1. Subtract 0.25 (males) or 0.65 (females) (the Lifetime Risk for the adult group) from the value found in step 1.

  2. Calculate the ratio of the one's dose estimate to the average dose for the age cohort.

  3. Multiply this ratio by the value from step 2 above.

  4. Add the adult value for Percentage Lifetime Risk (0.25 for males or 0.65 for females) to the number from step 4 above.

The number calculated by the above five steps would be an estimate of one's individual Percentage Lifetime Risk. It could be interpreted as the percent chance of developing thyroid cancer during their lifetime to 85+ years of age, or the proportion of 100 people who would develop the disease. For example, if the person was less than 1 year old at time of exposure and his or her individual dose estimate was about 50 rad, he or she would calculate a Percentage Lifetime Risk of about 1.1 (males) or 2.9 (females), meaning that the chance of developing thyroid cancer by age 85 would be about 1 percent (males) or 3 percent (females). Equivalently, for this 50 rad dose, it would mean that 1 of every 100 males or 1 of every 35 females like themselves would contract the disease because of the radiation exposure. Relatively few individuals likely received doses as high as 50 rad, thus relatively few persons would have chances of developing thyroid cancer this great.5

Two final points are worthy of note in this discussion of risk estimates. First, for those who are diagnosed with radiation related thyroid cancer, the prognosis, as previously discussed, is good. Second, the individuals that are diagnosed with thyroid cancer can have a relatively high probability that the disease was caused by radiation exposure even if the likelihood that they would get the disease was low. For example, the probability that a cancer is a result of radiation exposure (this probability is termed "probability of causation" or PC) (NIH 1985) can be calculated as [Relative Risk - 1]/[Relative Risk].6 Thus, for the child having received an actual dose of 10.3 rad (see Table 3.4), the PC would equal (1.67 - 1)/1.67 = 0.40. This corresponds to a 40 percent chance the cancer was a result of radiation exposure. This can be compared with a chance of only 0.42 percent (male) or 1.07 percent (female) that they would have developed the disease during their lives following a 10.3 rad exposure at 1 year of age. The PC becomes increasingly higher for larger radiation doses.

5  

The primary exception would be individuals who routinely drank goats' milk. NCI estimated the number of such individuals in the United States who were in childhood at time of exposure to have been about 20,000. These persons could have received doses greater than 100 rad at many locations.

6  

There are alternate ways to write this equation, all are equivalent: PC = (RR - 1)/RR = ERR/RR = ERR/(1 + ERR).



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