The Ribbon Newsletter

Why do women get breast cancer? Can it be prevented? We don't know precisely why a given woman gets breast cancer, nor can we prevent it. However, we do know some of the known risk factors that are associated with breast cancer.

It must be stressed that the risk for any individual woman cannot be determined by adding up the relative risks for a population of women. Having any elevated established risk factors, however, should be incentive for a woman to try to reduce any potential risks.

In the first edition of The Ribbon, BCERF provided a chart entitled "Human Breast Cancer Risk Factors." The first column of this chart, reprinted below, lists "established risk factors."

This list describes circumstances in populations of women that are associated with breast cancer. Having one or more of the risk factors for breast cancer does not mean an individual will get the disease, it means she is more susceptible to the disease than those without the risk factor in question.

Notice that we say "associated with breast cancer," rather than "causes of breast cancer." Research studies of populations help us understand that risk factors for a complex disease like breast cancer can at best determine an association, not an exact cause.

In this article and the accompanying table (page 3) we refer to 'relative risk.' Relative risk is the comparison of the incidence of breast cancer in a population of women with a particular risk factor to that of a population of women who do not have that risk factor. Or, it can be a comparison of people at one point along a continuum, such as age, to those at another point. In expressing relative risk, a woman without the risk factor in question has a risk of 1.0, and the risk of women with the risk factor is expressed in relation to this number. A risk factor that increases a woman's risk of breast cancer by 100% presents a relative risk of 2.0.

When reviewing the list of established risk factors for breast cancer, it is clear that these factors cannot be easily changed to reduce risk. Nothing can be done about the most important risk factor of all, aging. However, other established risk factors -- early menarche, late menopause, and late age at first birth -- represent trends that have been changing on a population level in the U.S.

What age women begin or cease menstruating (have their first and last periods) does not seem like an area in which women make an active choice. But there is increasing evidence that some lifestyle patterns, such as certain diets and level of physical activity, are intricately connected to women's hormonal cycle histories. BCERF is currently evaluating the research to date on related questions, such as how diet and exercise affect the age at which a girl gets her first period, as well as preparing materials to illustrate what is known about the relationship of hormones to breast cancer, and what changes women may consider making for themselves and their daughters.

The age at which a women gives birth to her first child, if she has children at all, is not typically influenced by considerations of breast cancer risk. U.S. women in general are giving birth to their first child at a later age. Importantly, this is largely attributable to social changes considered positive: more options available to women socially, in education, and in employment. Reports of the overall effect of changing childbearing trends vary, but one study estimates that later age at first birth, or women having no children, could account for almost 30% of U.S. breast cancer cases. These reports are leading some health professionals and researchers to challenge the idea that these risk factors cannot or should not be changed.

The Greatest Risk for Breast Cancer is Growing OlderAdvancing age, or simply growing older, is a risk factor for cancer in general. Cancer is often referred to as a "disease of aging." The longer one lives, the more likely one is to develop cancer, and, as a population, Americans are living longer. Cancer becomes more common as people age because of the increased time for DNA mutations in cells to accumulate and because DNA repair becomes less efficient. The longer the life span, the more time for the body to be exposed to any environmental risk factors.

It is important to note that breast cancer is a relatively uncommon disease for young women. By age 35 the risk is one in 622, and by age 45, one in 93. But, by 55 the risk of developing breast cancer is one in 33, and by 65 one in 17. The often cited 'one-in-eight' statistic refers to a lifetime risk for a woman who lives beyond age 85. Most of this risk is expressed in the later years of a woman's life. All other risk factors for breast cancer should be considered in combination with age.

How Does a Woman's Menstrual and Childbearing History Influence Breast Cancer Risk?

Researchers have come to understand that a woman's own hormones (for example, estrogen), play an important role in breast cancer. Early menarche, late menopause, and later or no childbearing are well-established risk factors for breast cancer, all influencing a woman's lifetime estrogen exposure.

