even greater than that received by the gland itself. Differences in growth rate and histology of the thyroid from birth through early childhood have led some to believe that the infant thyroid is more vulnerable than that of the adult to injury from ionizing radia- tion. In addition to differences in size and proportion of proliferating cells of the thyroid in infants, children, and adults, there may be significant alterations in absorption, metabolic turnover, and cell sensiti- vity with advancing age. Any of these factors might affect the amowt of biological damage resulting from a given radiation dose. Neither the relative importance nor the aggregate effect of these variables is known, HUMAN EXPERIENCES 1. Direct Effects of Iodine-131l It has been pointed out that iodine-131 entering the body is promptly and selectively concentrated in the thyroid gland, or is excreted, The direct radiobiological effects are thus confined to the gland itself. The principal effects, which have been observed only after high doses of radioactive iodine, include a decrease in the metabolic activity of the gland, and at still higher doses, an atrophy of epithelial cells, and the appearance of numbers of atypical cells. These changes in function and structure are seen only after doses of thousands of rads. Thyroid cancer is an uncommon result, described with certainty to date only in animals, CReport of the International Commission on Radiological Units and Measurements (ICRU), 1959. National Bureau of Standards Handbook 78, January 16, 1961, Washington, D. C. ‘silver S.: Radioactive Isotopes in Medicine and Blology. Lea and Febiger, Philadelphia, 2nd ed., 1962, page 117. DOE ARCHIVES Medicine.