What causes breast cancer?

If you or someone you love has been diagnosed with breast cancer, it’s normal to want to know what causes breast cancer to happen. But the exact cause of breast cancer is still unknown.

We do know that breast cancer develops when normal breast cells begin to grow out of control. Breast cancers usually start in the lobules or ducts of the breast. The lobules are glands that make breast milk. Ducts carry the milk from the lobules to the nipple. Groups of breast cancer cells are called malignant, or cancerous, tumors. Some breast cancer cells can spread, or metastasize, to other parts of the body.

We also know about many of the risk factors for breast cancer. Some risk factors, such as lack of exercise, can be controlled or changed. Others, such as a person’s sex at birth, age, and family history of breast cancer, can’t.

Only 10 percent of breast cancer diagnoses are linked to inherited genetic mutations such as BRCA1 and BRCA2. That means most breast cancer happens for other reasons.

Risk factors for breast cancer

Some things can raise a person’s risk of being diagnosed with breast cancer. The following list contains some breast cancer risk factors that can be controlled, and some that can’t.

Risk factors that cannot be controlled:

• Being born female
• Getting older (risk increases after age 55)
• Personal history of breast cancer
• Family history of breast cancer
• Inherited genetic mutations, such as BRCA1 or BRCA2
• Having Ashkenazi Jewish background
• Certain kinds of breast lesions (an abnormal change in the breast tissue that is felt by a doctor or patient or seen on a mammogram or other type of diagnostic imaging)
• Previous radiation therapy to the chest
• Having dense breast tissue
• Starting menstrual periods early
• Giving birth to a first child at an older age
• Beginning menopause at an older age
• Exposure to diethylstilbestrol (DES, a drug used in the 1940s through the early 1970s to lower risk of miscarriage)
• Socioeconomic factors. In all racial and ethnic groups, women who are more educated and have higher incomes are more likely to develop breast cancer. The reasons for this are unclear, but could be tied to things like diet and environmental exposures.

Risk factors that can be controlled:

• Lack of exercise
• Post-menopausal weight gain
• Drinking alcohol
• Taking some kinds of hormone replacement therapy after menopause (if taken for more than 5 years)

To learn more, visit Who gets breast cancer?

How race and ethnicity affect breast cancer development and treatment

A woman’s race or ethnicity affects how likely she is to be diagnosed with breast cancer, the kind of breast cancer she may be more likely to develop, and the kind of care she may receive. For example:

• White women have a higher chance of developing breast cancer than Black women.
• Women of Ashkenazi Jewish descent are more likely to develop breast cancer because they may have a higher rate of the BRCA1 mutation.
• Black women are more likely to develop a more aggressive form of breast cancer called triple-negative breast cancer before the age of 50.
• Black women and Hispanic women are less likely than white women to be diagnosed early and receive standard treatments.

For more information about the connection between race and ethnicity and breast cancer, visit Black with breast cancer and Sharsheret, an organization providing information and support for Jewish women facing breast and ovarian cancer.

Risk reduction

While total prevention of breast cancer is not currently possible, there are many ways to reduce risk. These include:

• Avoiding or limiting alcohol use
• Exercising regularly
• Maintaining a healthy weight, especially after menopause
• Limiting use of birth control that uses hormones
• Limiting postmenopausal hormone replacement therapy
• Breastfeeding, if you have children and if breastfeeding is possible for you, especially if you are able to breastfeed for one year
• Talking with your doctor about breast cancer screening and the best age to start

Risk-reduction tips for people at high risk

People at high risk have options to reduce the risk of a breast cancer diagnosis. These options include taking preventive medicines such as hormonal therapies (tamoxifen or raloxifene) or aromatase inhibitors. If you are at high risk, you and your health care team may also discuss the option of having prophylactic, or preventive, surgery, including mastectomy.

To learn more, visit What is breast cancer?

Drugs used to lower breast cancer risk

Preventive medicines can be used to lower the risk of developing breast cancer. These drugs include:

• Tamoxifen, which blocks estrogen in breast cells, which can be useful in lowering breast cancer risk
• Raloxifene, which blocks estrogen in breast cells but is only approved for women who have already gone through menopause
• Aromatase inhibitors, which lowers estrogen levels in women who have already gone through menopause

Ways to understand your risk for breast cancer

There are different ways to assess a person’s risk for breast cancer. They include:

• Talking with a genetic counselor (you can ask your doctor to refer you to one if you are concerned about risk)
• Talking with your doctor about how to estimate your risk, and risk-reducing options to consider
• Predicting risk with the National Cancer Institute’s Breast Cancer Risk Assessment Tool. This tool is for women who do not have a personal history of certain types of breast cancer or carry the BRCA1 or BRCA2 gene mutation.