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2 Other Studies of Radiation Exposure of Military Personnel INTRODUCTION The purpose of this chapter is to provide some context for later discussion of results in particular, to discuss whether the findings of our study are con- sistent with what is known about the effects of radiation exposure. However, because radiation exposure data have not been used in this report (see Chapter 7), and because our discussion of the effects of radiation exposure is necessarily indirect, we do not provide an extensive discussion of dose-related radiation effects in this chapter. Furthermore, there are several excellent, recent references on radiation risk which the interested reader may consult (ICRP, 1991; National Research Council, 1990; UNSCEAR, 1994~. We do discuss in some detail the results of four previous mortality studies of military veterans involved in nuclear weapons tests because the types of exposures to which they may have been subjected (both radiation and nonradiation exposures) are more likely to be similar to those experienced by the five-series cohort than those of nonmilitary cohorts. Among the studies of nonmilitary cohorts, the study of the survivor experi- ence following the atomic bomb exposures in Hiroshima and Nagasaki the Life Span Study (LSS:is of great importance despite the unique circumstances sur- rounding these exposures; most radiation protection recommendations are based primarily on LSS risk estimates (Ron et al., 1994~. The interested reader is referred to an extensive literature on cancer incidence rates and cancer mortality rates (fairly recent examples include Land, 1995; Mabuchi et al., 1994; Nagataki et al., 1994; Pierce et al., 1996; Preston et al., 1994; Ron et al., 1994; Thompson et al., 1994~; Schull (1995) provided a good overview ofthe entire LSS program. Other studies of nonmilitary populations exposed to relatively high levels of radiation include the following: the Andylosing Spondylitis Treatment Study (Derby et al., 19874; the Cervical Cancer Treatment Study (Boice et al., 1988~; the Canadian Fluoroscopy Study (Sherman et al., 1978~; the New York State Post 8
OTHER STUDIES OF RADIA TION EXPOSURE OF MILITARY PERSONNEL 9 parturn Mastitis Study (Shore et al., 1986~; and the Massachusetts Fluoroscopy Study (Boice et al., 1978, 1981~. The results of such studies are typically extrapo- lated mathematically to provide estimates of health effects at relatively low doses. While studies of relatively low-dose exposures can provide a basis for en- suring that radiation risk estimates based on higher doses neither underestimate nor overestimate the effects of lower doses, they have their unique problems. Chief among these are low statistical power (leading to lack of precision in risk estimates) and the strong potential for confounding that is present in studies that attempt to produce estimates of low relative risks. Studies of relatively low-level radiation exposure in nonmilitary populations include studies of exposures due to diagnostic radiology; fallout from nuclear weapons testing (populations of residents downwind from test sites); occupational exposures; and natural back- ground radiation. The results of these kinds of studies have been reviewed in detail elsewhere (Boice, 1996; Boice, et al., 1996; NRC, 1990; and Ron, 1998~. MILITARY POPULATIONS Several epidemiologic studies of military personnel possibly exposed to radiation during atmospheric nuclear weapons testing have contributed to the scientific debate regarding the adverse effects of radiation exposure on human health. These studies have reported modest elevations in risk for all-cause, all- cancer, and leukemia mortality in participants relative to comparison groups, but these elevations have not consistently reached statistical significance. Table 2-1 displays findings from several of the larger, controlled studies of military per- sonnel who participated in nuclear weapons testing. Watanabe and colleagues (1995) compared the mortality experiences of some 8,550 military participants at Operation Hardtack I, a 1958 U.S. test series in the Pacific Proving Ground, with a comparison group of roughly 14,600 military personnel. All-cause mortality (crude death rate ratio ERR] 1.10; 95% confidence interval [CI] 1.02-1.19) and digestive cancer mortality (RR 1.47; CI 1.06-2.04) were higher among participants than comparisons. Mortality rates due to all cancers, leukemia, and other suspected radiogenic cancers were not significantly elevated among participants. When stratified by gamma radiation doses the advisability of which the advisory committee of this report questions (see Appendix A)- participants in the highest dose group (>1,000 millirem fmrem]) had significant increases in mortality for all causes (RR 1.23; CI 1.04- 1.45), all cancers (RR 1.42; CI 1.03-1.96), and liver cancer (RR 6.42; CI 1.17- 35.33~. Participants in the middle dose group (250-1,000 mrem) did not demon- strate increased mortality rates for any conditions. Participants in the low-dose level (0-250 mrem) had significantly increased mortality rates due to digestive organ cancer. Among the digestive organs, esophageal cancer mortality showed the largest elevation in risk (RR 2.15), although neither it nor any other indi- vidually classified digestive organ reached statistical significance.
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12 ~, O THE FIVE SERIES STUDY Darby and colleagues (1993a,b) studied mortality and cancer incidence in some 21,000 military participants (and 22,000 comparison subjects) in nuclear weapons tests conducted by the United Kingdom in Australia and the Pacific during the 1950s and 1960s. The study included data on deaths occurring through 1990, extending the follow-up period reported in their earlier study an additional 7 years (Derby et al., 1988a,b). Test participants had significantly increased leukemia mortality during the entire follow-up period (RR 1.75; CI 1.01-3.06), with a stronger effect observed for the period 2-25 years after expo- sure (RR 3.38; CI 1.45-8.25) than for the entire follow-up period. The investi- gators suggested that this elevation may be due to the low rates of leukemia seen in the comparison group during both follow-up periods, but did not rule out the possibility that exposure to radiation from nuclear tests may have had an effect on the leukemia risk, particularly during the earlier post-exposure period. Pearce and colleagues (1996, 1997) investigated mortality and cancer inci- dence (through 1992) among 528 New Zealand participants (and 1,504 compari- son subjects) in United Kingdom nuclear weapons tests conducted in the Pacific in 1957 and 1958. Leukemia (RR 5.59; CI 1.04 41.7) and the total hematologic cancer group (RR 3.75; CI 1.36-10.8) mortality rates were statistically signifi- cantly elevated. All-cause and all-cancer mortality were slightly elevated, but did not reach statistical significance. Johnson and colleagues (1996) at the Medical Follow-up Agency of the Institute of Medicine investigated the mortality experience of more than 38,000 U.S. Navy personnel who participated in Operation Crossroads, a 1946 atmos- pheric nuclear test series that took place at the Bikini Atoll in the Pacific, and roughly 35,000 comparison personnel. The mortality experience of participants was evaluated relative to that of a comparison group, selected to be similar to the participants in several key ways such as branch of service, time and loca- tion of service, age, and paygrade but who had not participated in the Cross- roads nuclear test series. Analysis found a slight but statistically significant in- creased risk of all-cause mortality among participants (RR 1.05; CI 1.02-1.07~. Neither leukemia (RR 1.02; CI 0.75-1.39) nor all-cancer (RR 1.01; CI 0.9 1.07) mortality were significantly elevated among Navy Crossroads participants. In summary, the four studies of military personnel participating in atmos- pheric tests conducted by New Zealand, the United Kingdom, and the United States report rather consistent findings. For all-cause mortality, rate ratios from all four studies were slightly elevated (above 1.0~. Two of those studies showed sta- tistically significant estimated increased risk to participants. For all-cancer mortal- ity, three studies reported elevated mortality rates and one study showed a de- creased rate; none of these rates was statistically different from 1.0. Three studies reported elevated estimated risk of leukemia mortality among participants, relative to comparisons; two of these were statistically significant. Based largely on the findings from these studies of low-level radiation exposure in military populations, but also based on what is generally known about radiation effects, our study has focused on all-cause, all-cancer, and leukemia mortality as primary endpoints (see Chapter 9~.
