-7feel that cancer induction is not proportional to dose, but that a certain "threshold" level must be exceeded before cancers can be induced. If such a threshold exists and if the doses fall below this level, no induced cancers Will appear. It is possible that no cancers will result from the Utah exposures. The number of radiation-induced thyroid cancers will now be predicted for each of the following 3 assumptions: (A) a linear dose response with 175? irradiation equally as effective as x-irradiation, (B) a linear dose response, but 3? irradiation only 1/10 as effective as x-irradiation, and (C) a@ high threshald. is unlikely that I'3? Assumption (A) probably sets an upper limit, for it is more effective than x-rays. sets the Lower limit: Assumption Assumption (C) certainly the number of induced cancers cannot be less than zero! (B), while between the upper and lower limits is not necessardly the best estimate, but it yields reasonable values for planning the experimental search for thyroid cancer. Predicted thyroid cancers in the 250,800 irradiated Utah children are shown in Table 3. Table 3 PREDICTED THYROID CANCERS IN THE 250,000 IRRADIATED UTAH CHILDREN AVERAGE DOSE (1.3 rad - 10 rad) ASSUMPTION A) [35 Thyroid Cancers L1O® Infants given 1 rad. (Effect of x-rays; Beach & Dolphin) B) 1/10 of x-ray effect (I'?? “x-ray RBE in rats; Doniach) C) High Threshold —_— wee ee ee eee 11 - 88 1 - 9 0 eee eee eee "NATURAL" INCIDENCE BY 15 YEARS AGE (from Mustacchi & Cutler) 6 Similarly, it is instructive to predict the expected number of thyroid cancers in the 565 infants in Washington county, Utah, exposed to higher thyroid doses estimated at 120-440 rads 3) following the "Harry" shot of 19 May, 1953. These predictions are shown in Table 4.