group. Only adenomas(Fig. 7b) did not develop in persons exposed beyond their teenage years. 6 60 £ 2 — 50+ nes ge = Be Utirik a 405 mod Qe 30+ O £o Rongelapo a DH Ia SE 204 ~SC EH = 10+ o 1 o 5 - 0 / - og ow 4 1 a . | 20 = i 4 40 Years Post-Exposure Fig. & Relation of thyroid-absorbed dose to time of development of surgically conformed nodules in persons who were between 1 and 6 years of age at the time of exposure on Rongelap and Utirik. However, there is a marked disparity among the four graphs for the Rongelap group. The occurrence of many adenomatousnodulesin the younger Rongelap population (Fig. 7a) could have been a function of either age or high radiation dose, because the thyroid-absorbed dose was strongly age-dependent. However, none of the other three nodule types in the Rongelap exposed group (Fig. 7b-d) show the same preference for younger ages, although the number of observations is quite small. It is possible that the development of adenomas and occult papillary carcinomas, as well as overt carcinomas, was limited by thyroid cell injury resulting from the high radiation dose to the young Rongelap population. It is ironic that neoplastic nodules in the Rongelap group were numerically overshadowed by benign nonneoplastic lesions. While one might considerthis to be, in a sense, a mitigation of radiation injury, the significant morbidity of benign thyroid nodules in the exposed Marshallese has been discussed (Adams et al., 1988). 5) Did benign nodules antedate carcinomas? A highly significant correlation for time of development of benign nodules (either total benign nodules or adenomatous thyroid cancers was found using surgery for the specified groups (Ta r = 0,99 and p = <0.01). This striki which is present regardless of ageffor the use of thyroxine suppression, supports the§ notion that a benign lesion does not evolve into a [malignant one, nor do carcinomas, presumably poss@ssing a greater degree of autonomous growth, manifest themselves clinically any earlier than benign nafules. Ronet al. (1989) also noted a similarity fin time from radiation exposure to tumor fliagnosis for carcinomas, adenomas, and "noduleq” 6) Was the type of thyroid nodule induced by radiation a function of dose? It is thought that at thyroid dosds above 15002000 cGy the incidence of carcinoma is decreased due to extensive cell death which Iqaves few cells capable of becoming neoplastic (NCRP, 1985), although there are reports of undi thyroid cancer from external irradiatipbn with thyroid doses exceeding 3000 cGy (Tuckeretfal., 1991). For palpable solitary nodules in the gendral population the usual ratio of benign to maligfant lestons is about 6:1 and in some radiation-exlosed groupsit can be as high as 3:1 (DeGrootfet al., 1983), althoughthe ratio varies considerably depending on the definitions used. The ratios for the Marshallese are shown in Table 3, column B. [In the Utirik group where the total thyroid doge was relative small the ratios for persons under 1@fyears of age is 3.5:1 and for those who were older is 3.7:1. The high benign to malignant tumor rafio of 16:1 for Rongelap children who were exposed underthe age of ten years and whose thyroid dosesfexceeded 2000 cGy is consistent with most other sfudies, and the likely explanation is a decrease in [thyroid cancer due to extensive cell death or injuryjat the time of exposure. What mayalso be important, howpver,is the low benign to malignant tumor ratio (1)25:1) found in those Rongelap individuals with mid-range thyroid radiation doses(i.e., Rongelap indi age of 10 years at exposure, who probability in 2000 cGy). Thus, there was a hi this group of a detected nodule b ng malignant, whereas there was relatively lo probability of malignancy in persons whose dose —xceeded 2000 cGy. This may be relevant in clihical decision- making for nodules detected in [pther exposed populations. 21