T TT Marshallese | po vu 1 1 vw 3 | = 304 4 € vu = v Marshallese _ j Rochester 4 | Ann Arbor 0 © ~ Marshallese 0 | ! L 500 1000 | Thyroid dose (rads) 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!97 had an increased incidenceof thyroid tumors following a mean dose of only 30 rads to the gland (Figure 44), and more recently Modan et al.420 and Harley et al.!!2 have reported an in- creased incidence of thyroid tumorsin children who had received about 6.5 rads to the thyroid gland during x-ray treatment of the scalp for fun- gus 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, Hempelmann’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 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-inducedlesions occurat earlier ages in the exposed Rongelap people than in the Utirik or unexposed groups. In the latter groups almost all the thyroid nodularities develop in the older people. Recent data of Hempelmannet al.!99 indicate that the latent period may be > 30 years for 4 L | e ~ I L Thyroid dose (rads) 100 4 60 “ . ° “ e 0 9 a 1000 — - = 2a, . 5 ° a r e F 500 + | ° b L ot po 0 * ee 2) ° ro bo ~_ 4 | 5 10 13 Timeafter exposure (years) 4 20 Figure 45. Time of appearance of thyroid lesions in Marshallese. Exposed at age <10: >, benign: >. malig- nant. Exposed at age >10: @, benign; «, malignant. developmentofradiation-induced thyroid tumors. Thyroid cancer has been reported as long as +0 years after radiation exposure.!25 Correlation with sex showedthat slightly 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 thought to be moresensitive to radiation during childhood.129.121 Pochin1!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 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!?° theorizes that thyroid tumors are produced by an 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 maturitythe thv- roid weighs about 20 times as much asat birth. These factors would increase the chances for malignant transformation. The lower incidence of cancer of the thyroid in children than in adults mayberelated tc the lack of dose dependenceof the carcinogenic effect of radiation at high doses. Several investigators have shown that high doses of radiation (both x rays and radioiodine) are as- sociated with a lower incidence of thyroid malignancy than lower doses, possibly because the high doses produce enoughcell destruction to preclude malignant transformation.§5.120,121.126.127 This maybe the case with Marshallese children. It has been noted that following treatment of hyperthyroidism with large doses of radioiodine, although