A geographic portion of southern Sonoma County has been identified as one of two Northern California regions with higher than average rates of invasive breast cancer, researchers said this week.

The portion is defined by a single 2010 census tract that includes the southern edge of the city of Sonoma, though it is primarily rural.

The findings are detailed in a report released Tuesday by the Oakland-based Public Health Institute's California Breast Cancer Mapping Project. The report, which covers the period from 2000 to 2008, pinpoints four previously unidentified regions where cases of invasive breast cancer have been significantly higher than the state average.

These four "areas of concern" include two regions in Southern California, one in the South Bay and one that encompasses the northern Bay Area. The latter region -- which includes the Sonoma County census tract -- circles San Pablo Bay and includes portions of Marin, Napa, Solano and Contra Costa and Alameda counties.

Breast cancer is the most common cancer among women in California. Each year an average of 26,3000 women are diagnosed with breast cancer and about 4,175 die from it.

Unlike county-level surveillance data, the mapping project examined breast cancer rates within and across county boundaries by looking at data from the California Cancer Registry against census tracts.

The report does not explain why these four regions showed breast cancer rates that were higher than the state average. But authors of the report said they hoped the data would serve as an important epidemiological tool for county and state health officials.

"We found that the specific communities most impacted by breast cancer can fall within or across counties. By identifying these communities we can more efficiently and effectively direct resources to them," said Dr. Eric Roberts, principal investigator of the mapping project, in a statement.

Dr. Dennis McDonald, the former director of the Sutter Women's Health Center in Santa Rosa, said he had always heard that certain regions of Northern California have a higher incidence of breast cancer.

McDonald, who was recently transferred to Sacramento to head up Sutter Health's regional women's imaging program, said breast cancer diagnosis rates detected at the Sutter Women's Health Center in Santa Rosa were higher than the national average.

He is unsure if this was related to higher rates of breast cancer "or the fact that we used ultrasound liberally to further evaluate mammography or clinical abnormalities, especially in those women with dense breasts," he said.

The Sonoma County census tract that was included in the report is located just south and southeast of the city of Sonoma and continues all the way down to San Pablo Bay.

According to the 2010 Census, the tract has a population of 2,322 whose median age is about 49. About 87 percent of its residents are white, and 16.6 percent are Hispanic or Latino.

Diagnosis rates in this census tract were 10 to 20 percent higher than the state average in 2001. Any tract that logged rates significantly higher than the state average during any of the years between 2000 and 2008 was pinpointed.

Among the report's key findings for the North Bay Area region:

While diagnosis rates were higher than the state average, the region had a lower percent of women diagnosed with late-stage breast cancer.

White women accounted for 71 percent of invasive breast cancer, although this group comprised only half the region's female population in 2010.

Dr. Elizabeth Peralta, a breast surgeon with Sutter Pacific Medical Foundation, said the report could open new roads of study. She said using census tracts rather than county lines could prove useful in zeroing in on communities that share the same demographics, socioeconomics and other geographic characteristics.

"The bottom line is, can we map the rate of cancer in geographical areas that might reveal new questions to study?" she said, adding that countywide figures combine high risk communities with low risk areas.

Using a geographic approach to health intervention could yield better results, she said. "You try to tailor projects to fit a region."

You can reach Staff Writer Martin Espinoza at 521-5213 or martin.espinoza@pressdemocrat.com.