Breast cancer is a malignant tumor that starts from cells of the breast. A malignant tumor is a group of cancer cells that may invade surrounding tissues or spread (metastasize) to distant areas of the body. The disease occurs almost entirely in women, but men can get it, too. The remainder of this document refers only to breast cancer in women. For information on breast cancer in men, see the American Cancer Society document, Breast Cancer in Men.

The normal breast

In order to understand breast cancer, it helps to have some basic knowledge about the normal structure of the breasts.

The female breast is made up mainly of lobules (milk-producing glands), ducts (tiny tubes that carry the milk from the lobules to the nipple), and stroma (fatty tissue and connective tissue surrounding the ducts and lobules, blood vessels, and lymphatic vessels).

Most breast cancers begin in the cells that line the ducts (ductal cancers). Some begin in the cells that line the lobules (lobular cancers), while a small number start in other tissues.

The lymph (lymphatic) system

The lymph system is important to understand because it is one of the ways in which breast cancers can spread. This system has several parts.

Lymph nodes are small, bean-shaped collections of immune system cells (cells that are important in fighting infections) that are connected by lymphatic vessels. Lymphatic vessels are like small veins, except that they carry a clear fluid called lymph (instead of blood) away from the breast. Lymph contains tissue fluid and waste products, as well as immune system cells. Breast cancer cells can enter lymphatic vessels and begin to grow in lymph nodes.

Most lymphatic vessels in the breast connect to lymph nodes under the arm (axillary nodes). Some lymphatic vessels connect to lymph nodes inside the chest (internal mammary nodes) and those either above or below the collarbone (supraclavicular or infraclavicular nodes).

Knowing if the cancer cells have spread to lymph nodes is important because if it has, there is a higher chance that the cells could have also gotten into the bloodstream and spread (metastasized) to other sites in the body. The more lymph nodes that have breast cancer, the more likely it is that the cancer may be found in other organs as well. This is important to know because it could affect your treatment plan. Still, not all women with cancer cells in their lymph nodes develop metastases, and in some cases a woman can have negative lymph nodes and later develop metastases.

Benign breast lumps

Most breast lumps are not cancerous; that is, they are benign. Still, some may need to be sampled and viewed under a microscope to prove they are not cancer.

Fibrocystic changes

Most lumps turn out to be fibrocystic changes. The term "fibrocystic" refers to fibrosis and cysts. Fibrosis is the formation of fibrous (scar-like) tissue, and cysts are fluid-filled sacs. Fibrocystic changes can cause breast swelling and pain. This often happens just before a woman's menstrual period is about to begin. Her breasts may feel lumpy and, sometimes, she may notice a clear or slightly cloudy nipple discharge.

Other benign breast lumps

Benign breast tumors such as fibroadenomas or intraductal papillomas are abnormal growths, but they are not cancerous and do not spread outside of the breast to other organs. They are not life threatening. Still, some benign breast conditions are important because women with these conditions have a higher risk of developing breast cancer.

For more information see the section, "What Are the Risk Factors for Breast Cancer?" and the separate American Cancer Society document, Non-Cancerous Breast Conditions.

Breast cancer general terms

It is important to understand some of the key words used to describe breast cancer.

Carcinoma

This is a term used to describe a cancer that begins in the lining layer (epithelial cells) of organs such as the breast. Nearly all breast cancers are carcinomas (either ductal carcinomas or lobular carcinomas).

Adenocarcinoma

An adenocarcinoma is a type of carcinoma that starts in glandular tissue (tissue that makes and secretes a substance). The ducts and lobules of the breast are glandular tissue (they make breast milk), so cancers starting in these areas are sometimes called adenocarcinomas.

Carcinoma in situ

This term is used for the early stage of cancer, when it is confined to the layer of cells where it began. In breast cancer, in situ means that the cancer cells remain confined to ducts (ductal carcinoma in situ) or lobules (lobular carcinoma in situ). They have not invaded into deeper tissues in the breast or spread to other organs in the body, and are sometimes referred to as non-invasive breast cancers.

