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Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir VII Phase 2
(0.005–0.25 Gy) were somewhat dependent on the choice of comparison group. Cohort members in this low-dose category had a median life expectancy that was shorter than that of zero-dose survivors who were within 3 km of the hypocenter (229 d), shorter than the not-in-city group (365 d), but slightly longer (52 d) than survivors located 3 km or more from the hypocenter. These results do not support the hypothesis that life expectancy for atomic bomb survivors exposed at low doses is greater than that for comparable unexposed persons.
The LSS cohort of survivors of the atomic bombings in Hiroshima and Nagasaki continues to serve as a major source of information for evaluating health risks from exposure to radiation, and particularly for developing quantitative estimates of risk from exposure to ionizing radiation. Its advantages include its large size, the inclusion of both sexes and all ages, a wide range of doses that have been estimated for individual subjects, and high-quality mortality and cancer incidence data. In addition, the whole-body exposures received by this cohort offer the opportunity to assess risks for cancers of a large number of specific sites and to evaluate the comparability of site-specific risks. The full LSS cohort consists of approximately 120,000 persons who were identified at the time of the 1950 census. However, most recent analyses have been restricted to approximately 87,000 survivors who were in the city at the time of the bombings and for whom it is possible to estimate doses. Special studies of subgroups of the LSS have provided clinical data, biological measurements, and information on potential confounders or modifiers.
Mortality data for the period 1950–1997 have been evaluated in detail, adding 12 years to the follow-up period available at the time BEIR V (NRC 1990) was published. The longer follow-up period not only increases statistical precision, but also allows more reliable assessment of the long-term effects of radiation exposure, including modification or risk by attained age and time since exposure. Importantly, cancer incidence data from both the Hiroshima and the Nagasaki tumor registries became available for the first time in the 1990s. These data not only include nonfatal cancers, but also offer diagnostic information that is of higher quality that that based on death certificates, which is especially important for evaluating site-specific cancers. Although published evaluations described in Chapter 6 are based on DS86 dosimetry, a revised DS02 system—the result of a major international effort to reassess and improve survivor dose estimates—has recently become available and was used to develop BEIR VII risk models. An initial evaluation indicates that this revision will slightly reduce risk estimates.
The more extensive data on solid cancer that are now available have allowed more detailed evaluation of several issues pertinent to radiation risk assessment. Several investigators have evaluated the shape of the dose-response, focusing on the large number of survivors with relatively low doses. These analyses have generally confirmed the appropriateness of linear functions to describe the data. The modifying effects of sex, age at exposure, and attained age have also been explored in detail using both ERR and EAR models. The ERR/Sv has been found to decrease with both increasing age at exposure and increasing attained age, and it now appears that both variables may be necessary to provide an adequate description of the data. By contrast, the EAR shows a sharp increase with increasing attained age and a decrease with increasing age at exposure.
The availability of high-quality cancer incidence data has resulted in several analyses and publications addressing specific cancer sites. These analyses often include special pathological review of the cases and sometimes include data on additional variables (e.g., smoking for evaluation of lung cancer risks). Papers focusing on the following cancer sites have been published in the last decade: female breast cancer, thyroid cancer, salivary gland cancer, liver cancer, lung cancer, skin cancer, and central nervous system tumors. Special analyses have also been conducted of cancer mortality in survivors who were exposed either in utero or during the first 5 years of life.
Health end points other than cancer have been linked to radiation exposure in the LSS cohort. Of particular note, a dose-response relationship with mortality from nonneoplastic disease was demonstrated in 1992, and subsequent analyses in 1999 and 2003 have strengthened the evidence for this association. Statistically significant associations were seen for the categories of heart disease, stroke, and diseases of the digestive, respiratory, and hematopoietic systems. The data were inadequate to distinguish between a linear dose-response, a pure quadratic response, or a dose-response with a threshold as high as 0.5 Sv.