Marin County Breast Cancer Clues

The biggest difference between Marin County women with breast cancer and their neighbors without the disease is the amount of alcohol they consume—with the heaviest drinkers raising their risk almost fourfold, researchers report.

In the first study comparing a group of women in Marin County with breast cancer to a control group, researchers at UCSF found that the length of time spent living in that county had no bearing on their likelihood of developing the disease. That suggests that a mysterious toxin in the air, water or soil in Marin County is not a likely cause for the area’s high breast cancer rate, according to Margaret Wrensch, professor of epidemiology at UCSF.

Concern over Marin County’s high breast cancer rate prompted the study, which was co-authored by members of Marin Breast Cancer Watch, an advocacy group that is pressing for more research into why so many of the county’s women are diagnosed with the disease. Marin’s breast cancer rate is among the highest in the nation with 199 cases out of 100,000 women. The rate for the Bay Area in general is 155 per 100,000, and it is 144 per 100,000 for the U.S. as a whole.

The study was based on interviews with 285 women with breast cancer identified through a registry of cases at the Northern California Cancer Center. They were matched with 286 other breast cancer-free Marin County women of the same age, identified from random telephone calls. Most well known risk factors for breast cancer—family history of the disease, late childbearing and hormone replacement therapy—did not differ much between the women in the two groups, probably because those risk factors are so common among women in Marin County that it would be hard to pick up differences in their sample. Other factors that were associated with higher breast cancer risk by the study included later menopause, not using birth control pills, four or more mammograms from 1990 to 1994, beginning to drink after the age of 21, being a heavy smoker, and being raised in organized religion.

But by far the biggest difference seen was in the consumption of alcohol, with women who had at least two drinks a day being diagnosed with breast cancer at more than twice the rate of those who drank less. At three drinks a day, the risk rose almost fourfold.