Breast Cancer

Approved by the Cancer.Net Editorial Board, 10/2022

Anyone can develop breast cancer. In the United States, breast cancer is the most common cancer in women (excluding skin cancer). Male breast cancer is rare, accounting for less than 1% of all breast cancers. Learn more about male breast cancer in a separate guide on Cancer.Net.

The breast is made up of different tissues, ranging from very fatty tissue to very dense tissue. Within this tissue is a network of lobes. Each lobe is made up of small, tube-like structures called lobules that contain milk glands. Small ducts connect the glands, lobules, and lobes, carrying milk from the lobes to the nipple. The nipple is located in the middle of the areola, which is the darker area that surrounds the nipple.

Blood and lymph vessels also run throughout the breast. Blood vessels nourish the cells by delivering oxygen and nutrients and also removing waste and carbon dioxide. Lymph vessels, unlike blood vessels, only carry fluid away from tissues. They connect to lymph nodes and the lymphatic system, which drains bodily waste products. Lymph nodes are the small, bean-shaped organs that are part of the body’s immune system and help fight infection. Groups of lymph nodes are located in different areas throughout the body, such as in the neck, groin, and abdomen. Regional lymph nodes of the breast are those nearby the breast, such as the lymph nodes under the arm, which are called axillary lymph nodes.

Cancer begins when healthy cells in the breast change and grow out of control, forming a mass or sheet of cells called a tumor. A tumor can be cancerous or noncancerous, also called benign. A cancerous tumor is malignant, meaning it can grow and spread to other parts of the body. A benign tumor means the tumor can grow but has not spread.

This guide covers both non-invasive (stage 0) as well as early-stage and locally advanced invasive breast cancer, which includes stages I, II, and III. The stage of breast cancer describes how much the cancer has grown, and if or where it has spread.

Although breast cancer most commonly spreads to nearby lymph nodes, in which case the breast cancer is still considered a local or regional disease, it can also spread further through the body through the blood vessels and/or lymph nodes to areas such as the bones, lungs, liver, and brain. This is called metastatic or stage IV breast cancer and is the most advanced stage of the disease. However, the involvement of nearby lymph nodes alone is generally not stage IV breast cancer. Learn more about metastatic breast cancer in a separate guide on this website.

If breast cancer comes back after initial treatment, it can recur locally, meaning in the same breast and/or regional lymph nodes. It can also recur elsewhere in the body, called a distant recurrence or metastatic recurrence.

Breast cancer can be invasive or non-invasive. Invasive breast cancer is cancer that spreads into surrounding tissues and/or distant organs. Non-invasive breast cancer does not go beyond the milk ducts or lobules in the breast. About 80% of breast cancer is invasive cancer, and about 20% is non-invasive cancer. There are multiple types of breast cancers, which are classified based on how they look under a microscope.

• Ductal carcinoma in situ (DCIS). This is a non-invasive cancer (stage 0) that is located only in the duct and has not spread outside the duct.
• Invasive or infiltrating ductal carcinoma. This is cancer that has spread outside of the ducts. It is the most common type of invasive breast cancer.
• Invasive lobular carcinoma. This is a type of breast cancer that has spread outside of the lobules.

Less common types of invasive breast cancer include:

• Medullary
• Mucinous
• Tubular
• Metaplastic
• Papillary
• Micropapillary
• Apocrine
• Inflammatory breast cancer, which is an aggressive type of cancer that accounts for about 1% to 5% of all invasive breast cancers.

Paget’s disease is a rare type of cancer in the skin of the nipple or in the skin closely surrounding the nipple. It begins in the ducts of the nipple, then spreads to the nipple surface and the areola (dark circle of skin around the nipple). The nipple and areola often become scaly, red, itchy, and irritated. Often, Paget’s disease is mistaken for eczema or an infection before the correct diagnosis is made. Although it is usually non-invasive, it can also be an invasive cancer. It is usually found with an underlying breast cancer.

There are 3 main subtypes of breast cancer that are determined by doing specific tests on a sample of the tumor to determine its characteristics. These tests will help your doctor learn more about your cancer and recommend the most effective treatment plan.

Testing the tumor sample can find out if the cancer is:

• Hormone receptor positive. Breast cancers expressing estrogen receptors (ER) and/or progesterone receptors (PR) are called “hormone receptor positive.” These receptors are proteins found in cells. Tumors that have estrogen receptors are called “ER positive.” Tumors that have progesterone receptors are called “PR positive.” Only 1 of these receptors needs to be positive for a cancer to be called hormone receptor positive. This type of cancer may depend on the hormones estrogen and/or progesterone to grow. Hormone receptor-positive cancers can occur at any age, but they are more common after menopause. About two-thirds of breast cancers have estrogen and/or progesterone receptors. Cancers without these receptors are called “hormone receptor negative.” Hormone receptor-positive breast cancers are commonly treated using hormone therapy (see Types of Treatment).
• HER2 positive. About 15% to 20% of breast cancers depend on the gene called human epidermal growth factor receptor 2 (HER2) to grow. These cancers are called “HER2 positive” and have many copies of the HER2 gene or high levels of the HER2 protein. These proteins are also called “receptors.” The HER2 gene makes the HER2 protein, which is found on the cancer cells and is important for tumor cell growth. HER2-positive breast cancers grow more quickly. They can also be either hormone receptor positive or hormone receptor negative. HER2-positive early stage breast cancers are commonly treated using HER2-targeted therapies (see Types of Treatment). Cancers that have no HER2 protein are called “HER2 negative.” Cancers that have low levels of the HER2 protein and/or few copies of the HER2 gene are sometimes now called “HER2 low.”
• Triple negative. If a tumor does not express ER, PR, and HER2, the tumor is called “triple negative.” Triple-negative breast cancer makes up about 10% to 20% of invasive breast cancers. Triple-negative breast cancer seems to be more common among younger women, particularly younger Black women and Hispanic women. Triple-negative breast cancer is also more common in women with a mutation in the BRCA1 gene. Experts often recommend that people with triple-negative breast cancer be tested for BRCA gene mutations. See the Risk Factors and Prevention section for more information on these genetic mutations.