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Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir VII Phase 2
of radiation exposure on other health outcomes including benign tumors and mortality from causes of death other than cancer. These are discussed at the end of the chapter. In general, the committee has summarized papers on cancer incidence, cancer mortality, and noncancer mortality in the LSS cohort that have been published since BEIR V (NRC 1990).
This chapter is based on published material and does not include results of analyses conducted by the committee, which are described in Chapter 12. At the time of this writing, detailed analyses of mortality data covering the period 1950–1997 and of incidence data covering the period 1958–1987 had been published. The committee’s analyses were based on the most recent DS02 dosimetry system, whereas most of the published analyses described in this chapter were based on the earlier DS86 dosimetry system (see discussion of dosimetry below for further comment). Preston and colleagues (2004) recently evaluated the impact of changes in dosimetry on cancer mortality risk estimates using mortality data through 2000; these results are summarized in the discussion of dosimetry.
DESCRIPTION OF THE COHORT
The full LSS cohort consists of approximately 120,000 persons who were identified at the time of the 1950 census. It includes 93,000 persons who were in Hiroshima or Nagasaki at the time of the bombings and 27,000 subjects who were in the cities at the time of the census but not at the time of the bombings. This latter group has been excluded from most analyses since the early 1970s because of inconsistencies between their mortality rates and those for the remainder of the cohort.
Health End Point Data
Data on health end points are obtained from several sources. Vital status is updated in 3-year cycles through the legally mandated Japanese family registration system in which deaths, births, marriages, and divorces are routinely recorded. This ensures virtually complete ascertainment of death regardless of where individual subjects reside in Japan. Death certificates provide data on the cause of death. The Leukemia Registry has served as a resource for leukemia and related hematological disease (Brill and others 1962; Ichimaru and others 1978). In the 1990s, it became possible to link data from both the Hiroshima and the Nagasaki tumor registries to the LSS cohort, which allows the evaluation of cancer incidence (Mabuchi and others 1994). An advantage of the registry data, in addition to the inclusion of nonfatal cancers, is that diagnostic information is of higher quality than that based on death certificates. Both tumor registries employ active approaches for case ascertainment and provide high-quality data from 1958 onward. Published analyses based on these data cover the period 1958–1987 (Thompson and others 1994). Limitations of the incidence data are that they are not available before 1958 and do not include subjects who have migrated from Hiroshima or Nagasaki.1
The Adult Health Study (AHS) is a resource for data on health end points that require clinical data. The AHS cohort is a 20% subsample of the LSS, oversampled to provide greater representation of subjects in high-dose categories. Since 1958, AHS subjects have been invited to participate in biennial comprehensive health examinations at RERF. The level of participation has been between 70 and 85% for those living in the Hiroshima and Nagasaki areas (Ron and others 1995a).
Most results presented in this chapter were based on the dosimetry system adopted in 1986 (DS86). The committee’s analyses, described in Chapter 12, are based on the revised DS02 system, adopted in 2004. The DS02 system is the result of a major international effort to reassess and improve survivor dose estimates. This effort was initiated because reports in the early 1990s on thermal neutron activation measured in exposed material (e.g., Straume and others 1992; Shizuma and others 1993) were interpreted as suggesting that the then-current survivor dosimetry system (DS86) might systematically underestimate neutron doses for Hiroshima survivors who were more than about 1 km from the hypocenter. However, the revised estimates of neutron dose do not differ greatly from the DS86 estimates. The new dosimetry system also introduces improved methods for the computation of γ-radiation doses and better adjustments for the effects of external shielding by factory buildings and local terrain features.
Preston and colleagues (2004) analyzed mortality data on solid cancer and on leukemia using both DS86 and DS02 dose estimates. They found that both the risk per sievert for solid cancer and the curvilinear dose-response for leukemia were decreased by about 10% by the dosimetry revision. They also found that parameters quantifying the modifying effects of gender, age at exposure, attained age, and time since exposure were changed very little by the revision.
Table 6-1, based on Preston and colleagues (2003), shows the distribution of survivors in the LSS cohort by their estimated DS86 doses to the colon. The dose to the colon is taken to be the γ-ray absorbed dose to the colon plus the neutron absorbed dose to the colon times a weighting factor 10. This weighted dose is denoted by d, and its unit sieverts;2 such estimates were available for 86,572 survivors. The
Analyses of cancer incidence data have included an adjustment of person-years to account for migration (Sposto and Preston 1992).
Use of the symbol Sv for the unit of d is an extension of the convention to use sievert as a special name of the unit joules per kilogram (J/kg) with regard to the effective dose or the equivalent organ doses (i.e., the dose quantities that contain the radiation weighting factor recommended by ICRP 1991).