Breast cancer is a disease in which malignant (cancer) cells form in the tissues of the breast.

The breast is made up of lobes and ducts. Each breast has 15 to 20 sections called lobes. Each lobe has many smaller sections called lobules. Lobules end in dozens of tiny bulbs that can make milk. The lobes, lobules, and bulbs are linked by thin tubes called ducts.

Each breast also has blood vessels and lymph vessels. The lymph vessels carry an almost colorless, watery fluid called lymph. Lymph vessels carry lymph between lymph nodes. Lymph nodes are small, bean-shaped structures found throughout the body. They filter lymph and store white blood cells that help fight infection and disease. Groups of lymph nodes are found near the breast in the axilla (under the arm), above the collarbone, and in the chest.

The most common type of breast cancer is ductal carcinoma, which begins in the cells of the ducts and makes up about 70% to 80% of all breast cancer cases. The second most common type of breast cancer is lobular carcinoma, which begins in the lobes or lobules and makes up 10% to 15% of all breast cancer cases. Lobular carcinoma is more often found in both breasts at the same time than are other types of breast cancer. Inflammatory breast cancer is a rare type of fast-growing breast cancer in which cancer cells block lymph vessels in the skin of the breast.

For more information about breast cancer, see:

• Breast Cancer Prevention
• Breast Cancer Screening
• Breast Cancer Treatment During Pregnancy
• Male Breast Cancer Treatment
• Childhood Breast Cancer Treatment

A family history of breast cancer and other factors increase the risk of breast cancer.

Anything that increases your chance of getting a disease is called a risk factor. Having a risk factor does not mean that you will get cancer; not having risk factors doesn't mean that you will not get cancer. Talk to your doctor if you think you may be at risk for breast cancer.

Risk factors for breast cancer include the following:

• A personal history of invasive breast cancer, ductal carcinoma in situ (DCIS), or lobular carcinoma in situ (LCIS).
• A personal history of benign (noncancer) breast disease.
• A family history of breast cancer in a first-degree relative (mother, daughter, or sister).
• Inherited changes in the BRCA1 or BRCA2 genes or in other genes that increase the risk of breast cancer.
• Breast tissue that is dense on a mammogram.
• Exposure of breast tissue to estrogen made by the body. This may be caused by:
  • Menstruating at an early age.
  • Older age at first birth or never having given birth.
  • Starting menopause at a later age.
• Taking hormones such as estrogen combined with progestin for symptoms of menopause.
• Treatment with radiation therapy to the breast/chest.
• Drinking alcohol.
• Obesity.

Older age is the main risk factor for most cancers. The chance of getting cancer increases as you get older.

NCI's Breast Cancer Risk Assessment Tool uses a woman's risk factors to estimate her risk for breast cancer during the next five years and up to age 90. This online tool is meant to be used by a health care provider. For more information on breast cancer risk, call 1-800-4-CANCER.

Breast cancer is sometimes caused by inherited gene mutations (changes).

The genes in cells carry the hereditary information that is received from a person’s parents. Hereditary breast cancer makes up about 5% to 10% of all breast cancer. Some mutated genes related to breast cancer are more common in certain ethnic groups.

Women who have certain gene mutations, such as a BRCA1 or BRCA2 mutation, have an increased risk of breast cancer. These women also have an increased risk of ovarian cancer, and may have an increased risk of other cancers. Men who have a mutated gene related to breast cancer also have an increased risk of breast cancer. For more information, see Male Breast Cancer Treatment.

There are tests that can detect (find) mutated genes. These genetic tests are sometimes done for members of families with a high risk of cancer. For more information, see Genetics of Breast and Gynecologic Cancers.

The use of certain medicines and other factors decrease the risk of breast cancer.

Anything that decreases your chance of getting a disease is called a protective factor.

Protective factors for breast cancer include the following:

• Taking any of the following:
  • Estrogen-only hormone therapy after a hysterectomy.
  • Selective estrogen receptor modulators (SERMs).
  • Aromatase inhibitors.
• Less exposure of breast tissue to estrogen made by the body. This can be a result of:
  • Early pregnancy.
  • Breastfeeding.
• Getting enough exercise.
• Having any of the following procedures:
  • Mastectomy to reduce the risk of cancer.
  • Oophorectomy to reduce the risk of cancer.
  • Ovarian ablation.

Signs of breast cancer include a lump or change in the breast.

These and other signs may be caused by breast cancer or by other conditions. Check with your doctor if you have:

• A lump or thickening in or near the breast or in the underarm area.
• A change in the size or shape of the breast.
• A dimple or puckering in the skin of the breast.
• A nipple turned inward into the breast.
• Fluid, other than breast milk, from the nipple, especially if it's bloody.
• Scaly, red, or swollen skin on the breast, nipple, or areola (the dark area of skin around the nipple).
• Dimples in the breast that look like the skin of an orange, called peau d’orange.

