Researchers call for better studies on environmental links to breast cancer
BERKELEY – A much broader net needs to be cast in the search for environmental links to breast cancer, concludes a report released today (Monday, March 10) that stems from a landmark gathering last year of researchers, public health officials and activists.
According to the report of the International Summit on Breast Cancer and the Environment, current research methods and health initiatives are insufficient when it comes to understanding and preventing non-genetic causes of breast cancer.
The report was submitted to the Centers for Disease Control and Prevention (CDC) by researchers at the University of California, Berkeley, who organized the summit in Santa Cruz last May. The CDC funded the summit through a grant to the UC Berkeley Center for Family and Community Health.
Research needs to evaluate contributing risk factors at all ages of a person's life, including infancy and adolescence, the report says. The report also promotes the establishment of a national biomonitoring program to track exposures using breast milk and other body fluids, the improvement of lifetime exposure assessment for complex chemical mixtures and increased community involvement at all levels of prevention and research.
"This report is the closest anybody's come to developing a single voice on the issue of environmental links to breast cancer," said Patricia Buffler, principal investigator for the summit, professor of epidemiology and former dean of UC Berkeley's School of Public Health. "It was born out of a process that brought together groups with different perspectives, backgrounds and agendas for a productive dialogue on a difficult topic."
The report highlights the most current research on breast cancer presented at the summit by leading experts in the field. Based partly on evidence presented at the summit, more than 100 participants generated thousands of recommendations that were eventually narrowed down to a high priority list of 28 in the areas of research, education and communication, and policy.
The incidence of breast cancer has increased significantly over the past several decades, with some figures indicating that less than half of all cases in the United States are explained by known risk factors such as reproductive patterns and genetic traits. At the same time, other research on breast cancer cases worldwide suggests that up to 85 percent of the variation in rates can be explained by duration of breastfeeding and the number of children a woman has borne.
In advocating for the expansion of breast cancer research to cover risk factors over a woman's lifetime, the report points to key periods such as the onset of menarche, the first full-term pregnancy and menopause. Researchers know that the early onset of menarche is a risk factor for breast cancer and are investigating why girls today are getting their periods at an earlier age than before.
Some clues come from research that says more exercise leads to physiological changes that may subsequently affect breast cancer risk. Researchers at the University of Southern California (USC) found that young girls who are physically active start their periods at an older age, and that moderate amounts of increased exercise reduce the frequency of ovulatory menstrual cycles among teenage girls.
"This results in less exposure to those hormones that are important for breast cancer risk," said Leslie Bernstein, professor of preventive medicine at USC and one of the researchers at the summit. "We have also shown that physically active women have a lower risk of breast cancer. What we need to understand better, however, is exactly how much and what type of exercise is needed to have a beneficial effect."
Another position advocated by the report was to use breast milk to monitor levels of bio-accumulative chemicals in the body. In contrast to blood and urine samples, breast milk is easier to collect and can indicate exposure to chemicals that accumulate in fat cells. After a hearing last fall in which some of these summit recommendations were presented, two California legislators agreed to introduce a bill this year that would provide funding for a program that would check breast milk samples for carcinogenic toxins.
As for which toxins should be studied, the report highlights the need for better biomarkers that can indicate exposure to xenoestrogens and compounds associated with the development of mammary tumors in animals. In addition, science needs to move beyond measuring single agents to complex chemical mixtures, the report says.
"Researchers have tended to look at the body burdens of things that stick around," said Shelia Zahm, deputy director of the division of Cancer Epidemiology and Genetics at the National Cancer Institute. "But there are other compounds that pass through the body quickly that should be investigated, even if the research is based on a questionnaire. We can ask about what's being used on nearby land or what kind of pesticides and fertilizers are used on the lawns."
The effects of smoking also emerged as a major concern at the summit, leading to the recommendation that passive smoking exposures be eliminated nationwide.
Stanton Glantz, professor of medicine at UC San Francisco, said the importance of tobacco's role in breast cancer risk has been underplayed until now. Prior studies have failed to show conclusive links between smoking and breast cancer, but Glantz said those studies were flawed because they did not separate people who were regularly exposed to secondhand smoke from people in a smoke-free environment.
"Conventional wisdom has held that there is no link between smoking and breast cancer," said Glantz. "But, if you think about it, many of the major chemicals people are worried about in the environment are found in secondhand smoke. One cigarette is like a mini toxic waste dump in your mouth."
Glantz said some recent studies have shown that secondhand smoke increases breast cancer risk by about 40 percent, but the evidence is still being debated in the scientific community.
Nevertheless, under the "precautionary principle" espoused by the summit attendees that it is better to be safe than sorry, enough evidence exists for the increased risk from secondhand smoke to warrant protective action.
What is clear in the report is the overall need for researchers to improve communications and collaborations with members of the community. The summit came about through the dissatisfaction many community activists had with the design of the well-publicized Long Island Breast Cancer Study. The study, which failed to find a link between a limited number of pollutants and breast cancer, did not look at timing of exposure or cumulative exposure to a mixture of chemicals.
"One lesson we have learned from the Long Island study design was the importance of involving the community early and on an ongoing basis as equal partners," said UC Berkeley's Buffler. "Not only does it increase the community's confidence in the final results, it may help researchers select a study design that better fits the circumstances of the project."
"The planning and the conference were only the beginning," said Lisa Wanzor, associate director of San Francisco-based Breast Cancer Action. "Now, the hard work starts of implementing the recommendations that emerged from the scientists, advocates and community members."