100 — | Marshailese e Percent incidence - cc a F 1000+ — = L 2 t 7 H 23 e - 3 3 , 3 é Ss 3 vol ~ 3 500 = -_ 0 / Rochester | Ann Arbor oleMarshallese 0 300 Thyroid dose (rads) ~ L_ 1000 , Figure 44. Incidence of benign thyroid nodules. partly related to the smaller contribution to the dose from short-lived isotopes of iodine. Nevertheless the Ann Arbor children in Hempeimann’s study207 had an increased incidenceof thyroid tu- mors following a mean dose of only 30 rads to the . gland (Figure 44), and more recently Modan et al.t10 and Harley et al.111 have reported anin- creased incidence of thyroid tumors in children who had received about 6.5 rads to the thyroid gland during x-ray treatmentofthe scalp for fungus infection. Modanetal. state that ‘“‘one would strongly suspect a mini-epidemic of thyroid adenoma” in the group studied. In considering the risk data of Hempelmann’s and Modan’s groups, the large elementofJewish people (who haveincreased susceptibility to thyroid tumors) should be kept in mind. Also, Hempeimann’s wereirradiated as infants and therefore represent only a narrow age range. 1. Latent Period In Figure 45 the developmentof thyroid abnormalities is plotted according to radiation dose and umeafter exposure; the latent period appears to be longer with lower doses. Figures 46 and 47 show the relationship between dose and age at developmentofthyroid lesions. These data indicate that the radiation-induced lesions occur at earlier ages in the exposed Rongelap people than in the Utink or unexposed groups. In the latter groups almost all the thyroid nodularities develop in the older people. Recent data of Hempelmannetal. 199 in- dicate that the latent period may be > 30 years for 4 7 7 _ = f Z Marshailese | 900b151 ee 0 - * * Dt 5 10 . 4 =] deed 15 Time after exposure (years) 20 Figure 45. Time of appearance of thyroid lesions in Marshallese. Exposed at age <10: 2, benign: =, maitgnant. Exposed at age > 10: @, benign; *, malignant. developmentof radiation-induced thyroid tumors. Thyroid cancer has been reported as long as +0 years after radiation exposure,!75 Correlation with sex showedthatslightly more exposed Rongelap females (18 of 45) developed thyroid lesions than did males (11 of 41). All three cases of cancerof the thyroid were in females. The thyroid gland is generally thoughtto be more sensitive to radiation during childhood. 120.121 Pochin?18 estimates a percent incidence per 100 rads of 0.5 to t.0 for adults and 1.3 for children. Certainly the largest numberof benign lesions occurred in the Marshallese children. This is probably due mainly to the smaller size of the thyroid glands resulting in larger doses per gram ofgland. Doniach!2° theorizes that thyroid tumors are produced byan initiating factor (radiation) plus a promoting factor (TSH) which increases mitosis and enhances the expression of a possible malignant clone. In addition, in children the growth factor may be important, since at maturity the thyroid weighs about 20 times as much asat birth. These factors would increase the chances for ma- lignant transformation. The lowerincidence of cancer of the thyroid in children than in adults may be related tc the lack of dose dependence of the carcinogenic effect of radiation at high doses. Several investigators have shown that high doses of radiation (both x rays and radioiodine) are associated with a lower incidence of thyroid malignancy than lower doses, possibly because the high doses produce enough ceil destruction to preciude malignant transformation.85.120,121.126,.127 This may be the case with Marshallese children. [t has been noted that following treatmentof hyperthvroidism with large doses of radioiodine, although