Tests that examine the breasts are used to diagnose breast cancer.

Check with your doctor if you notice any changes in your breasts. The following tests and procedures may be used:

• Physical exam and health history: An exam of the body to check general signs of health, including checking for signs of disease, such as lumps or anything else that seems unusual. A history of the patient’s health habits and past illnesses and treatments will also be taken.
• Clinical breast exam (CBE): An exam of the breast by a doctor or other health professional. The doctor will carefully feel the breasts and under the arms for lumps or anything else that seems unusual.
• Mammogram: An x-ray of the breast.
• Ultrasound exam: A procedure in which high-energy sound waves (ultrasound) are bounced off internal tissues or organs and make echoes. The echoes form a picture of body tissues called a sonogram. The picture can be printed to be looked at later.
• MRI (magnetic resonance imaging): A procedure that uses a magnet, radio waves, and a computer to make a series of detailed pictures of both breasts. This procedure is also called nuclear magnetic resonance imaging (NMRI).
• Blood chemistry studies: A procedure in which a blood sample is checked to measure the amounts of certain substances released into the blood by organs and tissues in the body. An unusual (higher or lower than normal) amount of a substance can be a sign of disease.
• Biopsy: The removal of cells or tissues so they can be viewed under a microscope by a pathologist to check for signs of cancer. If a lump in the breast is found, a biopsy may be done.

  There are four types of biopsy used to check for breast cancer:

  • Excisional biopsy: The removal of an entire lump of tissue.
  • Incisional biopsy: The removal of part of a lump or a sample of tissue.
  • Core biopsy: The removal of tissue using a wide needle.
  • Fine-needle aspiration (FNA) biopsy: The removal of tissue or fluid, using a thin needle.

If cancer is found, tests are done to study the cancer cells.

Decisions about the best treatment are based on the results of these tests. The tests give information about:

• how quickly the cancer may grow.
• how likely it is that the cancer will spread through the body.
• how well certain treatments might work.
• how likely the cancer is to recur (come back).

Tests include:

• Estrogen and progesterone receptor test: A test to measure the amount of estrogen and progesterone (hormones) receptors in cancer tissue. If there are more estrogen and progesterone receptors than normal, the cancer is called estrogen and/or progesterone receptor positive. This type of breast cancer may grow more quickly. The test results show whether treatment to block estrogen and progesterone may stop the cancer from growing.
• Human epidermal growth factor type 2 receptor (HER2/neu) test: A laboratory test to measure how many HER2/neu genes there are and how much HER2/neu protein is made in a sample of tissue. If there are more HER2/neu genes or higher levels of HER2/neu protein than normal, the cancer is called HER2/neu positive or HER2 positive. This type of breast cancer may grow more quickly and is more likely to spread to other parts of the body. The cancer may be treated with drugs that target the HER2/neu protein, such as trastuzumab and pertuzumab.
• Multigene tests: Tests in which samples of tissue are studied to look at the activity of many genes at the same time. These tests may help predict whether cancer will spread to other parts of the body or recur (come back).

  There are many types of multigene tests. The following multigene tests have been studied in clinical trials:

  • Oncotype DX: This test helps predict whether early-stage breast cancer that is estrogen receptor positive and node negative will spread to other parts of the body. If the risk that the cancer will spread is high, chemotherapy may be given to lower the risk.
  • MammaPrint: A laboratory test in which the activity of 70 different genes is looked at in the breast cancer tissue of women who have early-stage invasive breast cancer that has not spread to lymph nodes or has spread to 3 or fewer lymph nodes. The activity level of these genes helps predict whether breast cancer will spread to other parts of the body or come back. If the test shows that the risk that the cancer will spread or come back is high, chemotherapy may be given to lower the risk.

Based on these tests, breast cancer is described as one of the following types:

• Hormone receptor positive (estrogen and/or progesterone receptor positive) or hormone receptor negative (estrogen and/or progesterone receptor negative).
• HER2 positive or HER2 negative.
• Triple-negative (estrogen receptor, progesterone receptor, and HER2 negative).

This information helps the doctor decide which treatments will work best for your cancer.

Certain factors affect prognosis (chance of recovery) and treatment options.

The prognosis and treatment options depend on:

• The stage of the cancer (the size of the tumor and whether it is in the breast only or has spread to lymph nodes or other places in the body).
• The type of breast cancer.
• Estrogen receptor and progesterone receptor levels in the tumor tissue.
• Human epidermal growth factor type 2 receptor (HER2/neu) levels in the tumor tissue.
• Whether the tumor tissue is triple-negative (cells that do not have estrogen receptors, progesterone receptors, or high levels of HER2/neu).
• How fast the tumor is growing.
• How likely the tumor is to recur (come back).
• A woman’s age, general health, and menopausal status (whether a woman is still having menstrual periods).
• Whether the cancer has just been diagnosed or has recurred (come back).