¥ s2 = Oe am = J Marshallese LL Ann Arbor 0 | 500 Thyroid dose (rads) L000 — i 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 Hempelmann’s study?07 had anincreased incidenceof thyroid tumors following a mean dose of only 30 rads to the gland (Figure 44), and more recently Modan et al.t20 and Harleyet al.1!1 have reported an increased incidenceof 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 element ofJewish people (who have increased susceptibility to thyroid tumors) should be kept in mind. Also, Hempe!mann’s were irradi- ated as infants and therefore represent only a narrow age range. 1. Latent Period In Figure 45 the developmentof thyroid abnor- malities is plotted according to radiation dose and time after exposure, the latent period appears to be longer with lower doses. Figures 46 and 47 show the relationship between dose and age at developmentof thyroid lesions. These data indicate that the radiation-induced lesions occurat earlier ages in the exposed Rongelap people thanin the Utirik or unexposed groups. In the latter groups almost all the thyroid nodularities develop in the older people. Recent data of Hempelmann etal.19 in- dicate that the latent period may be > 30 years for J ee a 8 ] ; : : 500/0 | , ® 0 [ Rochester 0 Le Marshallese J i Marshallese e J T Thyroid duse (rads) 100 -— bop a td 5 . * dt 10 15 Timeafter exposure (years) 20 Figure 45. Time of appearance of thyroid lesions in Marshallese. Exposed at age <10: o, benign; +, malignant. Exposed at ige > 10: @, benign; *, malignant. developmentof radiation-induced thyroid tumors. Thyroid cancer ras been reported as long as 40 years after radiation exposure.125 Correlation with sex showed thatslightly more exposed Rongelap females (18 of 45) developed thyroid lesions than did males (11 of 41). All three cases of cancer of the thyroid were in females. The thyroid gland is generally thought to be moresensitive to radiation during childhood.12%121 Pochin!18 estimates a percent incidence per 100 rads of 0.5 to 1.0 for adults and 1.3 for children. Certainly the largest numberof benignlesions occurred in the Marshallese children. This 1s probably due mainly to the smaller size of the thyroid glands resulting in larger doses per gram ofgland. Doniach!?9 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 matunty thethyroid weighs about 20 times as muchas atbirth. These factors would increase the chances for ma- lignant transformation. The lower incidence of cancerof the thyroid in children than in adults maybe 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 radioicdine) are associated with a lower incidence of thyroid malignancythan lower doses, possibly because the high doses produce enoughcell! destruction to preclude malignant transformation.85-120.221,126,127 This may be the case with Marshallese children. It has been noted thatfollowing treatmentof hyperthyroidism with large doses of radioiodine, although