Correspondence to: Victor E. Archer, M.D., Rocky Mountain Center for Occupational and Environmental Health, Department of Family and Preventive Medicine, University of Utah School of Medicine, 75 South, 2000 East, Salt Lake City, UT 84112-5120 (e-mail: archer{at}dfpm.utah.edu).
The sophisticated analysis of thyroid cancer and nuclear fallout by Gilbert et al. (1) overlooked some important confounders. Mortality from thyroid cancer varies markedly by ethnicity and region (2) and by population density. A strong hint of confounding by these factors is present in their Tables 3 and 4 (1), where many of the calculated risk values were negative. The base value (from persons least exposed) appears to have been too high. The reason may be that the lowest exposed groups lived mainly in densely populated eastern or coastal areas, in contrast to the largely white, rural population in the western mountain states who received the highest exposures. This reason applies to both their incidence and mortality data.
To check on the potential of the confounders noted above, a sampling of counties and states
has been used. Population data from 1954 (3) and cancer data (4) from 1950 through 1959 were used. The white and nonwhite division
was the same as that used by Riggan et al. (4), and included Hispanics
with whites. This time frame was the period of greatest fallout but before any fallout-induced
cancers would have been manifested. In Table 1, with the use of three
states from each of six different regions in the United States, the thyroid cancer death rates among
whites are found to be markedly different, rising from 0.45 deaths per 100 000 people in
one area (southeast) to 0.77 deaths per 100 000 people in the northeast, with the
mountain, south, and central states having lower than average rates. Nonwhite thyroid cancer
rates exhibit a similar regional variation but do not appear to be substantially different from white
rates.
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These data from large county and state samples suggest that ethnicity was probably not an important problem, but they emphasize that region and population density probably were problematic. To be valid, the analyses of Gilbert et al. must consider both of these confounders. The regional problem can probably be solved by limiting the analysis to those regions with the higher exposures and nearby less exposed regions or by matching the higher exposed counties with less exposed counties in regions with similar background rates. The population density problem might be solved by simply excluding all counties with a population of more than 200 000 people in 1954. If the authors can revise their analyses to consider these two confounders, it is likely that their radiation risk calculations will approximate those obtained by other studies (1).
The overall average exposure in 1951 was given as 0.0 Gy. This exposure rate seems strange because there were 12 atmospheric tests that year that had an equivalence of about 112 kilotons (3). This rate contrasts with 0.7 and 0.8 Gy attributed to testing in 1952 and 1958 from smaller kiloton-equivalent yields (3). Large radioiodine releases from Hanford, WA, reactors at about the same time must have given additional exposures to people in some of the counties in Washington, Idaho, Oregon, or Montana, but they were not mentioned.
REFERENCES
1
Gilbert ES, Tarone R, Bouville A, Ron E. Thyroid cancer rates
and 131I doses from Nevada atmospheric nuclear bomb tests. J Natl Cancer
Inst 1998;90:1654-60.
2 Collin SL. Etiopathogenesis of thyroid cancer. In: Falk S, editor. Thyroid disease. New York (NY): Raven Press; 1990. p. 434.
3 National Cancer Institute. Estimated exposures and thyroid doses received by the American people from iodine-131 in fallout following Nevada atmospheric nuclear bomb tests. Bethesda (MD): National Institutes of Health, National Cancer Institute; 1997 Report No.: DHHS Publ No. (NIH) 97-4264 and appendices.
4 Riggan WB, Van Bruggen J, Acquavella JF, Beaubier J, Mason TJ. United States cancer mortality rates and trends, 1950-1979. Vol 3. Washington (DC): Environmental Protection Agency and National Cancer Institute, EPA-600/1/-83-015a, Government Printing Office; 1983.
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