A new study finds Marin County isn't the only part of the Bay Area where breast cancer rates are higher than the state average.
Researchers with the California Breast Cancer Mapping Project say they've also found a higher rate of breast cancer -- defined as 10 to 20 percent above statewide average -- in parts of Alameda, Sonoma, Napa, Solano and Contra Costa Counties.
Researchers looked at breast cancer rates by census tract, a more targeted approach than other numbers which are collected county-wide.
“This is an opportunity for us to think about communities affected by breast cancer in new ways,” said Dr. Eric Roberts, of Oakland's Public Health Institute and principal investigator for the study [PDF]. “And one of them is that -- although no one argues that there isn't an excess of breast cancer in Marin County -- when you look at it this way, it appears more that Marin County is part of an extended area.”
The study also found portions of the South Bay have higher than average breast cancer rates. Those areas include communities in San Mateo County, northern Santa Clara and southern Alameda Counties.
Roberts says the maps are not designed to determine the causes of breast cancer rates or detect specific environmental triggers, but will point researchers in new directions. As the San Francisco Chronicle reports:
(T)he study is an important tool for further research, said Janice Barlow, executive director of Zero Breast Cancer, a nonprofit advocacy group based in Marin County. She served on the project's advisory group.
"This opens up whole new areas to look at and explore," Barlow said. "It's an opportunity to advance our understanding of why there are such geographic variations in breast cancer incidence."
About 26,300 new cases of breast cancer are diagnosed in women in California each year, leading to about 4,175 deaths annually.
Marin County has long been studied for its high rates of breast cancer, but many questions remain unanswered. Risk factors for the disease include having a family history of breast cancer, being white, hormone use after menopause, being an older mother or never having children, alcohol consumption, and having higher socioeconomic status. Still, many health experts say those factors don't explain the rate disparities along geographic lines.
"We can say definitively that breast cancer is caused by a combination of genetics, behavioral risk factors and the environment," said Dr. Eric Roberts, a research scientist at Public Health Institute and principal investigator of the California Breast Cancer Mapping Project. "The state of the science is that we really don't know what the mix is."
While the new study doesn't delve into the reasons behind the clusters, it offers a more specific geographic picture of the problem. Its authors suggest the results may be used by lawmakers and local health officials to redirect education, outreach and screening efforts.
The study also found that parts of Los Angeles County, Orange County and a small portion of western Riverside County are also seeing higher rates of breast cancer.