Vol. 04 Issue 1, Winter 1999

Research Commentary: New Evidence on Dieldrin in Study of Danish Women
The Ribbon 

Organochlorine Exposure and Risk of Breast Cancer A. P Hoyer, P. Grandjean, T. Jorgensen, J. W. Brock and H. B. Hartvig.The Lancet, 352: 1816-1820, 1998.

There had been considerable interest in the role that certain organochlorine chemicals may play in breast cancer risk. Much of this interest is because some of these chemicals have been shown to act like weak estrogens. The hormone estrogen has been shown to play a role in breast cancer risk. Organochlorines include environmental pollutants such as polychlorinated biphenyls (PCBs), and some insecticides that are banned from use, but that stay in the environment for long periods of time. Organochlorines can be stored in fat, and contaminated fish and fatty meats can be a source of low-level exposure to humans. Levels of organochlorines in the blood can reflect the amount of organochlorines stored in body fat.

Researchers in Denmark worked with scientists in the U.S. to see if there was a relationship between levels of many different kinds of organochlorines in the blood and the risk of breast cancer. This study obtained blood samples from 7712 women from Copenhagen, Denmark. The blood was frozen and stored for later analysis. Over the next 17 years, 240 of these women developed breast cancer. From the larger group of women (called a cohort), 477 women were selected that did not have breast cancer, but that were the same age as the women with breast cancer. This type of epidemiological study is called a prospective, nested case-control study*.

The blood samples from the women with and without breast cancer were analyzed for many types of organochlorines by scientists at the Centers for Disease Control in Atlanta, Georgia. The researchers did not find a significantly higher risk of breast cancer in women with high serum (blood) levels of the organochlorine insecticide DDT or its breakdown product DDE, or the insecticides beta-hexachlorocyclohexane, lindane, kepone, chlordane metabolites, or various forms of PCBs.

They did, however, find that breast cancer risk was two fold higher in women with the highest blood levels of an insecticide called dieldrin compared to women that had the lowest blood levels of this chemical. This is the first report that has found an association between the blood levels of dieldrin and breast cancer risk. There are only two other very small studies that have looked at breast cancer risk and body levels of dieldrin in women. Neither study found significantly higher levels of dieldrin in women with breast cancer compared to women without breast cancer. One of these studies was not able to detect dieldrin in any of the breast fat samples.

Strengths of the Danish study are that it had moderately high number of subjects, it determined the blood levels of many types of organochlorines, and it obtained blood samples before the women were diagnosed with breast cancer. The researchers also looked at patterns of other breast cancer risk factors, called confounding factors, in the women with and without breast cancer. Confounding factors that were considered in the study included: weight, height, number of full-term pregnancies, alcohol consumption, smoking, physical activity, menopausal status and education. One important confounder, history and duration of breast-feeding, was not considered.

This study begs the question, why would dieldrin affect breast cancer risk? The authors of the Danish study cited evidence from another study that showed that dieldrin was a weak environmental estrogen when evaluated in the "E-SCREEN" test. (The E-SCREEN test measures estrogenicity by seeing if a chemical can stimulate the growth of estrogen-dependent human breast tumor cells.) Others have done additional tests to see if dieldrin is an environmental estrogen. These other tests either did not show that dieldrin was estrogenic or found that dieldrin was an extremely weak estrogen. So, we really don't know how dieldrin may affect breast cancer risk.

It is not possible to conclude that breast cancer risk is affected by levels of dieldrin in the body from the results of the Danish study. More studies are needed to confirm the results of this study. It also needs to be determined if there are different patterns of dieldrin exposure in Scandinavian countries compared to other countries. It is possible that the high fish and fat consumption in Denmark may result in a higher exposure to dieldrin than in other countries where residents eat less fat and less fish.

Program on Breast Cancer and Environmental Risk Factors
Cornell University, College of Veterinary Medicine
Vet Box 31, Ithaca, NY 14853-6401
Phone: 607.254.2893; Fax: 607.254.4730
Email: breastcancer@cornell.edu
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