Menarche and Menopause. Consider the relative risks presented in the table on page 3. Studies suggest that women who have their first menstrual period (menarche) before age 14 has a relative risk of about 1.3 (a 30% increase in risk) compared to a woman who has her first period at age 16. Women who experience menopause at age 55 or older, has a relative risk of about 1.5 (a 50% increase in risk) compared to a woman who experiences menopause between ages 45 and 54. It seems that the more menstrual periods women have over their lifetime, or, in other words, the more of their own estrogen they are exposed to, the greater the risk of breast cancer.

Childbearing. The effect of having children early adds another dimension to this picture of breast cancer risk. Pregnancy and breast-feeding interrupt the hormonal cycles, so women who give birth to one or more children reduce the number of hormonal cycles they experience. But there seems to be another effect as well: a pregnancy carried to term before age 30 causes the breast cells to change in a way that helps prevent them from growing abnormally. Some studies show a woman's risk of breast cancer almost doubling when she has her first child after age 30, or has no biological children, compared to women who have their first child before age 20. The period of time between menarche and the first full term pregnancy - how many years this is and events that take place during that period- appears to influence breast cancer risk.

Women's exposure to their own reproductive hormones has increased dramatically over time. A much later age of menarche and having many more children were the norm for most of human history. In the last 200 years the average age of menarche has dropped from age 17 to 12.8. Improved nutrition, a different diet, less exercise, and a very different social environment are all probable reasons for women's reproductive histories changing so much.

Family history, or, having a first degree relative (mother, sister, or daughter) with breast cancer, increases a woman's risk of getting breast cancer. Having other relatives with breast cancer, such as an aunt or grandmother, is not associated with an increased risk. A woman's risk increases more if her first degree relative's breast cancer developed before menopause or if it affected both breasts. If a woman's mother was diagnosed with breast cancer before age 60, the daughter's relative risk has been shown to be 2.0, or twice as high as a woman with no first degree relatives with breast cancer. It is important to remember that 85% of women who get breast cancer have no family history of the disease.

There has been much publicity surrounding the identification of two inherited gene alterations, BRCA1 and BRCA2, which predispose women to early-onset breast cancer. Those who do carry the alterations face a greatly increased risk of developing breast cancer, estimated to be an 85% lifetime risk. However, these gene alterations are expected to account for only 5 - 10% of the total number of breast cancer cases in the United States.

Family history of breast cancer in a particular woman's family may or may not be related to an inherited gene. Some proportion of family histories of breast cancer may be related to other, possibly environmental, risk factors that the family shares, such as environmental exposures where they live, or eating a common diet. Lifestyles can be 'inherited,' too. It is important for anyone with a family history of breast cancer to avoid considering herself "destined" to develop breast cancer, and to identify possible means of risk reduction.

These well established risk factors are estimated to only account for 30-40% of breast cancer in the United States. BCERF is helping to determine what may explain the remainder by working to understand the relationship between breast cancer and environmental risk factors. None of these possible environmental risk factors for breast cancer are considered 'established' at this point. Subsequent articles will provide The Ribbon readers with further background on environmental risk factors, and present a range of opportunities for risk reduction.

**Established Risk Factors for Breast Cancer**
Women who have:	Relative risk is:	Compared to women with:
Menarche at age 11-14	1.3	Menarche at age 16
Menarche at age 15	1.1
First child born at age 20-24	1.3	First child born before age 20
First child born at age 25-29	1.6
First child born at age 30 or older	1.9
No biological children	1.9
Menopause at age 55 or older	1.5	Menopause at age 45-54
Menopause before age 45	0.7
First degree relative diagnosed with breast cancer before age 60	2.0	No first-degree relatives (mother, sister, or daughter) with breast cancer
First degree relative diagnosed with breast cancer after age 60	1.4
Two first-degree relatives with breast cancer	4.0-6.0

"Established" Risk Factors for Breast Cancer The Ribbon

"Established" Risk Factors for Breast Cancer
The Ribbon