before a cancer occurred at that site, not the 50-year committed dose used in radiation protection. When an inhaled radionuclide has a half-life or biological half-time in the body of a few years or less, there is little difference between the dose received before the time of occurrence of a cancer, assuming that the cancer did not occur before a minimum latent period after intake, and the 50-year committed dose resulting from a given intake. However, when a radionuclide is long-lived and tenaciously retained in the body, there can be a significant difference between the dose received before a cancer occurred and the 50-year committed dose. In inhalation exposures of atomic veterans, the difference is potentially important mainly for plutonium and, to a lesser extent, 90Sr. The difference between the dose received and the 50-year committed dose is most important for organs and tissues other than those in the respiratory and GI tracts, excluding respiratory lymphatic tissues where long-lived and insoluble radionuclides are assumed to be tenaciously retained (ICRP, 1979a; 1994b).

Consider a hypothetical example in which the disease of concern in an atomic veteran is bone or liver cancer and the dose to bone surfaces or liver was due primarily to inhalation of insoluble plutonium. Suppose that the cancer was diagnosed 35 years after exposure and that the latent period for the cancer is 10 years (Eckerman et al., 1999). In this case, the dose that could have caused the cancer is the dose received within the first 25 years after exposure, and use of 50-year committed doses to bone surfaces or liver would overestimate the dose that could have caused the cancer, mainly because of the biological half-time of plutonium in bone or the liver of several decades (ICRP, 1979a; 1993; 2002). The retention half-time of insoluble plutonium in the lung of a few years when inhaled particles are respirable (AMAD, 1 μm) is less important. Thus, even if inhaled plutonium were rapidly transferred to bone or liver, the 50-year committed dose would overestimate the dose received in the first 25 years by about a factor of 2, and the degree of overestimation would increase somewhat if the inhaled plutonium was respirable and the low rate of absorption of insoluble forms of inhaled plutonium from the respiratory tract into blood is taken into account.21

Again, the difference between the 50-year committed dose and the dose received in an organ or tissue is potentially important only if the dose was due mainly to intakes of long-lived radionuclides that are tenaciously retained in the body. Thus, the importance of this difference in dose reconstructions for atomic veterans depends on the activities of particular radionuclides inhaled.

The committee also notes, however, that use of 50-year committed doses from inhalation of long-lived radionuclides that are tenaciously retained in the body, such as plutonium, could result in underestimates of the dose that could

21  

The relatively rapid mechanical clearance of some inhaled material to the GI tract in the case of insoluble plutonium in respirable form would not affect the degree of overestimation of dose to a significant extent, because the fraction of ingested insoluble plutonium that is assumed to be absorbed into blood from the GI tract is very low (typically 10−4–10−5).



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