In the late 1990s and early 2000s, Marin County had some of the highest breast cancer rates in California and the US. The County of Marin responded to community concerns about these rates by examining existing data on breast cancer rates, mammography screening rates, and risk factors, and eventually by conducting the Marin Women’s Study (MWS) at Marin mammography sites to understand the causes of breast cancer in women in general, as well as providing keys to why Marin’s rates were so high. While the latest data from CCR show that age-adjusted rates for all races combined are significantly higher in Marin than in California, our racial distribution is very different from that of other counties and the state, and breast cancer incidence rates vary by race. In Marin’s non-Hispanic White women, which is the group for which we have enough numbers to get a stable rate for a relatively short period of time, breast cancer incidence rates are now statistically similar to the average incidence rate among white non-Hispanic females in California and the U.S. Marin's invasive breast cancer incidence rates peaked in 2001, and while these lower rates have fluctuated throughout the time since 2001, rates in 2012 were the lowest since recording began in 1988.
That is taken into account in the age adjustment we apply when we calculate rates.
The rates peaked in 1999-2001 and have since decreased from that peak. Rates in 2012 were the lowest since recording began in 1988. This trend toward lower rates is promising, and we hope this continues.
There has been a continued decline in breast cancer mortality in California, the Bay Area and Marin over the past two decades; fewer women are dying of breast cancer. Mortality rates in Marin have not been significantly greater than average rates in California during the past 20 years.
While there is no way to explain the trends with certainty, there are some factors that likely contribute to the changes in breast cancer incidence. Combination Hormone Replacement Therapy (HRT), or estrogen plus progesterone HRT, is associated with breast cancer incidence and is the type of HRT prescribed to women who have not had a hysterectomy. To date, the most important result from the Marin Women's Study was the finding of a large drop in combination HRT use in Marin, which preceded a sharp drop in breast cancer incidence rates in the Marin population.
While the extent to which women in a population participate in mammographic screening is associated to some extent with incidence rates, screening patterns do not appear to explain all of the changes in breast cancer incidence, as incidence rates have decreased since 2001 while screening rates have either not changed or have increased over the same time period. Four separate analyses conducted in the early 2000s showed that the use of mammography is only 0%-5% higher among women in Marin County than among women in California and the U.S.
Changes in other risk factors could also be contributing to the changes in breast cancer incidence rates. We will continue to monitor available data sources and to use the MWS data to try to uncover reasons for changes in incidence with the goal of providing women with concrete information about how to reduce their risk of breast cancer.
For 2008-2012, rates in Asian and African American women were lower, but not significantly different, than rates for White Non-Hispanic women, while rates for Hispanic women were significantly lower than rates in white Non-Hispanic women.
When we mapped breast cancer incidence in Marin among white women from 1988-1992, the high rates of breast cancer were not concentrated in one area and did not vary widely throughout Marin. We obtained our cases from the Cancer Prevention Institute of California, and analyzed them by 1990 Census Tract. There was no statistical difference in incidence among tracts.
Breast Cancer Risk Factors
Risk factors are characteristics that some people have that may make them more susceptible to a disease. The risk factors for breast cancer are not as clear-cut as they are for other diseases like heart disease or the flu; many women who get breast cancer have none of the known risk factors except age and being female, and most women with one or more of the risk factors do not go on to develop breast cancer.
There is an association with many risk factors and breast cancer such as gender, age, diet, genetics, family history, age at menarche/menopause, age at first pregnancy, parity, breastfeeding, breast density, hormone replacement therapy, radiation, exercise, race and socioeconomic status. Click here for more detailed information on breast cancer risk factors.
There has historically been a high prevalence of several known breast cancer risk factors in Marin. Compared to the average California population, women in Marin have more risk factors including higher socioeconomic status, later childbearing, fewer children, more frequent alcohol consumption, and higher levels of education. Based on this information, we would expect Marin County to have a somewhat higher breast cancer incidence rate than other geographic regions with fewer risk factors.
