Two Bay Area regions are among four in California with substantially higher than average rates of invasive breast cancer, researchers revealed Monday.

Breast cancer rates in the four regions were 10 to 20 percent higher than the state average from 2000 to 2008, according to a study that pinpoints the location of cancer cases with a new level of precision. What caused the higher rates remains a mystery.

"Breast cancer comes from a mix of genetic and environmental causes, and we don't know what the mix is," said Dr. Eric Roberts, principal investigator for the study.

"This is a way that state and county officials can understand better where the susceptible populations are," he said.

In the Bay Area, the high-rate regions include the western half of Contra Costa County, the northernmost part of Alameda County, and a swath running around the southern end of the bay through Alameda, Santa Clara and San Mateo counties.

The report stops short of calling the areas "cancer clusters," instead referring to "areas of concern."

A breast cancer organization advocate said the report raises important questions.

"What are people exposed to? Are they exposed to higher levels than people in other communities?" asked Connie Engel, science and education manager for the Breast Cancer Fund, which seeks to uncover environmental causes of breast cancer.

But, Engel said, "I see this as empowerment via knowledge, rather than something to be alarmed about."

The previously unidentified areas were discovered by looking at breast cancer rates by census tract, instead of by county, as is typically done, said researchers from the California Breast Cancer Mapping Project, part of the Oakland-based Public Health Institute.

Marin County has long been known to have high breast cancer rates. The new study reveals that it trends higher primarily in the eastern portion of that county. High rates were also found in portions of Sonoma, Napa and Solano counties.

The two other regions were in Southern California and include sections of Ventura, Los Angeles, Riverside and Orange counties.

Known breast cancer risk factors include having certain genetic mutations, using combined estrogen-progestin hormone replacement therapy, having no children or fewer children later in life, consuming two or more alcoholic drinks per day and spending little or no time breast-feeding during a lifetime.

Such behaviors are more common among higher-income women. Regions of the nation with a large proportion of white women in upscale areas often have higher-than-average breast cancer rates, noted Dr. Robert Hiatt, associate director of population sciences at the UC San Francisco Helen Diller Family Comprehensive Cancer Center.

That description fits many Bay Area communities. But it wouldn't explain why Richmond and most of Oakland, for example, also have high breast cancer rates, said Hiatt, who was not involved with the study.

He noted that researchers at UCSF and elsewhere are looking at the effect environmental exposures might have on girls before puberty.

Women living in the affected areas can reduce their risk by talking to their doctors about when to begin mammography, losing weight, exercising and limiting hormone therapy and alcohol consumption, said Christina Clarke, a research scientist with the Fremont-based Cancer Prevention Institute of California. She was not involved with the study, but her organization provided some of the data used.

So were there other common themes that might hint at what is happening?

Mirroring statewide trends, the study found that white women were over-represented in breast cancer cases in these areas, compared to their presence in the population.

In the Bay Area, the report also noted that women in these areas were slightly more likely to have been diagnosed earlier than women statewide and were more likely to have private insurance.

The study is based on where women lived at the time of their diagnosis, not where they were during puberty or their child-bearing years.

Sandy Kleffman covers health. Contact her at 510-293-2478. Follow her at Twitter.com/skleffman.