Risk Factors:

About 12% of women develop breast cancer in their lifetime. Each year in the United States, about 180,000 women are diagnosed with invasive breast cancer and about 68,000 women are diagnosed with pre-invasive breast cancer. (Although breast cancer in men is rare, about 2,000 American men are diagnosed each year with invasive breast cancer.)

About 40,000 American women die from breast cancer each year. Breast cancer death rates have declined significantly since the 1990s, especially for women younger than age 50. The earlier breast cancer is diagnosed, the earlier the opportunity for treatment. In the United States, there are currently more than 2.5 million breast cancer survivors.

Risk factors for breast cancer include:

Age

Most cases of breast cancer occur in women older than age 60. According to the American Cancer Society, about 1 in 8 cases of invasive breast cancer are found in women younger than age 45, while 2 in 3 cases of invasive breast cancer occur in women age 55 and older.

Race and Ethnicity

Breast cancer is slightly more common among white woman than African-American, Asian, Latina, or Native American women. However, African-American women tend to have more aggressive types of breast cancer tumors and are more likely to die from breast cancer than women of other races. It is unclear whether this is mainly due to biologic or socioeconomic reasons. Social and economic factors make it less likely that African-American women will be screened, so they are more likely to be diagnosed at a later stage. They are also less likely to have access to effective treatments.

Breast cancer is also more prevalent among Jewish women of Eastern European (Ashkenazi) descent (see Genetic Factors).

Family and Personal History

Women who have a family history of breast cancer are at increased risk for developing breast cancer themselves. Having a first-degree relative (mother, sister, or daughter) who has been diagnosed with breast cancer doubles the risk for developing breast cancer.

Women who have had ovarian cancer are at increased risk for developing breast cancer. And, a personal history of breast cancer increases the risk of developing a new cancer in the same or other breast.

Genetic Factors

About 5 - 10% of breast cancer cases are due to inherited genetic mutations.

BRCA Genes. Inherited mutations in genes known as BRCA1 or BRCA2 are responsible for most cases of hereditary breast cancers, ovarian cancers, or both in families with a history of these cancers.

BRCA gene mutations are present in only about 0.5% of the overall population. However, certain ethnic groups -- such as Jewish women of Eastern European (Ashkenazi) descent -- have a higher prevalence (2.5%) of BRCA gene mutations. BRCA gene mutations are also seen in some African-American and Hispanic women.

Screening Guidelines for BRCA Genes. The U.S. Preventive Services Task Force (USPSTF) recommends that women at high risk should be tested for BRCA genes, but does not recommend routine genetic counseling or testing in low-risk women (no family history of BRCA 1 or 2 genetic mutations). Risk assessment is based on a womanâ ' s family history of breast and ovarian cancer (on both the maternal and paternal sides).

The relevance of the inherited BRCA1 or BRCA2 mutations to survival is controversial. Some studies have suggested that these mutations are linked to less lethal breast cancer. Others suggest that they do not change prognosis or may worsen it. Women with these genetic mutations do have a greater risk for a new cancer to develop. Patients with BRCA1 mutations tend to develop tumors that are hormone receptor negative, which can behave more aggressively.

Other Genetic Mutations. Other genes associated with increased hereditary breast cancer risk include p53, CHEK2, ATM, and PTEN.

Exposure to Estrogen

Because growth of breast tissue is highly sensitive to estrogens, the more estrogen a woman is exposed to over her lifetime, the higher her risk for breast cancer.

Duration of Estrogen Exposure. Early age at menarche (first menstrual period) or later age at menopause may slightly increase a womanâ ' s risk for breast cancer.

Pregnancy. Women who have never had children or who had their first child after age 30 may have a slightly increased breast cancer risk. Having children at an early age, and having multiple pregnancies, reduces breast cancer risk. Evidence does not show an association between abortion and increased breast cancer risk.

Studies have been mixed on whether breast-feeding decreases breast cancer risk. Breast-feeding reduces a womanâ ' s total number of menstrual cycles, and thereby estrogen exposure, which may account for its possible protective effects. Some studies suggest that the longer a woman breast-feeds, the lower her risk.

Oral Contraception. Although studies have been conflicting about whether estrogen in oral contraception increases the chances for breast cancer, the most recent research indicates that current or former oral contraceptive use does not significantly increase breast cancer risk. Women who have used oral contraceptives may have slightly more risk for breast cancer than women who have never used them, but this risk declines once a woman stops using birth control pills.

