How Was the Map Produced?

We created age-adjusted Standardized Incidence Ratios (SIRs) of invasive breast cancer for white, non-Hispanic women by census tract (click to see definition of census tract). This required several data elements: 1) breast cancer incidence rates for Marin County by age category (collapsed over census tract for years 1988-1992; and 2) the population size of each census tract by age category from the 1990 Census. Because there are few non-white women living in Marin County, we did not calculate ratios for other races. To do so would have resulted in unstable estimates due to insufficient numbers. To determine the number of estimated cases in each census tract, referred to more specifically as "expected," we multiplied the age-specific breast cancer incidence rate for women in Marin County by the number of women reported in each census tract from the 1990 Census. The SIR is then the number of cases divided by the number of expected cases.
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How Can I Find the Census Tract I Lived in for the 1990 Census?

To find your census tract from the 1990 Census, go to http://www.census.gov

• Under data sets, third bullet down, click on "1990 Summary Tape File 1"
• Highlight radio button 1990 Summary Tape File
• On right side of section, click on Reference Maps
• Scroll down to enter address
• Census tract is bordered and labeled in a burnt orange color

Census tracts are small, relatively permanent statistical subdivisions of a country. Census tracts usually have between 2,500 to 8,000 persons, and are designed to be homogenous with respect to population characteristics economic status, and living conditions. The spatial size of a census tract varies varies widely depending on the density of settlement, and are delineated with the intention of being maintained over a long time, so that statistical comparisons can be made from census to census.
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What Does the Map Show?

The map shows some color variation, as we would expect, but cannot conclude causal effect on where a woman lives and breast cancer risk. The map shows relative similarity in the ratio of observed to expected cases of breast cancer between census tracts in Marin County, 1988-1992. The lavender and purple areas correspond to areas where women are more likely to reside at the time of diagnosis.
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What the Map Does Not Show?

This approach serves as a first step to describe the distribution of breast cancer in Marin County between 1988-1992. This map does not show how long a woman was living at the location of residence at diagnosis. This map doesn't show incidence rates of invasive breast cancer in marin County overall or by census tracts. The age-adjusted incidence rate for invasive breast cancer in Marin County for white-non Hispanic females, 1988-1992 was 132/100,000, and for the Bay Area counties, the rate was 130/100,000.  The map does not show environmental or other risk factors that might be related to developing breast cancer.
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Specifically, the map does not show:

• That location of residence is a risk factor for breast cancer.
• Any of the known risk factors associated with developing breast cancer. For example, age at menarche, age at first birth, recent hormone replacement therapy and family history of breast cancer are known to be associated with the development of breast cancer, but cannot be depicted ton the map, and may vary with each census tract.
• The reasons why women get breast cancer
• How risk factors could be distributed in Marin County
• Why the breast cancer rates are higher in Marin County as a whole

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Why Not Map the Number of Breast Cancer Cases in Each Area?

There are some areas within the County that have many residents and some that have fewer residents. Generally speaking, areas with more people in them will have more cases of breast cancer. Areas where older women live would be expected to have more breast cancer cases in each area, then there might be some areas that have more cases of breast cancer just because more people live there and/or because many older residents live there. Secondly, in sparsely populated areas, showing the number of cases on a map may infringe on an individual's privacy.
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Since Age is the Number One Risk Factor for Women, Would the Purple Colored Areas Mean That There are Just More Older People Living There?

No. We removed the effect of age at diagnosis by a procedure called age-adjustment. Each SIR represents the observed to expected number of cases controlling for age. While we know that variation in SIRs is not due to differences in age of residents, we do not know if the differences may be due to other risk factors, such as hormone replacement therapy, the average number of children born to each woman, the age when the woman began to having children, or behavioral or environmental factors. Thus, several factors could be related to breast cancer occurrence except age, since age has been controlled for statistically.
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How Did You Determine Where There Were More or Fewer Cases than the Standard?

A Standardized Incidence Ratio (SIR) is calculated using the actual number of cases of breast cancer compared to the statistically expected number of cases. when the SIR is higher or lower that one, we conclude that there are more (SIR >1.0) or fewer (SIR<1.0) cases than expected. When the observed number of cases equals the expected number of cases, then the SIR equals 1.0.
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What Does it Mean if I Live in a Purple-Colored Area?

