Histologic definitions: In interpreting the program data on Brookhaven thyroid Appendix D lists all exposed persons who have had thyroid surgery which confirmed a thyroid lesion. medical nodules the histopathological classification of thyroid nodules used by the expert panel of pathologists needs to be reviewed. This classification, based on diagnostic categories recommended by the World Thyroid nodules in the Comparison group: The examination of the Comparison group has been invaluable in interpreting the thyroid nodule risk data among the exposed population. Even though they do not constitute an ideal "controi" group, it is not likely that a better comparison population could have been obtained without initiating a formal, prospective research effort. It is appropriate that the voluntary cooperation of the Heaith Organization (Hedinger and Sobin, 1974) and modified in 1981 by Dr. Donald Paglia of the Department of Surgical Pathology, University of California, Los Angeles, for the panel’s use, has been applied to all thyroid specimens obtained at surgery since the beginning of the program: Adenomatous nodule: a focal proliferative lesion consisting of changes typical of adenomatous goiter; the lesions are hyperplastic and do not fulfill criteria of true neoplasms. Adenoma: an encapsulated proliferative lesion with a uniform internal growth pattern and benign clinical course. Occult papillary carcinoma: a small nonencapsulated sclerosing carcinoma; considered to be clinically benign even if associated with positive regional lymph nodes. Papillary carcinoma:larger, infiltrating carcinoma, usually containing both papillary and follicular components. The four pathologists on the panel that review the Marshallese specimens are: Dr. L.V. Ackerman, Health Sciences Center, SUNY,Stony Brook, NY; Dr. W.A. Meissner, formerly with New England Deaconess Hospital, Boston, MA; Dr. A.L. Vickery, Massachusetts General Hospital, Boston, MA; Dr. L.B. Woolner, Mayo Clinic, Rochester MN. Histologic sections of all surgically removed thyroid tissue have been examined by these authorities. Although most diagnoses have been unanimous, some were controversial. In the following analysis and discussion of Marshallese nodules, the “most neoplastic” diagnosis has been selected when there has been a split decision, with the "least neoplastic” being the adenomatous nodule, next being the adenoma, the third being occult papillary cancer, and the most neoplastic being the carcinoma. members of this group be gratefully acknowledged by all who have relied on the Marshallese thyroid nodule data to interpret the role of radiation in causing thyroid disease. In that the unexposed Comparison group comprises persons of Rongelap ancestry and was quite closely age- and gendermatched whenselected in 1957, this group is more representative of the exposed Rongelap population than any other Marshallese community and certainly more so than a population of non-Marshallese. The development of thyroid nodules in the Comparison population is similar to the spontaneous thyroid nodule incidence reported elsewhere. Maxon et al. (1977) concluded that the rate of development of benign thyroid nodules and thyroid carcinomas in western countries is 0.07% and 0.01% of the population per year, respectively, and that the incidence is linear with respect to age. In 1990 the numberof person-years of observation of the Comparison group was 10,400. Therefore, based on the conclusions of Maxonetai. (1977), the expected number of thyroid nodules, benign and malignant, would be 8.3, of which 6 or 7 would be benign and 1 or 2 would be carcinomas.* In fact, 8 nodular thyroids were detected, of which 6 were benign and 2 were carcinomas (see Table 2). Possible sources of inaccuracy include the following: (1) Only surgically confirmed nodules are included. Therefore, since several unoperated nodules have been diagnosed in the Comparison group, the “observed” number may underestimate the true numberof thyroid nodules. However, the palpated nodules may have been lipomas or neuromas, for example, and therefore appropriately excluded. (2) One of the two occult papillary carcinomas diagnosed in this group was not detected prior to surgery. This "nodule" is therefore excluded. Thus the total number of nodules is given as 8 rather than 9, as listed in Table 2. (3) 1984 is the latest 16