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Last Edited: 5/26/2004

Breast Cancer

Breast cancer is the most common type of cancer among women in both California and the US (other than skin cancer).  Though it can occur in men also, this is rare.  Breast cancer is a malignant tumor that starts in the breast tissue and, if left untreated, may spread (metastasize) to other parts of the body.  The National Cancer Institute website provides excellent information on what you need to know about breast cancer including information about detection, symptoms, diagnosis and treatment.


The breast cancer incidence rates in California have been fairly stable since 1988 however more cancers are being diagnosed at an early stage (while they are still very small).  Deaths from breast cancer in California have declined by more than 20% since 1988, due to the combined effects of better treatment and earlier diagnosis.  The shift to earlier-stage diagnoses reflects, in part, the successful efforts of doctors and cancer awareness programs to increase the number of women who receive regular breast cancer screening.  While this is very good news for California women, breast cancer incidence may begin to rise in the 2000-2010 decade as the large number of women born after World War II reach the age where breast cancer becomes more common.  This group of women may also be at higher risk for developing breast cancer than their mothers, due to earlier menarche (the beginning of menstruation), smaller family size, delayed childbearing and other factors.


For the past several years, breast cancer incidence rates in California, compared other areas in of the US, have been 26% lower among Asian/Pacific Islanders, 7% lower among African Americans, about the same among Hispanics and 5% higher among non-Hispanic white women.  Compared to other parts of the state, breast cancer incidence rates within California tend to be highest in the San Francisco Bay Area and next highest in the Southern Coastal area.  For reasons that are not completely understood, being highly-educated and financially well-off are associated with a higher risk of developing breast cancer.  Non-Hispanic white women in the highest socioeconomic status category are at highest risk.  Some geographic variation in breast cancer rates within California may be related to these factors.


The Environmental Health Investigations Branch (EHIB), in collaboration with many other agencies and organizations, has been actively involved in the investigation of breast cancer incidence trends and patterns within California.  Much of this research has occurred in response to public concern over the need to understand why breast cancer incidence appears to be higher in certain parts of the state.  Additionally, public concern over the increasing exposures to environmental factors such as electromagnetic fields, pesticides, traffic, cigarette smoking and endocrine disrupting chemicals (see topic links below) has spurred EHIBís research to investigate the associations between breast cancer incidence and living or working near sources of such exposures.





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