Breast Cancer: It's Not All In The Genes

by Catherine Zandonella, M.P.H

October is Breast Cancer Awareness Month, and before it ends an additional 14,800 women in the U.S. will be diagnosed with the disease. Roughly one of every nine women will develop breast cancer in her lifetime. While genes play a role in many cases of breast cancer, roughly 70 percent of diagnosed women have non-inherited cancer. For these, we have to look at what we do and the world around us.

Some environmental causes are established. Having children late in life and early onset of puberty both increase breast cancer risk. Exposure to radiation from chest x-rays during childhood and taking hormone replacement therapy are also known to increase risk. Breast cancer rates are higher in women who are obese, women who gain weight during adulthood and those who drink alcohol routinely. On the bright side, vigorous exercise for 45 to 60 minutes five or more days per week can lower breast cancer risk in all women, studies have shown. And for postmenopausal women, any regular exercise performed can help.

Recent research into other factors, however, has returned intriguing results on contaminants ranging from pesticides to tobacco smoke. Here's a roundup:

Chemicals in the Environment

Increasingly, studies are finding that chemicals common in the world around us play a role in the development of breast cancer. These chemicals may contribute to breast cancer risk by damaging DNA, promoting tumor growth, or altering mammary gland development both before birth, during puberty, and during and after pregnancy.

In a review of the studies by the Silent Spring Institute, researchers identified 216 chemicals that were linked to breast cancer in at least one animal study. These included pesticides, dyes, pharmaceuticals and hormones as published in the journal Cancer in May 2007. Twenty-nine of these chemicals are produced in the U.S. at greater than one million pounds per year, 35 are air pollutants and 73 have been found in consumer products or as food contaminants.

Dioxins

Dioxins in the fat of milk, meat and fish are among those chemicals most strongly linked to breast cancer. Known to affect mammary gland architecture in animal studies, dioxin can have long-lasting effects on breast development if exposure occurs in utero, during a time corresponding to the first trimester of pregnancy, when some women may not even know they are pregnant. If animals with altered mammary gland development due to in utero dioxin exposure go on to mate and rear offspring, the mammary glands of their offspring are also altered, indicating that exposure to this environmental toxicant can alter breast development in multiple generations, according to the June 2006 issue of Endocrinology. Dioxin can also harm breast development if exposure happens during the other two critical periods, puberty and during lactation.

Air Pollution

Air pollution contains polyaromatic hydrocarbons (PAHs) among many other chemicals linked to breast cancer in laboratory animals. "When people think breast cancer, they don't immediately think air pollution," says Julia Green Brody, PhD, a scientist at the Silent Spring Institute in Massachusetts. "But there is an increasing body of research that suggests to us that air pollution might play a role in breast cancer risk because it contains chemicals that are known to cause breast cancer in animals."

Pesticides

Another potential source of chemical exposures is pesticides. Among women living on Long Island, NY, breast cancer risk is higher in those with lifetime self-reported use of residential pesticides, a study in the March 2007 American Journal of Epidemiology found. Long-banned DDT still has a place on the list of potential exposures linked to breast cancer. Millions of American women were exposed to DDT from insect control programs in the 1940s and 1950s. The study found that women exposed to DDT in childhood (as measured by the presence of DDT in their blood donated during the 1960s as part of an unrelated study) had a greater risk of breast cancer than women born before DDT was used. Whereas previous studies of DDT exposure and breast cancer found no link, this study was able to test for DDT in the blood of women closer to the time of exposure. "Women who could have been exposed under the age of 14 to DDT are the ones that had the largest risk ... [and] nearly every women in the U.S. during those years would have been exposed," says Barbara Cohn, Ph.D., of the Public Health Institute in Berkeley, California.

The critical aspect of this study is that it addresses when exposures occurred. Evidence is accumulating that exposure during childhood leads to a greater increase in risk of breast cancer. For example, girls exposed to radiation in Japan during World War II went on to have a higher rate of breast cancer than women who were exposed as adults. "The time of exposure seems to be important," says Mary S. Wolff, Ph.D., an environmental scientist at Mount Sinai School of Medicine. "Exposure very early in life or post-natally can cause changes that you don't see if animals are exposed after birth or after pubertal development."

Second-hand smoke

The link between second-hand smoke and breast cancer remains controversial. Scientists at the California Environmental Protection Agency, however, are convinced that environmental tobacco smoke can cause breast cancer, at least for younger, premenopausal women, based on a review of the studies published in a February 2007 issue of Preventative Medicine. For reasons that are still unclear, the risk of breast cancer in women who actively smoke is not all that much greater than women who are exposed to passive smoke.

Better Choices

Disturbingly, while the incidence of new breast cancer cases has dropped recently among white females, researchers think this may be due to fewer women getting mammograms. Fewer screenings mean fewer diagnoses, and this trend is backed up by the fact that African American women have an unchanged incidence of breast cancer and have unchanged levels of using mammograms. (Another reason for fewer cases among whites is that they have decreased their use of hormone replacement therapy, linked to an increased breast cancer risk in postmenopausal women, whereas blacks have not.)

The American Cancer Society recommends that women age 40 and older should have a mammogram annually for as long as they are in good health.

Get rid of pesticides inside and outside the house and take up organic lawn care practices instead, see "Grass Roots: Easy, Organic Lawn Care" www.thegreenguide.com

Never smoke tobacco and avoid secondhand smoke.

To reduce dioxin intake, drink skim milk rather than whole and trim fat from meat.

Avoid personal care products containing ingredients linked to cancers; for suspect compounds, see Greenerpenny and "The Dirty Dozen Chemicals in Cosmetics."