Invasive (infiltrating) carcinoma

An invasive cancer is one that has already grown beyond the layer of cells where it started (as opposed to carcinoma in situ). Most breast cancers are invasive carcinomas -- either invasive ductal carcinoma or invasive lobular carcinoma.

Sarcoma

Sarcomas are cancers that start from connective tissues such as muscle tissue, fat tissue, or blood vessels. Sarcomas of the breast are rare.

Types of breast cancers

There are several types of breast cancer, although some of them are quite rare. In some cases a single breast tumor can have a combination of these types or have a mixture of invasive and in situ cancer.

Ductal carcinoma in situ

Ductal carcinoma in situ (DCIS; also known as intraductal carcinoma) is the most common type of non-invasive breast cancer. DCIS means that the cancer cells are inside the ducts but have not spread through the walls of the ducts into the surrounding breast tissue.

About 1 in 5 new breast cancer cases will be DCIS. Nearly all women diagnosed at this early stage of breast cancer can be cured. A mammogram is often the best way to find DCIS early.

When DCIS is diagnosed, the pathologist (a doctor specializing in diagnosing disease from tissue samples) will look for areas of dead or dying cancer cells, called tumor necrosis, within the tissue sample. If necrosis is present, the tumor is likely to be more aggressive. The term comedocarcinoma is often used to describe DCIS with necrosis.

Lobular carcinoma in situ

Although not a true cancer, lobular carcinoma in situ (LCIS; also called lobular neoplasia) is sometimes classified as a type of non-invasive breast cancer, which is why it is included here. It begins in the milk-producing glands but does not grow through the wall of the lobules.

Most breast cancer specialists think that LCIS itself does not become an invasive cancer very often, but women with this condition do have a higher risk of developing an invasive breast cancer in the same breast or in the opposite breast. For this reason, women with LCIS should make sure they have regular mammograms.

Invasive (or infiltrating) ductal carcinoma (IDC)

This is the most common type of breast cancer. Invasive (or infiltrating) ductal carcinoma (IDC) starts in a milk passage (duct) of the breast, breaks through the wall of the duct, and grows into the fatty tissue of the breast. At this point, it may be able to spread (metastasize) to other parts of the body through the lymphatic system and bloodstream. About 8 of 10 invasive breast cancers are infiltrating ductal carcinomas.

Invasive (or infiltrating) lobular carcinoma

Invasive lobular carcinoma (ILC) starts in the milk-producing glands (lobules). Like IDC, it can spread (metastasize) to other parts of the body. About 1 out of 10 invasive breast cancers are ILCs. Invasive lobular carcinoma may be harder to detect by a mammogram than invasive ductal carcinoma.

Less common types of breast cancer

Inflammatory breast cancer: This uncommon type of invasive breast cancer accounts for about 1% to 3% of all breast cancers. Usually there is no single lump or tumor. Instead, inflammatory breast cancer (IBC) makes the skin of the breast look red and feel warm and gives the skin a thick, pitted appearance that looks a lot like an orange peel. Doctors now know that these changes are not caused by inflammation or infection, but by cancer cells blocking lymph vessels in the skin. The affected breast may become larger or firmer, tender, or itchy. In its early stages, inflammatory breast cancer is often mistaken for infection (mastitis). Because there is no defined lump, it may not show up on a mammogram, which may make it even harder to find it early. It tends to have a higher chance of spreading and a worse outlook than typical invasive ductal or lobular cancer.

For more information, see the separate American Cancer Society document, Inflammatory Breast Cancer.

Triple-negative breast cancer: This term is used to describe breast cancers (usually invasive ductal carcinomas) whose cells do not have estrogen receptors and progesterone receptors, and do not have an excess of the HER2 protein on their surfaces. (See "How is breast cancer diagnosed?" for more detail on these receptors.) Breast cancers with these characteristics tend to occur more often in younger women and in African-American women, and they tend to grow and spread more quickly than most other types of breast cancer. Because the tumor cells lack these receptors, neither hormone therapy nor drugs that target HER2 are effective against these cancers (although chemotherapy may be useful if needed).

