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. Breast cancer occurs mainly in women, but men can get it, too. Many people do not realize that men have breast tissue and that they can develop breast cancer.

Normal breast structure

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

The breast is made up mainly of lobules (milk-producing glands in women), 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).

Until puberty (usually around age 13 to 14), young boys and girls have a small amount of breast tissue consisting of a few ducts located under the nipple and areola (area around the nipple). At puberty, a girl's ovaries make female hormones, causing breast ducts to grow, lobules to form at the ends of ducts, and the amount of stroma to increase. In males, hormones made by the testicles restrict further growth of breast tissue. Men's breast tissue contains ducts, but only a few if any lobules.

Like all cells of the body, a man's breast duct cells can undergo cancerous changes. But breast cancer is less common in men because their breast duct cells are less developed than those of women and because their breast cells are not constantly exposed to the growth-promoting effects of female hormones.

The lymph (lymphatic) system

The lymph system is important to understand because it is one of the ways that 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 near the breast bone (internal mammary nodes) and either above or below the collarbone (supraclavicular or infraclavicular nodes).

It's important to know if the cancer cells have spread to lymph nodes. If they have, there is a higher chance that the cells could have gotten into the bloodstream and spread (metastasized) to other sites in the body. This is important to know when you are choosing a treatment. The more lymph nodes that contain breast cancer, the more likely it is that the cancer may be found in other organs as well. Still, not all men with lymph nodes that contain cancer develop metastases, and in some cases a man can have negative lymph nodes and later develop metastases.

Benign breast conditions

Some breast disorders that can affect men are benign (not cancerous).

Benign breast tumors: There are many types of breast tumors (abnormal lumps or masses of tissue) that are benign, such as papillomas and fibroadenomas. Benign breast tumors do not spread outside of the breast and are not life threatening. Benign tumors are common in women but are very rare in men.

Gynecomastia: Gynecomastia is the most common male breast disorder. It is not a tumor but rather an increase in the amount of a man's breast tissue. Usually, men have too little breast tissue to be felt or noticed. A man with gynecomastia has a button-like or disk-like growth under his nipple and areola, which can be felt and sometimes seen. While gynecomastia is much more common than breast cancer in men, both can be felt as a growth under the nipple, which is why it's important to have any such lumps checked by your doctor.

Gynecomastia is common among teenage boys due to changes in the balance of hormones in the body during adolescence. It is also common in older men due to changes in their hormone balance.

In rare cases, gynecomastia occurs because tumors or diseases of certain endocrine (hormone-producing) glands cause a man's body to make more estrogen (the main female hormone). Men's glands normally make some estrogen, but it is not enough to cause breast growth. Diseases of the liver, which is an important organ in male and female hormone metabolism, can change a man's hormone balance and lead to gynecomastia. Obesity (being extremely overweight) can also cause higher levels of estrogens in men.

Some medicines can cause gynecomastia. These include some drugs used to treat ulcers and heartburn, high blood pressure, and heart failure. Men with gynecomastia should ask their doctors about whether any medicines they are taking might be causing this condition.

Klinefelter syndrome, a rare genetic condition, can lead to gynecomastia as well as increase a man's risk of developing breast cancer. This condition is discussed further in the section, "What are the risk factors for breast cancer in men?"

Breast cancer general terms

It may help 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 in women), 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 abnormal cells remain confined to ducts (ductal carcinoma in situ, or DCIS) or lobules (lobular carcinoma in situ, or LCIS). 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 invaded 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.

Types of breast cancer in men

Ductal carcinoma in situ (DCIS)

In DCIS (also known as intraductal carcinoma), cancer cells form in the breast ducts but do not invade through the walls of the ducts into the fatty tissue of the breast or spread outside the breast. DCIS accounts for about 1 in 10 cases of breast cancer in men. It is almost always curable with surgery.

Infiltrating (or invasive) ductal carcinoma (IDC)

This type of breast cancer breaks through the wall of the duct and invades the fatty tissue of the breast. At this point, it can spread (metastasize) to other parts of the body. IDC (alone or mixed with other types of invasive or in situ breast cancer) accounts for at least 8 out of 10 male breast cancers. Because the male breast is much smaller than the female breast, all male breast cancers start relatively close to the nipple, so spread to the nipple is more likely. This is different from Paget disease as described below.

Infiltrating (or invasive) lobular carcinoma (ILC)

This type of breast cancer starts in the breast lobules (collections of cells that, in women, produce breast milk) and invades the fatty tissue of the breast. ILC is very rare in men, accounting for only about 2% of breast cancers. This is because men do not usually have much lobular tissue.

Lobular carcinoma in situ (LCIS)

In LCIS, abnormal cells form in the lobules, but they do not invade into the fatty tissue of the breast or spread outside the breast. Although LCIS is sometimes classified as a type of non-invasive breast cancer, most breast specialists think it is a pre-cancerous condition rather than a true non-invasive cancer. As with invasive lobular carcinoma, LCIS is very rare in men.

Paget disease of the nipple

This type of breast cancer starts in the breast ducts and spreads to the nipple. It may also spread to the areola (the dark circle around the nipple). The skin of the nipple usually appears crusted, scaly, and red, with areas of itching, oozing, burning, or bleeding. Using the fingertips, a lump may be detected within the breast.

Paget disease may be associated with DCIS or with infiltrating ductal carcinoma. It accounts for about 1% of female breast cancers and a higher percentage of male breast cancers.

Last Medical Review: 09/24/2008
Last Revised: 05/13/2009