About breast cancer

Breast cancer is the most common non-skin cancer among American women. An estimated 279,100 new cases of breast cancer will be
diagnosed in women in the United States in 2020, according to the American Cancer Society. Breast cancer accounts for 15 percent of all new cancer diagnoses and 7 percent of all cancer deaths each year.

What causes breast cancer?

Breasts are made of a variety of different tissues, including ducts, lobes and lobules and glands that produce milk and carry it to the nipple. The breasts also contain lymph nodes and fatty tissue. Cancer develops when cells in the breast mutate and grow out of control, forming a tumor. Most breast cancers—about 80 percent—are ductal carcinomas, which begin in milk ducts. About 10 percent of all breast cancers are lobular carcinomas, which develop in the lobes or glands that produce milk.

Other factors that may increase a woman’s risk for developing breast cancer include:

Obesity
Breast density
Menstrual history
A sedentary lifestyle
Heavy drinking
Previous medical treatments

Learn more about risk factors for breast cancer

Who gets breast cancer?

The risk for developing breast cancer increases with age. According to the National Cancer Institute:

The average age of a woman diagnosed with breast cancer is 62.
The average age of a woman who dies from breast cancer is 68.
Breast cancer is the most common cancer diagnosed in women between age 55 and 64.
About 10 percent of breast cancers occur in women younger than 45.

Women with a family history of breast cancer may be at a higher risk for developing the disease. For example:

Women whose mother, sister or daughter has or had breast cancer may have double the risk.
Women who have inherited mutations in the BRCA1 or BRCA2 gene are at higher risk.

Breast cancer also occurs in men, but is very rare. Approximately 2,670 American men will learn they have breast cancer in 2019, the American Cancer Society estimates. Male breast cancer accounts for 1 percent of all breast cancer diagnoses.

Get answers to top breast cancer questions

Types of breast cancer

There are many types and subtypes of breast cancer, but most are adenocarcinomas of the breast. Adenocarcinoma tumors are found in many common cancers, including prostate, lung, and colorectal. These types of tumors form in glands or ducts that secrete fluid. Breast adenocarcinomas form in milk ducts or milk-producing glands called lobules. Each type of breast cancer may be determined based on where in the breast it develops, whether it is considered invasive or non-invasive and whether it is driven by hormones or proteins. Types of breast cancer include:

Adenocystic carcinoma
Angiosarcoma
Ductal carcinoma
Inflammatory breast cancer
Lobular carcinoma
Metaplastic carcinoma
Phyllodes tumor

Subtypes of breast cancer include those driven by specific hormones, such as estrogen, progestogen or the protein HER2. Sixty percent of breast cancers are estrogen-positive. Twenty percent of breast cancers are HER2-positive. Another 20 percent are triple-negative breast cancers, a type of breast cancer that tests negative for estrogen, progesterone and HER2. Triple-negative breast cancer is among the more aggressive forms of the disease.

Breast cancer cells can spread into the lymph nodes in and around the breasts and, from there, travel and form tumors in distant parts of the body. When that occurs, it is called metastatic breast cancer. When it spreads, breast cancer is most often found in the brain, bones, liver and lungs. It is still considered breast cancer even if it is found on other parts of the body.

Learn more about breast cancer types