Environment Department, Electric Power Research Institute, Palo Alto, CA 94304
In their recent Journal article, Charles et al. (1) examined the association between prostate cancer mortality and occupational exposure to magnetic fields and polychlorinated biphenyls. The authors took advantage of the availability of a high-quality data set from a well-described occupational cohort. Numerous studies describing results from analyses of the same data set (e.g., Savitz and Loomis (2), Savitz et al. (3)) have contributed significantly to the understanding of associations between occupational exposure to magnetic fields and mortality from various causes.
The authors of this study (1) carefully weighed its strengths and limitations. They failed, however, to discuss one of the most serious limitations of their analyses. The study examined prostate cancer mortality rather than incidence. Mortality from any type of cancer is affected by not only the incidence of the cancer (which ideally should be used for etiologic research) but also the survival of cases after diagnosis. The survival of cancer cases could depend on various factors, such as the stage of cancer at diagnosis, the type of treatment, and the presence of any comorbidity.
For prostate cancer, the distinction between incidence and mortality is especially important. As a result of relatively good survival, there is a large discrepancy between prostate cancer incidence and mortality rates. On the basis of data from the Surveillance, Epidemiology, and End Results (SEER) program (4), during 19731988 (a period included in the Charles et al. study (1)), the annual age-adjusted prostate cancer mortality rates were, on average, only 29 percent of the corresponding annual age-adjusted prostate cancer incidence rates among White men (range, 2435 percent) and 38 percent among Black men (range, 3541 percent).
To illustrate the unreliability of mortality data as a substitute for incidence data, we can look at racial differences in incidence and mortality rates for prostate cancer in the United States. Based on SEER data (4), during 19731988, the annual age-adjusted incidence rates were, on average, 1.51-fold higher among Black men than among White men (range, 1.381.63). During the same period, the annual age-adjusted mortality rates were, on average, 1.99-fold higher among Black men than among White men (range, 1.822.15). The twofold increase in the death rate among Black men compared with White men is only partially explained by the higher incidence among Black men. On the basis of available evidence, it seems that the additional increase in the mortality rate among Black men is the result of their poorer survival because of a combination of later-stage diagnosis for Blacks and differences in treatment between Blacks and Whites (5, 6). Racial differences in stage at diagnosis and treatments are likely to stem from cultural and socioeconomic differences between the two ethnic groups (6, 7).
Similarly, men in typical high-magnetic-field-exposure occupations (e.g., linemen) could also systematically differ in many social, economic, and cultural ways from other, unexposed workers. If any of those characteristics is associated with survival, the observed difference in prostate cancer mortality between exposed and unexposed workers may not reflect real differences in prostate cancer incidence rates.
Although the study (1) was well designed and executed, and the results were clearly presented, the value and interpretation of the results are questionable. Because of the use of mortality data, these results provide no reliable support for a possible etiologic relation between magnetic field exposure and prostate cancer development. Interpretation of the results becomes even more problematic when we consider the lack of convincing laboratory evidence in support of a relation between magnetic field exposure and carcinogenesis in general, and prostate cancer development in particular (8).
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