Many risk factors for breast cancer are related to prolonged exposure to estrogen and other hormones that play a role in a woman’s menstrual cycle. These risk factors include early menarche, late menopause, having children late in life, never bearing children, and never breastfeeding.
Many studies have shown an increased risk associated with recent use of certain pharmaceutical hormones, including oral contraceptives and hormone replacement therapy. Other drugs have complex estrogen-related effects. Tamoxifen, for example, is used as a breast cancer therapy because it blocks estrogen in the breast. At the same time, however, it increases the risk of uterine cancer.
Other factors that appear to increase estrogen levels—including alcohol use, a lack of physical exercise, a higher body mass after menopause, and obesity—are also associated with a higher breast cancer risk.
Ionizing radiation—including exposure from x-rays and CAT scans—is an established environmental risk factor for breast cancer. Exposure in girls has a greater effect on risk than exposure later in life.
Genetic susceptibility and family history of the disease also have been associated with breast cancer risk. The high-risk inherited breast cancer genes identified thus far—BRCA1 and BRCA2—account for an estimated 5 to 10 percent of cases.
In assessing risk factors for breast cancer, it is important to keep in mind the interplay among multiple factors across a lifetime. Our genetic susceptibilities interact with varying doses of a range of environmental toxics that we’re exposed to for days, weeks, months, or years at a time. Many environmental chemicals have become pervasive, and we receive multiple exposures at a time.
Many of the risk factors for breast cancer are ones women cannot change. But that doesn’t mean we’re helpless. If future research is to offer genuine hope for defeating the breast cancer epidemic, science must ask new questions in a search for additional causes.
Given the role that hormones play in the development of breast cancer, scientists at Silent Spring Institute believe that other hormonally active compounds—including synthetic estrogens in consumer products and pesticides, natural phytoestrogens in food, and other compounds that affect hormone signaling—deserve careful study.
More than a hundred synthetic compounds in industrial and commercial products have already been identified as estrogenic, including many that have been specifically shown to make estrogen-dependent human breast cancer cells grow in the laboratory. While many of these chemicals are relatively weak estrogen mimics, exposure to complex mixtures of them is ubiquitous. As a result, Silent Spring Institute has made estrogens in the environment a priority in its breast cancer research.
For Further Information
The Environment and Breast Cancer: Science Reviews is a comprehensive report on what we know now about the links between environmental pollutants and breast cancer risk.