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type of radiation received at each age. These models provide a method for calculation of radiogenic-cancer risks based on a critical review of data on the Japanese atomic-bomb survivors and other study groups and methods of applying radiation risk estimates across populations.
The cancer sites considered are the esophagus, stomach, colon, liver, lung, bone, skin, breast, ovary, bladder, kidney, thyroid, red marrow (leukemia), and residual (all remaining cancer sites combined). An absolute-risk model is applied to bone, skin, and thyroid; that is, it is assumed for these sites that the radiogenic cancer risk is independent of the baseline cancer mortality rate (cancer mortality death rate for a given site in an unexposed population). For the other cancer sites, a relative-risk model is used; it is assumed that the likelihood of a radiogenic cancer is proportional to its baseline cancer mortality rate. The baseline cancer mortality rates are calculated from US cancer mortality data for 1989–1991 (NCHS 1993a; 1993b; 1992).
The computation of sex-and site-specific values for the lifetime cancer risk per unit absorbed dose involves an integration over age, beginning at the age at which the dose is received, of the product of the age-specific risk-model coefficient (times the baseline mortality of the cancer in the case of a relative-risk model) and the survival function. The survival function is used to account for the possibility that the exposed person will die of a competing cause before a radiogenic cancer is expressed.
Estimates of the site-specific cancer mortality for a hypothetical low dose, low dose rate, uniform irradiation of the whole body by low-LET and high-LET radiations are given in table 4.6. Some organs such as male breast or brain are not explicitly included as these sites have not shown definitive dose responses (Pierce and others 1996). To the extent that these sites contribute to the total cancer response, they are included in the Residual category.
The risk estimates of table 4.6 are based on the risk-model coefficients in Federal Guidance Report 13 (EPA 1998). The estimates are age-averaged values for the hypothetical stationary population. For details regarding the method of computation, see Federal Guidance Report 13.
2. Absorbed-dose rates as a function of time after an acute intake
Age-specific biokinetic models for radon and its decay products are used to calculate the time-dependent inventories of activity in various regions of the body after acute intake of a unit activity of 222Rn. This calculation is performed for each of six "basic" ages at intake: infancy (100 d); 1, 5, 10, and 15 y; and maturity (20 y). The biokinetic model for radon used in the calculations was described above (see appendix A for further details). The biokinetic models for the radon decay products were taken from ICRP's recent series of documents on age-specific doses to members of the public from intake of radionuclides (ICRP 1996; ICRP 1995; ICRP 1993; ICRP 1989).