Causes of Breast Cancer

About 50% of women who develop breast cancer have no risk factors other than age and sex.

• Sex is the biggest risk because breast cancer occurs mostly in women.

• Age is another critical factor. Breast cancer may occur at any age, though the risk of breast cancer increases with age.

• The average woman at age 30 years has 1 chance in 280 of developing breast cancer in the next 10 years. This chance increases to 1 in 70 for a woman aged 40 years, and to 1 in 40 at age 50 years. A 60-year-old woman has a 1 in 30 chance of developing breast cancer in the next 10 years.

Genetic causes

Family history has long been known to be a risk factor for breast cancer. The risk is highest if the affected relative developed breast cancer at a young age or if she is a close relative such as a mother, sister, daughter, or aunt.

There is great interest in genes linked to breast cancer.

• BRCA1 is an abnormal gene that, when inherited, markedly increases the risk of breast cancer to a lifetime risk of almost 85%. Women with this abnormal gene also have an increased likelihood of developing ovarian cancer. Women who have the BRCA1 gene tend to develop breast cancer at an early age.

• A second abnormal gene, BRCA2, increases the risk of developing breast cancer but not ovarian cancer.

• Testing for these genes is expensive and frequently not covered by insurance. In addition, women who test positive may have trouble getting or keeping health insurance.

• The issues around testing are complicated, and women who are interested in testing should discuss this with their health care providers.

Hormonal causes

Hormonal influences play a role in the development of breast cancer.

• Women who start their periods at an early age or experience a late menopause have a higher risk of developing breast cancer.

• Conversely, being older at your first menstrual period and early menopause tend to protect one from breast cancer.

• Having a child before age 30 years may provide some protection, and having no children may increase your risk for developing breast cancer.

• Oral contraceptives have not been shown to increase or decrease a woman's the lifetime risk of breast cancer.

• A large study conducted by the Women's Health Initiative showed an increased risk of breast cancer in postmenopausal women who were on a combination of estrogen and progesterone for several years. Therefore, women who are considering hormone therapy for menopausal symptoms need to discuss the risk versus the benefit with their health care providers.

Dietary causes

Breast cancer seems to occur more frequently in countries with high dietary intake of fat.

• This link is thought to be an environmental influence rather than genetic. For example, Japanese women, at low risk for breast cancer while in Japan, increase their risk of developing breast cancer after coming to the United States.

• Several studies comparing groups of women with high- and low-fat diets, however, have failed to show a difference in breast cancer rates.

Benign breast disease

Fibrocystic breast changes are very common.

• Fibrocystic breasts are lumpy with some thickened tissue and are frequently associated with breast discomfort, especially right before your menstrual period.

• This condition does not lead to breast cancer.

• However, certain types of benign breast changes, such as those diagnosed on biopsy as proliferative or hyperplastic, do predispose women to the later development of breast cancer.

Environmental causes

Radiation treatment seems to increase the likelihood of developing breast cancer but only after a long delay. For example, women who received radiation therapy to the upper body for treatment of Hodgkin disease before age 15 years have a significantly higher rate of breast cancer than the general population.