different tendency for mitotic activity will be quite evident. After review of many thyroid preparations it appears that labeling is more often seen in areas|where normal follicles have given way to minute microfollicles or to solid cellular argas beneen follicles. The preliminary impression is gained that these areas repre of “nodules” or actual neoplasms. igi Such an example is seen in Figure 19A histologic pattern was seen in at least 2 of the Marshallese on whom we and studied extensively. It is impossible to be classifiec as malignant. A search has been gross dissection, the massive thyroid neoplasm is densely adherent to the trachea and larynx. found outside of the neck thus far. invading, structuresof the neck. judge whether many of the Jesiions shoul made in the lung and liver has invaded msculature of No proven distant metastages have beer Figure 20 illustrates a very extensive : There are so many interrelated observations in these experiments that] relationships are not yet appreciated. Several conclusions thus far seem l. There is an increased incidence of thyroid neoplasms following 3. A dose level of 131] mist be small enough that it does not i cripple the capacity for replication but it must be large enough Lesim ail flurte clear. administration of 131] to rats. produce some intrinsic damage to the cell. 4. Neoplasms are produced morereadily in the 7000 to 18,000 rad dose range. Although rat thyroids are not totall, destroyed with dose up to 80,000, an occasional tumor has occurred above this level. S. With increasing doses of 1311 there is a decreased labeling with tritiated thymidine. There is an immediate cessation of mitotic activity for a brief period of time. With recovery from ron destructive doses there is a supranormal rate of mitotic activity. As time passes there is then a decline below the normal level. 6. The permanent decline in inducable mitotic activity occurs over weeks or months depending on the magnitude of the original radiatipn insult. ° 7. The capacity to develop a hypertrophied gland under chronic thi Stimulation is gradually impaired or lost depending on the size o the dose of 117, onan Participation in Other Work on Radiation Effects: Coincident with the long term studies carried on under this contract, principal investigator (along with some others) has devoted a great deal of] several national problems concerned with radiation effects on the thyroid. as follows: 1. : tome > These sre Chairman of the Steering Committee for analysis and study of almosf 38,00 patients in the Cooperative Thyrotoxicosis Therapy rollow-up Study} of National Center for Radiological Heaith of the USPHS. a