Vol. 06 Issue 3, Fall 2001
Research Commentary: Thoughts on Recent Finds Regarding Organochlorines and Breast Cancer Risk
The Ribbon
Kirsten Moysich, Ph.D.
Assistant Professor
Cancer Prevention, Epidemiology and Biostatistics
Roswell Park Cancer Institute
Results from a recent publication of a combined analysis of five large epidemiological studies indicated that there was no link between blood levels organochlorine compounds, such as DDT and polychlorinated biphenyls (PCBs), and increased risk of breast cancer (Laden, et al., 2001). This publication has led to some strong reactions from the media in which it was stated that there is now convincing evidence for the lack of an association between these chemicals and breast cancer. Some of these media reports also discredited members of the environmental and breast cancer advocacy community, as well as the scientific community, who are not entirely convinced that chemicals like PCBs and DDT do not play a role in the development of breast cancer.
As an environmental epidemiologist and a co-author of the recent study, I would like to make the following comments regarding these media reactions:
The number of well-designed epidemiological studies on the role of organochlorines in the development of breast cancer is still relatively small. Unlike laboratory studies, in which scientists have tight control over the research experiments, epidemiological studies are subject to many sources of bias that can influence the results of our studies. Because of the possibility that so many factors other than the exposure of interest could explain the findings in our line of research, studies on a given exposure (e.g., PCB and DDT blood levels) and a disease (e.g., breast cancer) have to be carried out numerous times in different populations, using different research methods. Thus, as epidemiologists we do not consider the first study that demonstrates a link between an exposure and a disease to be the most important, but rather the 20th or 50th study that shows the same association. This is true for studies on diet and cancer, exercise and cardiovascular disease, as well as smoking and reproductive health. For instance, there is still debate over whether or not a diet high in fat is related to greater risk of this cancer even though a large number of epidemiological studies have been published on this issue. It is baffling why the limited number of studies on organochlorines and breast cancer risk should be evaluated differently. Having said this, I do not believe that the current body of evidence points to a role of chemicals such as PCBs and DDT in the development of breast cancer in general, but I am bothered by the labeling of those who might disagree with me as environmental fanatics.
Although the existing body of evidence appears to suggest that there is no overall association between organochlorine chemicals and breast cancer risk, there may be subgroups of women who are more susceptible to a potentially harmful effect of these chemicals. First, two out of two studies that addressed this research question among African-American women have demonstrated some evidence that, in this ethnic subgroup, higher blood levels of PCBs were related to greater risk of breast cancer (Krieger, et al., 1994; Millikan, et al., 2000). It should be pointed out that in one of these studies (Krieger, et al., 1994), it could not be ruled out that chance was an explanation for the observed increase in risk. Second, two out of two studies that investigated the effect of PCBs on breast cancer risk among women with genetic characteristics that may make them more vulnerable to a possible adverse effect of PCBs on breast cancer risk, have shown that women with high PCB blood levels and this genetic trait are at higher risk of this disease than women with lower PCB levels, without the genetic trait (Moysich, et al., 1999, unpublished data). Although these findings are very interesting and suggestive, it is possible, just like any other epidemiological association, that they were the result of the numerous sources of bias that cannot be avoided in epidemiological studies. However, these initial findings certainly should be further investigated. In contrast to the views of some, I believe that we do not know the full story of the role of organochlorines in the development of breast cancer at this point and do need future studies in these potentially susceptible subgroups of women. Even though these studies are expensive and time consuming, they deserve the same attention and scientific vigor from the scientific community as do studies on exposures associated with less industry and advocacy interests.
References
Krieger, N., M.S. Wolff, R.A. Hiatt, M. Rivera, J. Vogelman, N. Orentreich. "Breast Cancer and Serum Organochlorines: A Prospective Study Among White, Black, and Asian women." Journal of the National Cancer Institute 86 (1994): 589-599.
Laden, F., G. Collman, K. Iwamoto, A.J. Alberg, G.S. Berkowitz, J.L. Freudenheim, S.E. Hankinson, K.J. Helzlsour, T.R. Holford, H.Y. Huang, K.B. Moysich, J.D. Tessari, M.S. Wolff, T. Zheng, D.J. Hunter. "1,1-Dichloro-2,2-bis(p-chlorophenyl)ethylene and Polychlorinated Biphenyls and Breast Cancer: Combined Analysis of Five U.S. Studies." Journal of the National Cancer Institute 93 (2001): 768-775.
Millikan R., E. Devoto, E. Duell, C.K. Tse , J. Beach, S. Edmiston, S. Jackson, B. Newman. "DDE, PCBs and breast cancer among African-American and white residents of North Carolina." Cancer Epidemiology Biomarkers & Prevention Nov 9 (2000): 1233-40.
Moysich K.B., P.G. Shields, J.L. Freudenheim, J.E. Vena, P. Kostyniak, H. Greizerstein, E.F. Schisterman, J.R. Marshall, S. Graham, C.B. Ambrosone. "Polychlorinated biphenyls, cytochrome P4501A1 polymorphism, and Postmenopausal Breast Cancer Risk." Cancer Epidemiology Biomarkers and Prevention 8 (1999): 41-44.
Program on Breast Cancer and Environmental Risk Factors