- 6 The significance of these exposures is not the size of the average dose (which is small) but in the enormous number of irradiated children. To those of you who object to my estimates, I applaude you: may your doubts lead you to establish more reliable limits! PREDICTED THYROID CANCERS The natural incidence of childhood thyroid cancer is extremely low. Values from Mustacchi and Cutler indicate that by age 15 years only 25 thyroid cancers are expected per 1,000,000 children ©) , Thus, only about 6 "natural" childhood thyroid cancers are anticipated in the 250,000 exposed Utah children. X-rays can induce thyroid cancer. About 20-30 years ago, it was "fashionable" in some hospitals to x-irradiate infants in the neck region for benign conditions. Thyroid cancer has followed in an unpleasantly large number of these exposed children. Beach and Dolphin ©’ ) found reports of 132 post-irradiation thyroid malignancies in the published medical literature: the additional number of unpublished cases remains unknown. They analyzed the relation between incidence and dose ina special population of 4673 exposed children for which the individual doses were available. of thyroid cancer increased with dose to 1.7% at 500 rads. The incidence Assuming incidence proportional to dose, they derived an incidence of 35 cancers per million infants each receiving 1 rad. However, they were careful to point out that their analysis did not exclude the possibility of a curved dose-response relation: the data in their figure 5 can also be interpreted to suggest a "threshold" at perhaps 50-100 rads, below which cancers would not be induced. There is some experimental evidence that I'°! x-rays. is less effective than Doniach found that the I’3! dose had to be about 10 times greater than an abrupt dose of x-rays to cause equal effects in adult rats ©) , However, rats are not men”, and the relative effectiveness of I'9) vs. x-rays in children is yet to be established. Although the I'°? exposures in Utah were unplanned for this purpose, perhaps they can provide information of fundamental importance in radiobiology. The dose-response relation is very difficult to establish at very low doses, because the incidence of effects is so very small. % Although some women claim that men are rats. Some radiobiolgists