In addition, more women in Marin report being physically active, eating fruits and vegetables, being at a recommended body weight, and breast-feeding than the rest of California and the U.S. These behaviors are all protective factors against breast cancer.
The MWS is researching the impact of these established risk and protective factors on breast health.
An early analysis of data (prior to the MWS), the Traditional Risk Factor Study (TRIFS), suggested that approximately 50% of the breast cancer cases in Marin County were attributable to younger age at menarche (under 12 years old), delaying childbirth until after age 30, having no children, family history, older age at menopause (over 55 years old), and high postmenopausal body mass index. About 30% of the breast cancer cases in Marin were attributable to a later age (after age 29) at first child birth/nulliparity.
The Marin Women’s Study (MWS) is a research study that investigates how reproductive, lifestyle, demographic, and biologic factors affect breast cancer risk in Marin women.
The MWS will further our understanding of previous breast cancer research in Marin by building upon our population-based data. We know that women in Marin have had more of certain established risk factors for breast cancer. However, we do not know if individuals getting breast cancer were among those with more of the risk factors. In short, we need to find out if the established risk factors were also more prevalent among cases and then examine how the risk factors relate to breast health in Marin.
In addition, the MWS will help us to investigate the factors that underlie breast health in all women. The MWS has allowed the investigation of known risk factors (including alcohol consumption, use of hormone replacement therapy, reproductive history, and higher socioeconomic status) and how they associate with breast health. In addition, we have been able to analyze saliva for markers that could be associated with breast cancer risk. The saliva analysis has allowed us to look at some gene-environment interactions in the development of breast cancer as well as hormone levels that could be associated with breast health. Investigations of known and suspsected risk factors and their relationship to breast health continue.
Most importantly, the MWS is a data bank that, in confidential form, will be available to researchers from all over to continue advancing breast cancer research in the future.
Over 13,000 Marin women filled out the MWS survey at the time of their mammogram and more than 8,000 women provided saliva samples. The survey data has been processed and checked for quality. The size of the sample combined with the genetic material and information on personal risk factors will enable researchers to analyze breast cancer risk in detail. Multiple analyses have already been conducted that have provided concrete information about the distribution of risk factors in our population as well as novel hypotheses that can be followed up by researchers in other populations, including:
An early MWS analysis documented a striking drop in combination hormone replacement therapy (HRT) after nationwide press about the results of the Women's Health Initiative study that showed an increased risk of breast cancer among women taking combination HRT. In Marin, the drop in HRT utilization documented in the MWS population preceeded a drop in breast cancer incidence rates. These findings are being followed up by examining additional years of data and the types of hormones being used by MWS participants.
We have found associations between breast health and the characteristics of women's pregnancies, including pregnancy induced hypertension (PIH), breastfeeding, and age at first birth. One of the more interesting and novel findings of this study was that a history of PIH was associated with decreased breast density years later, which in turn is associated with a lower risk of breast cancer. Based on these findings, we investigated whether the effect of PIH on breast density varies by certain genetic markers, and found that there was variation by two SNPs. We found that women with a certain genetic profile and a history of PIH not only had lower breast density, but were protected against breast cancer. These findings are being replicated outside of the MWS to determine whether the associations are the same in other populations.
The MWS has also allowed examination of how risk factors differ in women who began the MWS after having been diagnosed with breast cancer compared to women who had not been diagnosed with breast cancer, as well as the association between factors such as socioeconomic status, alcohol, and diet and breast density.
Finally, the MWS has already begun serving as a data resource to nationally-recognized researchers, who have begun using MWS data in a confidential fashion to answer cutting edge questions related to breast cancer prevention.
Established Risk Factors: a Family history of breast cancer, Ashkenazi Jewish heritage, and having children later in life - are all associated with breast cancer in the MWS population, as they have been in other study populations.