Hormone Replacement Therapy. Many studies have reported a higher risk for breast cancer in postmenopausal women who take combination hormone replacement therapy (HRT), which contains both estrogen and progestin. A combination of estrogen and testosterone also increases breast cancer risk.

Several studies of women who had a hysterectomy indicate that estrogen alone does not increase overall breast cancer risk when the drug is used for 7 years or less. However, women who take estrogen for 10 - 15 years or more do have an increased risk, especially women who are already at higher risk for breast cancer. In addition, HRT increases breast cancer density, making mammograms more difficult to read. This can cause cancer to be diagnosed at a later stage. Women who take estrogen HRT should be aware that they need frequent mammogram screenings.

The North American Menopause Society recommends that women who are at risk for breast cancer should avoid hormone therapy and try other options to manage menopausal symptoms such as hot flashes. Most doctors recommend that women use HRT only for short-term relief of menopausal symptoms. [For more information, see In-Depth Report #40: Menopause.]

Infertility and Infertility Treatments. Despite some concerns that infertility treatments using the drug clomiphene may increase the risk for breast cancer, most studies do not show an association. Some studies indicate that ovulation induction with clomiphene may actually decrease breast cancer risk. (Clomphine is related to tamoxifen, a drug that is used for breast cancer prevention in high-risk women.)

Breast Abnormalities

Abnormalities or Breast Conditions Suggesting a Higher Risk. Certain factors and breast conditions may increase the risk for breast cancer:

• Dense breast tissue is associated with a higher risk for breast cancer. Studies suggest that women with highly dense tissue have 2 - 6 times the risk of women with the least dense tissue. Genetic factors play a large role in breast density. Hormone replacement therapy also increases breast density. In addition, dense breasts make mammograms more difficult to read, which increases the likelihood of missing early signs of cancer.
• Benign proliferative breast disease, or unusual cell growth known as atypical hyperplasia, is a significant risk factor for breast cancer.

Some common benign breast abnormalities that pose few or no risks include the following:

• Cysts. These mostly occur in women in their middle-to-late reproductive years and can be eliminated simply by aspirating fluid from them.

• Fibroadenoma. These are solid benign lumps that occur in women ages 15 - 30.
• Breast abscesses during breast-feeding.

• Nipple discharge. Discharge from the nipple is worrisome to patients, but is unlikely to be a sign of cancer. Unexplained discharge still warrants evaluation, however.

• Mastalgia. This is breast pain that occurs in association with, or independently from, the menstrual cycle. About 8 - 10% of women experience moderate-to-severe breast pain associated with their menstrual cycle. In general, breast pain does not need assessment unless it is severe and prolonged.

Physical Characteristics

The following physical characteristics have been associated with increased risk:

• Obesity increases the risk for all types of estrogen receptor-positive breast cancers. Women who gain weight after menopause are most at risk. (On a positive note, losing weight after menopause decreases breast cancer risk.) In postmenopausal women, estrogen is produced in fat tissue. High amounts of fatty tissue increase levels of estrogen in the body, leading to faster growth of estrogen-sensitive cancers.
• Estrogen is involved in building bone mass. Therefore, women with heavy, dense bones are likely to have higher estrogen levels and to be at greater risk for breast cancer.
• Some studies have found a greater risk for breast cancer in taller women, possibly due to the higher estrogen levels associated with greater bone growth.

Environmental Factors

Exposure to Estrogen-like Industrial Chemicals. Chemicals with estrogen-like effects, called xenoestrogens, have been under suspicion for years. There has been particular concern with pesticides containing organochlorines (DDT and its metabolites, such as dieldrin) and pyrethroids (permethrin), but at this time evidence of any causal association is very weak.

Exposure to Diethylstilbestrol. Women who took diethylstilbestrol (DES) to prevent miscarriage have a slightly increased risk for breast cancer. There may also be a slightly increased risk for their daughters (commonly called "DES daughters"), who were exposed to the drug when their mothers took it during pregnancy.

Radiation Exposure. Heavy exposure to radiation is a significant risk factor for breast cancer. Girls who received high-dose radiation therapy for cancer face an increased risk for breast cancer in adulthood. Low-dose radiation exposure before age 20 may increase the risk for women with BRCA genetic mutations.

Disproven Risk Factors

Antiperspirants or use of deodorants after shaving have not been linked with any higher risk for breast cancer. There is also no evidence that bras increase breast risk. Abortion does not increase risk.