Breast Cancer in Marin: Solving the Mystery

Just across the Golden Gate Bridge from San Francisco, breast cancer rates among women in Marin were greater than in any other California county.  It had increased 60% between 1991 and 1999, and skyrocketed 20% between 1998 and 1999 alone, statistics that sent a wave of quiet panic through the community.
“There is no logical reason Marin County—where women hike pristine trails, eat organic and see their doctors regularly—has more breast cancer than almost any place on Earth,” the Chronicle reported.

Early studies suggested links to the affluence level of Marin County’s women that resulted in late or no child-bearing and above-average consumption of wine.
So closely watched were the results that each new research finding yielded a new Chronicle headline.  Marin County breast cancer rate blamed on lifestyle, demographics.  Breast cancer amid affluence: High rate in Marin County appears tied to wealth, education.  Marin County breast cancer clues:  Length of residence not significant, alcohol main risk factor.
But those headlines were premature.  After 20 years of studying data from several organizations and databases, CPIC scientists had a Eureka moment.
 
Researchers had been discounting a cancer-causing role for hormone therapy because data showed that women in Marin did not take more hormones overall than other California counties. 
But they realized Marin women were taking more combined (estrogen and progesterone) hormone therapy than women in other parts of the state, at a rate that roughly paralleled their increased incidence of breast tumors. 

CPIC made this discovery by tracing the low rates of hysterectomies in California.  To treat menopause, women who have not had hysterectomies must also use a second hormone in addition to estrogen—progesterone.  That hormone therapy regimen was precisely the one most often practiced by women in Marin and was strongly linked to breast cancer.  In addition, women in Marin were drinking 2 or more glasses wine per day, further increasing their cancer risk.

After a nationwide study found a causative link between breast cancer and hormone therapy, women quit hormone therapy like it was a bad habit.  Within a few months, the breast cancer rate in Northern California declined by 10% and a miraculous 18% in Marin County.

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To view the Cancer Prevention Institute’s research findings on ways to reduce cancer risks across lifestyle, genetics, environment and occupation, visit www.cpic.org