Effects of Pregnancy: Later age at first pregnancy is associated with breast cancer in the MWS population. Breast feeding and high blood pressure during pregnancy (PIH) may be associated with changes in breast density and thus may affect the risk of developing breast cancer.
Adolescent Risk Factors: Analyses are underway to examine the association between factors such as smoking early in life (before the age of 20) and alcohol intake during high school breast cancer later in life in the MWS group.
Environmental Agents: Carcinogens including the heavy metal cadmium and bisphenol A (widely used in the manufacturing of plastic water bottles, food containers and toys) may play a role in breast cancer. We assessed the feasibility of using saliva samples to measure cadmium and bisphenol A levels for use in breast cancer risk prediction. Use of saliva samples to assess breast cancer risk has practical applications both locally and in developing countries.
Risk Prediction: Early results suggest improvements in risk prediction can be achieved when looking at women at high risk for breast cancer. A statistical risk model developed using MWS data was tested on another population for validation, which could lead to more effective targeting of prevention and screening.
The MWS is moving beyond traditional risk factors by collecting information on stress, bovine leukemia virus, residential history, preterm birth, and gene/environment interactions to name a few.
Importantly, this was the first prospective study in Marin County to collect individual level risk factor and biospecimen data on women for whom we also have information on breast health. County health departments do not traditionally undertake this type of large research study; the County of Marin partnered with community members and breast cancer survivors, local and national researchers and providers, and local advocates to undertake a study that would allow us to answer the questions important to the women and families of Marin and to conduct research that would inform breast cancer prevention efforts in all women.
Many scientists believe that environmental risk factors such as exposures to everyday chemicals could be attributing to the disease worldwide. Much of the research and evidence linking breast cancer to a particular environmental factor is in its infancy. Since the disease is so complex and can take years to develop, it is difficult to isolate environmental exposures in humans. Often enough, we want to pinpoint one specific factor that caused our disease, but with most cancers, the cause is most likely due to a variety of lifetime factors that make some of us more susceptible to the disease. Nonetheless, environmental causes should not be ignored. With the Marin Women’s Study, we hope to bring more understanding to this puzzle and provide future researchers with biospecimens that can be analyzed using new tools as they are developed.
Although there is no doubt environmental factors may play a role in the development of breast cancer, several studies have provided indirect evidence that the Marin County environment is not making a larger contribution to breast cancer rates in Marin than in other places. The Marin County Residence, School, and Workplace Study did not find sufficient evidence to conclude that years lived or worked in Marin were associated with an increased risk of developing breast cancer. A map of breast cancer rates by residence at time of diagnosis in Marin found that census tracts in Marin had similar rates of breast cancer, and that tracts with higher rates were not geographically related, nor related to the location of prior military bases or other areas of suspected environmental toxins. Extensive testing of the water supply in Marin has failed to reveal evidence of pesticides, estrogenic substances, or other known carcinogens.
The MWS has examined the utility of saliva for measuring environmental factors, including cadmium and bisphenol A. We have also collected data on residential history, which can be used to examine environmental-related research.
As far as we know, the best answer is to eat healthy and stay active, and to open communication with your personal physician about how to reduce your personal risk. Be informed about choices you make. Since the risk factors for breast cancer do not adequately describe the majority of the cases, we do not fully know the best steps for prevention. Based on the current research in breast cancer and health in general we recommend the following:
- Eat a healthy, well-balanced diet with plenty of fruits and vegetables
- Be active every day
- Lose excess weight
- Don’t smoke
- Breast feed your baby
- Limit alcoholic drinks to less than one a day
- Reduce alcohol and increase folic acid intake for women on estrogen therapy
- Consult a healthcare practitioner about hormone replacement therapy
- Minimize ionizing radiation to the chest
- Protect yourself from unnecessary exposure to harmful chemicals
Early detection is not prevention, but being aware of your body and detecting cancer early can lead to healthier outcomes. We recommend all women work with their personal physicians to determine a regular screening schedule that includes breast self-exams, clinical breast exams, and mammograms for women aged 40 and over.