The Development of Neoplasms in Irradiated Animal and Human Thyroids From the world wide experience now available, it seems clear that the Bevelopment of a neoplasm, especially a malignant one, is not a common sequella of 1317] irradiation to the thyroid in adults. Not enough is known about their occurrence in chpldren because few children have been so treated. The reports of Sheline, Lindsayland their associates suggest that the occurrence of nodules following 13 I is considefably more frequent in children than adults. OthersSsuch as Crile and his associates agd Starr and his associates have no concern about the possibility of the development of geoplasms the doses used are so large and cellular replication so impaired that the cliances of in children. Because of continued concern, the mmber of children treated pith ly remains small. Thus adequate experience is not yet available. Although thd occurrence of necplasms may be very low following 1511 treatment for hyperthyroidism 14 may be that neoplasm formation is almost precluded. The experimental designs of the studies to be described have taken intd consideratiam several factors about which we mst learn more. 1.) The age at which neoplsms are most likely to be produced. 2.) The dose of 131] which is sufficiently disruptihg to the cell to induce abnormal replication but not so great that it destroys the cabacity for replication. Presumably the most effective doses which produce neoplasms lif between the equivalent of the tracer dose and the therapeutic dose used in man. 3.)] Determine what other factors might promote neoplasm formation. 4.) Observe neoplasms during the early stage of their development and the related features in the remainder of which are giving rise to neoplasms. the gland A number of large series of long-term rat experiments have been set up fiuring the latter half of this contract to study the development of neoplasms of thyroid in rats treated with various doses of It is well known from our own bxperiments and those of others that thyroid neoplasms in the raf require more than 4 ear to develop after continuous administration of a goitrogen or I. Hence the problem of housing and the cost of feeding have been seriously limiting factors. In addition, very large series of animals with many controls has been necessary because of losses and (in our earlier experience) a rather low incidence of necplasms. fhe need for natural ]Far these reasons and the limitation of funds, only one series of animals has been posgible at a time. In the past there has been controversy concerning the ease with which ngoplasms may be produced in animals with 1317, One of the failings in mst of_the published experiments designed to test neoplasm formation in animals given 131r has bean the primary attention to how much of the isotope was given to the animals and nog how much and how long it was retained. It has become apparent over a long period of qxperimentatia that the amount retained in the thyroid may be enormously different than that] anticipated from the amount injected and the variations in the amounts retained amng anifmls may be very great. As a result we have resorted to individually identifying each arfimal and Measuring its uptake and retention by in vivo amd in vitro counting. Weighing the gland of representative animals to determine a mean thyroid weight has contributed [bo the reliability of calculating rad dose. The same basic experimental pattern has been followed on 5 series of rat over the past 10 years. The same strain of Sprague-Dawley Rats from Charles River Brebding Colony have been used exclusively through the past 5 years. A Remington diet}has been