aThese categories are defined using the estimated dose to the colon, obtained as the sum of the γ-ray dose to the colon plus 10 times the neutron dose to the colon.
SOURCE: Based on data from Preston and others (2003).
37,458 survivors (43%) with doses less than 0.005 Sv were primarily survivors who were located more than 2.5 km from the hypocenter. Only 2101 (2.4%) had doses exceeding 1 Sv. Table 6-1 also shows the number of solid cancer deaths and noncancer disease deaths in the period 1950–1997.
The material in the sections that follow draws heavily on results presented by Thompson and colleagues (1994) and Preston and colleagues (1994, 2003). Here, features of the statistical methods that were used for most analyses in these papers are described. Readers should consult the source papers for details. In nearly all cases, analyses were based on Poisson regression using the AMFIT module of the computer software EPICURE (Preston and others 1991).
Most recent analyses have been based on either excess relative risk (ERR)3 models, in which the excess risk is expressed relative to the background risk, or excess absolute risk (EAR)4 models, in which the excess risk is expressed as the difference in the total risk and the background risk. The age-specific instantaneous risk is given either by
where λ denotes the background rate at zero dose and depends on city (c), sex (s), attained age (a), and birth year (b), and the excess may depend on sex (s), age at exposure (e), attained age (a), and time since exposure (t). Not all variables are included in all models; in fact, any two of the variables e, t, and a determine the third. Parametric models are used for the ERR and EAR. The most recent analyses of solid cancer mortality (Preston and others 2003) have been based on models of the form
Earlier analyses (Thompson and others 1994; Pierce and others 1996) were based primarily on ERR models of the form
The function ρ(d) is usually taken to be a linear or linear-quadratic function of dose, although threshold and categorical (nonparametric) models have also been evaluated. With the linear function, ρ(d)=βsd, and βs is the excess relative risk per sievert (ERR/Sv), which provides a convenient summary statistic. The parameters γ and η measure the dependence of the ERR/Sv on age at exposure and attained age.
Preston and colleagues (2003) and Thompson and colleagues (1994) used parametric models for the background risks. Some past analyses, such as those by Pierce and coworkers (1996) treated the background risk in ERR models by including a separate parameter for each category defined by city, sex, age at risk, and year. Thompson and colleagues did not fit EAR models; however, average EARs were estimated by dividing the estimated number of excess cancers by the total person-year-Sv.
Analyses of leukemia are based on bone marrow dose; analyses of the combined category of all solid cancers are based on colon dose; and analyses of site-specific cancers are based on specific organ doses. Dose is expressed in sieverts and is a weighted dose obtained as the sum of the dose of γ-radiation and 10 times the neutron dose. This approach is based on the assumption of a constant relative biological effectiveness (RBE) of 10 for neutrons. In most
The ERR is the rate of disease in an exposed population divided by the rate of disease in an unexposed population minus 1.0.
The EAR is the rate of disease in an exposed population minus the rate of disease in an unexposed population.