Researchers already knew (and were baffled as to why) Marin County’s women experienced higher rates of breast cancer, but a new study using census tract data from the Public Health Institute’s California Breast Cancer Mapping Project found an elevated incidence of breast cancer in Sonoma, Napa, Solano and Contra Costa Counties. Some South Bay communities, including San Mateo County, northern Santa Clara and southern Alameda Counties, also have higher than average rates.