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Scientists May Have Uncovered a Key to Understanding Why Many Women Get Breast Cancer

We may be one step close to understanding why some women with no family history develop breast cancer.

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Editor's Note: This is the first in a two-part series. Read Part 2.

Deep in a laboratory freezer, 100,000 vials of blood have been frozen for the better part of five decades.

For scientist Barbara Cohn, it’s a treasure trove. Collected from more than 15,000 San Francisco Bay Area women after they gave birth in the 1960s, each vial of blood holds a woman’s lifetime of secrets.

The blood bears the chemical signature of environmental pollutants, some long banned, that the women were exposed to decades ago. Cohn, who directs the research in Berkeley, Calif., believes these early-life exposures may hold the key to understanding a woman's risk of breast cancer today.Scientists say these vials could help them unravel one of the most enduring medical mysteries: Why do some women, with no family history, develop breast cancer?

The women's blood is being tested for traces of dozens of pollutants – used by industry and found in many consumer products – that can impersonate estrogen and other hormones. The theory is that early exposure to these chemicals, even before birth, inside the mother’s womb, may fundamentally alter the way that breast tissues grow, triggering cancer decades later.

Cancer patients and their doctors have long puzzled over what factors in a woman’s environment may raise her risk of breast cancer.  One of every eight women in the United States is diagnosed with breast cancer during her lifetime, with more than 232,000 new cases diagnosed yearly, according to the American Cancer Society. Only  5 to 10 percent can be accounted for by genetics; other known risk factors include age, obesity and low physical activity.

Earlier this month, a science advisory  panel urged the federal government to fund more projects aimed at uncovering the environmental causes of breast cancer because eliminating these factors may provide the greatest opportunity to prevent it.

It’s particularly vexing for scientists because it’s difficult to unlock a woman’s exposures during her most critical times for breast development: in the womb and during puberty and pregnancy.

“As researchers looking at adult outcomes of disease processes such as breast cancer, one of the biggest challenges we face is trying to get a handle on prenatal exposures and what is going on in the prenatal environment,” said Shanna Swan, an environmental health scientist at Mount Sinai School of Medicine in New York.

Many scientists have been looking for connections between various environmental exposures and the disease – with mixed results. Some findings suggest links to a few chemicals, including the banned pesticide DDT. But others have found no link.

And in 2011, an institute of the National Academies of Sciences reported “a possible link” between breast cancer and some common ingredients of vehicle exhaust, benzene and 1,3-butadiene. But the  report said the jury is still out for most other widespread chemicals, such as pesticides, ingredients of cosmetics and bisphenol A (BPA).For example, experts from the American Cancer Society, reviewing previous studies, in 2002 found  no association between breast cancer and chlorinated chemicals including DDT.

Nevertheless, absence of evidence isn’t evidence of absence, said Elizabeth Ward, National Vice President of Intramural Research at the  American Cancer Society. Many of the biggest risk factors remain unknown, she said.

The problem with most studies is that they measured levels of chemicals in women later in life, after they were diagnosed with cancer, not during periods when the breast is most susceptible, said Suzanne Fenton, a reproductive toxicologist at the  National Institute of Environmental Health Sciences in North Carolina.

“The research doesn’t prove that the link doesn’t exist or that these chemicals are safe for the breast,” Fenton said. “It shows that we may not have been asking the right question.”

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