Mixed tumors: Mixed tumors are those that contain a variety of cell types, such as invasive ductal cancer combined with invasive lobular breast cancer. In this situation, the tumor is treated as if it were an invasive ductal cancer.

Medullary carcinoma: This special type of infiltrating breast cancer has a rather well-defined boundary between tumor tissue and normal tissue. It also has some other special features, including the large size of the cancer cells and the presence of immune system cells at the edges of the tumor. Medullary carcinoma accounts for about 3% to 5% of breast cancers. The outlook (prognosis) for this kind of breast cancer is generally better than for the more common types of invasive breast cancer. Most cancer specialists think that true medullary cancer is very rare, and that cancers that are called medullary cancer should be treated as the usual invasive ductal breast cancer.

Metaplastic carcinoma: Metaplastic carcinoma (also known as carcinoma with metaplasia) is a very rare type of invasive ductal cancer. These tumors include cells that are normally not found in the breast, such as cells that look like skin cells (squamous cells) or cells that make bone. These tumors are treated like invasive ductal cancer.

Mucinous carcinoma: Also known as colloid carcinoma, this rare type of invasive breast cancer is formed by mucus-producing cancer cells. The prognosis for mucinous carcinoma is usually better than for the more common types of invasive breast cancer.

Paget disease of the nipple: This type of breast cancer starts in the breast ducts and spreads to the skin of the nipple and then to the areola, the dark circle around the nipple. It is rare, accounting for only about 1% of all cases of breast cancer. The skin of the nipple and areola often appears crusted, scaly, and red, with areas of bleeding or oozing. The woman may notice burning or itching.

Paget disease is almost always associated with either ductal carcinoma in situ (DCIS) or, more often, with infiltrating ductal carcinoma. If no lump can be felt in the breast tissue and the biopsy shows DCIS but no invasive cancer, the prognosis is excellent.

Tubular carcinoma: Tubular carcinomas are another special type of invasive ductal breast carcinoma. They are called tubular because of the way the cells are arranged when seen under the microscope. Tubular carcinomas account for about 2% of all breast cancers and tend to have a better prognosis than most other infiltrating ductal or lobular carcinomas.

Papillary carcinoma: The cells of these cancers tend to be arranged in small, finger-like projections when viewed under the microscope. These cancers are most often considered to be a subtype of ductal carcinoma in situ (DCIS), and are treated as such. In rare cases they are invasive, in which case they are treated like invasive ductal carcinoma, although the outlook is likely to be better. These cancers tend to be diagnosed in older women, and they make up no more than 1% or 2% of all breast cancers.

Adenoid cystic carcinoma (adenocystic carcinoma): These cancers have both glandular (adenoid) and cylinder-like (cystic) features when seen under the microscope. They make up less than 1% of breast cancers. They rarely spread to the lymph nodes or distant areas, and tend to have a very good prognosis.

Phyllodes tumor: This very rare breast tumor develops in the stroma (connective tissue) of the breast, in contrast to carcinomas, which develop in the ducts or lobules. Other names for these tumors include phylloides tumor and cystosarcoma phyllodes. These tumors are usually benign but on rare occasions may be malignant.

Benign phyllodes tumors are treated by removing the mass along with a margin of normal breast tissue. A malignant phyllodes tumor is treated by removing it along with a wider margin of normal tissue, or by mastectomy. While surgery is often all that is needed, these cancers may not respond as well to the other treatments used for invasive ductal or lobular breast cancer.

Angiosarcoma: This is a form of cancer that starts from cells that line blood vessels or lymph vessels. It rarely occurs in the breasts. When it does, it is usually seen as a complication of radiation to the breast. It tends to develop about 5 to 10 years after radiation treatment. However, this is an extremely rare complication of breast radiation therapy. Angiosarcoma can also occur in the arm of women who develop lymphedema as a result of lymph node surgery or radiation therapy to treat breast cancer. (For information on lymphedema, see the section, "How Is Breast Cancer Treated?") These cancers tend to grow and spread quickly. Treatment is generally the same as for other sarcomas (see the separate American Cancer Society document, Sarcoma - Adult Soft Tissue Cancer).

Last Medical Review: 09/04/2008
Last Revised: 05/13/2009