Purple shaded census tracts show that the observed to expected ratio exceeds 1.0 by 25% to 50%. Living in a purple-colored area does not mean that you are at risk for breast cancer because of where you live or how long you have lived there. These areas show ratios that may be due to random chance or may be due to personal risk factors, such as number of full-time pregnancies, hormone-replacement therapy, alcohol consumption, and age when a woman had her first child. There are many other risk factors for breast cancer and this map cannot tell us about why one woman develops breast cancer and another woman does not.
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What Does It Mean if I Live in a Green-Colored Area?

In both green-shaded areas (light green and pale green), observed to expected ratios show that the observed number of cases is lower than expected for 1988-1992, suggesting no elevated incidence compared to expected. If you live in a light green-shaded area (25-50% below standard), the observed to expected ratio is a least .75 or lower relative to 1.0. If you live in a pale green-shaded area (less than 25% below standard), the observed to expected ratio is greater than 0.76 but less than1.0.
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What Does It Mean if I Live in a Lavender-Colored Area?

In the lavender area (less than 25% above standard), the observed to expected ratio is no greater than 25% above 1.0 (i.e., between 1.0-1.25). Random chance cannot be ruled out as a likely explanation for ratios in these areas, and living in a lavender-colored area does not mean you are at risk for breast cancer because of where you live. There are many factors that cause breast cancer and this map cannot tell us about why one woman develops breast cancer and another woman does not.
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Why Does this Map Refer Specifically to the Time Period of 1988-1992?

We calculated incidence rates using cases that were diagnosed between 1988 and 1992, because it is is the five-year period surrounding the 1990 Census. Summarizing five years of invasive breast cancer data is needed for analysis to reduce errors due to small numbers. The California Cancer Registry has not released cancer data after year 2000 for analysis, so the 2000 Census figures were not applied in this analysis.
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I was diagnosed with breast cancer during the study period. Will my personal data be released?

We operate under a strict code of research ethics that prohibits the release of personal information about the breast cancer cases included in the data we use. Information that could identify individuals, including the addresses of breast cancer cases, will be kept confidential. In addition to protecting women's confidentiality by carefully choosing how we display data, we are also committed to protecting confidentiality by strictly limiting computer access to the data to County employees involved in investigating breast cancer, each of whom is required to keep the data confidential.
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What Can I Do to Protect Myself and Reduce the Chance that I will Develop Breast Cancer?

If you are concerned about your risk factor for getting cancer, there are a few things you can do. The American Cancer Society (http://www.cancer.org) suggest the following as some important steps in early detection and prevention:

• Participate in breast cancer screening, including breast self-exam, clinical breast exam by a health care provider, and mammography.
• Reduce your exposure to known risk factors whenever possible, by taking steps to increase your physical activity, minimizing intake of alcohol, maintaining a healthy weight, and eating a diet low in fat and high in fresh fruits and vegetables.
• Talk with your health care provider about your personal risk factors, including any history in your family of specific types of cancers.
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What Has Marin HHS done to address this issue?

During 2001, the Marin County Department of Health and Human Services (HHS) developed an epidemiology program within the county, hired two epidemiologists and breast cancer program and research coordinators, and began the map project during the same year. The County of Marin assembled a scientific advisory committee that has provided recommendations for ongoing research projects, and we are actively working with community organizations, federal agencies, and university colleagues to explore collaboration opportunities.
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What are the Next Steps?

The Marin County HHS is currently seeking state and federal funding to expand our research efforts with epidemiological, biological, and community-based activity with our community and academic partners. The objective of HHS is to be a catalyst for research addressing breast cancer issues that have relevance to the national agenda.

Below are some of our specific research objectives:

• Increase capacity for epidemiologic, biologic, and environmental data analysis
• Analyze data to describe more fully incidence and mortality data
• Analyze and compare demographic groups from California and non-California locations
• Investigate possible risk factors unique to Marin County and other known risk factors through new data collection efforts
  • This would include expanding the Marin County Health Survey (MCHS)
  • Identify a cohort of women from the MCHS for further follow-up
  • Continue and Expand the Adolescent Risk Factor Survey (ARFS)
• Collaborate with existing researchers and studies that may help us understand risk factors for breast cancer in Marin County
  • California Teachers Study
  • Cape Cod Breast Cancer and Environmental Study
  • Nurses Health Study
  • Women's Care Study

  For additional information, please call 415-473-4077.

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