Are A-bomb survivor studies an appropriate basis for nuclear worker compensation?

Wakeford (2003) and Little (2003) wrote in response to our comments about the use of A-bomb survivor studies as a basis for U.S. nuclear worker compensation decisions (Wing and Richardson 2002).

Wakeford (2003) disagrees with our statement (Wing and Richardson 2002) that there is discrepancy between childhood cancer risk estimates from in utero radiation in A-bomb and diagnostic X-ray studies. Doll and Wakeford (1997) stated that "children exposed in utero to radiation from atomic bomb explosions have not experienced any corresponding risk of cancer," the discrepancy "being the most serious reason for doubt" about findings from the Oxford Survey of Childhood Cancers (OSCC) (Stewart et al. 1956; Bithell 1993), the first and largest study to demonstrate the link between childhood cancer and in utero irradiation. Doll and Wakeford (1997) concluded,

   only one reason would appear to be serious:
   namely lack of any comparable excess in cohorts
   of children irradiated in utero, most notably in
   those exposed to radiation from the explosion of
   the atomic bombs in Japan.

Although we raised questions about potential sources of bias in the A-bomb data, Wakeford (2003) argues that risk estimates for the in utero cohort of A-bomb survivors are "compatible with" findings from the OSCC, given the imprecision of these estimates. There are indeed few childhood cancers among those exposed in utero. However, the OSCC suggests approximately a 40% excess of childhood cancers following exposure to a prenatal X ray, the majority of cases being childhood leukemias (Bithell 1993). The work of McMahon et al. (1962) supports this finding but suggests an excess exclusively of childhood leukemia (Bithell 1993). In contrast, among Japanese survivors exposed in utero, there were no childhood leukemias and no childhood cancers before the age of 10 years (the age range of most OSCC cases). One case of childhood cancer in a 10-year-old has been reported (nephroblastoma) and a second case was reported in a 14-year-old (hepatoma) (Kato et al. 1989; Yoshimoto et al. 1988, 1991). Wakeford (2003) argues that, based on these two cases, a dose-response estimate for childhood cancer occurring at ages < 15 years following in utero exposure is compatible with, although smaller than, the effect estimate derived using OSCC data. More than imprecision, the absence of childhood leukemia and childhood cancers before 10 years of age may be indicative of selection bias in the A-bomb cohort (Stewart 2000).

Little (2003) wrote that uncertainties in external radiation dose estimates for A-bomb survivors are of comparable magnitude to those for badge-monitored nuclear industry workers. Because there was no instrumentation in place for measuring the survivors' doses at the time of exposure, survivors' dose estimates rely on information derived from a questionnaire. Therefore, issues related to the survey administration are fundamental to the accuracy of these data. The survey was conducted under direction of an occupying military force (Lindee 1994). Respondents had experienced a nuclear holocaust and potentially were suffering posttraumatic stress disorder. It is suspected that not all survivors truthfully reported their proximity to the epicenters of the bombings (Watts 2000). Estimates of uncertainty in radiation doses derived under simple statistical assumptions about the distributions of errors do not account for such problems.

Little (2003) also questioned concerns about selective survival in the A-bomb study. Shimizu et al. (1999) described evidence of selective survival in the Life Span Study based on analyses of noncancer mortality, and Stewart and Kneale (2000) noted differences in radiation-mortality dose-response relationships among survivors with and without reports of acute injuries, suggesting evidence of selective survival. Little's assertion that simple assumptions about errors in dose estimates reduce the statistical significance of differences in dose-response relationships between these groups (Little 2003) is premised on a misplaced reliance on statistical significance testing. Furthermore, Little fails to address Stewart and Kneale's central premise (Stewart and Kneale 2000): selection effects varied with age, being greatest for the very old and the young at the time of bombing. Stewart (2000) argued that a more pronounced survival of the fittest at the extremes of age has influenced evidence of radiation effects.

Critical evaluation of potential sources of bias in the A-bomb studies is timely because of proposals that this study provide the basis for judgments in worker compensation.

The authors declare they have no conflict of interest.

David Richardson

Steve Wing

Department of Epidemiology

School of Public Health

University of North Carolina

Chapel Hill, North Carolina

E-mail: [e-mail]

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