Risk of Breast Cancer

For an excellent and succinct summary of risk factors for breast cancer, see http://www.cancer.org/cancer/breastcancer/detailedguide/breast-cancer-risk-factors

The National Cancer Society also provides an online breast cancer risk calculator.


BRCA1 and BRCA2 Mutations

As noted above, the BRCA1 and BRCA2 mutations can be inherited and lead to an increased risk of breast and other types of cancer. The National Cancer Institute Fact Sheet on BRCA1 and BRCA2 states:

"A woman's lifetime risk of developing breast and/or ovarian cancer is greatly increased if she inherits a harmful mutation in BRCA1 or BRCA2. Such a woman has an increased risk of developing breast and/or ovarian cancer at an early age (before menopause) and often has multiple, close family members who have been diagnosed with these diseases. Harmful BRCA1 mutations may also increase a woman's risk of developing cervical, uterine, pancreatic, and colon cancer (1, 2). Harmful BRCA2 mutations may additionally increase the risk of pancreatic cancer, stomach cancer, gallbladder and bile duct cancer, and melanoma (3).

Men with harmful BRCA1 mutations also have an increased risk of breast cancer and, possibly, of pancreatic cancer, testicular cancer, and early-onset prostate cancer. However, male breast cancer, pancreatic cancer, and prostate cancer appear to be more strongly associated with BRCA2 gene mutations (2–4).

The likelihood that a breast and/or ovarian cancer is associated with a harmful mutation in BRCA1 or BRCA2 is highest in families with a history of multiple cases of breast cancer, cases of both breast and ovarian cancer, one or more family members with two primary cancers (original tumors that develop at different sites in the body), or an Ashkenazi (Central and Eastern European) Jewish background. However, not every woman in such families carries a harmful BRCA1 or BRCA2 mutation, and not every cancer in such families is linked to a harmful mutation in one of these genes. Furthermore, not every woman who has a harmful BRCA1 or BRCA2 mutation will develop breast and/or ovarian cancer."

Nevertheless, it is estimated that 85% of breast cancers are sporadic, i.e., not do to an inherited mutation.

Hormonal Factors

A woman's risk of developing breast cancer depends on hormonal and reproductive history. In essence, any factor that increases a women's total lifetime exposure to estrogens (either endogenous or exogenous) increases her risk. Estrogens stimulate growth of breast tissue and appear to have a role in the development & growth of breast cancer. Estrogens promote the development of mammary cancer in rodents. They also stimulate proliferation of human breast cancer cells grown in cell culture.

Factors that would contribute to lifetime exposure to endogenous estrogens would include:

Exogenous Sources of Estrogen (from an external source, i.e., medications and birth control)

Oral Contraceptives and Cancer

From National Cancer Institute:

"A 1996 analysis of epidemiologic data from more than 50 studies worldwide by the Collaborative Group on Hormonal Factors in Breast Cancer found that women who were current or recent users of birth control pills had a slightly higher risk of developing breast cancer than women who had never used the pill (2). The risk was highest for women who started using oral contraceptives as teenagers. However, 10 or more years after women stopped using oral contraceptives, their risk of developing breast cancer had returned to the same level as if they had never used birth control pills...."






Tamoxifen - An Estrogen Blocking Drug

Tamoxifen is a drug that has an estrogen-blocking effect in the breast, but estrogen-like effects on some other tissues, e.g. uterus & bone.


The Breast Cancer Prevention Trial studied effect of Tamoxifen for 5 years in a large group of women at increased risk. Women ages 35 and older who took a daily dose of 20 mg Tamoxifen for up to 5 years had 50% reduction in risk of breast cancer.

  • In women ages 35-49, risk of serious side effects was similar to placebo.
  • Over age 50 there was increased risk of uterine cancer and blood clots when taking Tamoxifen


Carcinogens and Breast Cancer

In the video clip below, Dr. David Sherr, from the environmental health department at BUSPH, discusses the potential role of environmental pollutants as risk factors for breast (and other) cancers. Dr. Sherr's research is exploring the molecular mechanisms that initiate and maintain breast cancer. Much of his research has focused on the role of an environmental chemical receptor (the aryl hydrocarbon receptor - AhR) and the role of polycyclic aromatic hydrocarbons (PAHs), which are atmospheric pollutants generated by burning fuel and by cooking meat at high temperatures (e.g., grilling). PAHs are also found in smoked fish. In this video, Dr. Sherr also discusses the difficulties in establishing a causal link between environmental pollutants and cancer.

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Other Risk Factors for Breast Cancer