Normal Breast Structure

In order to understand breast cancer, it is helpful 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, 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 prevent 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. Because women have many more breast cells than men do and perhaps because their breast cells are constantly exposed to the growth-promoting effects of female hormones, breast cancer is much more common in women.

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 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 (cells that are important in fighting infections). Breast cancer cells can enter lymphatic vessels and begin to grow in lymph nodes. This becomes important when we talk about staging (see the section, "How Is Breast Cancer in Men Staged?").

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).

Knowing if the cancer cells have spread to lymph nodes is important because 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 are involved with the breast cancer, the more likely it is that the cancer will eventually be found in other organs as well. But not all men with lymph node involvement develop metastases, and it is not unusual for a man to have negative lymph nodes and later develop metastases.

Benign Breast Conditions

Many types of breast disorders can affect both men and women.

Benign breast tumors: Most breast tumors (abnormal lumps or masses of tissue) are benign (not cancerous). Benign breast tumors do not spread outside of the breast and are not life threatening. Benign tumors, such as papillomas and fibroadenomas, are common in women but are extremely 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 present 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 hormone balance during adolescence. The same condition is also common in older men due to changes in their hormone balance.

Rarely, gynecomastia occurs because tumors or diseases of certain endocrine (hormone-producing) glands cause a man's body to produce more estrogen (the main female hormone). Although men's glands normally produce some estrogen, 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 may be another cause of elevated estrogens in men.

Many commonly prescribed medicines can sometimes cause gynecomastia, too. 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 and 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 (car-sin-o-ma): 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 (add-no-car-suh-NO-muh): 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 (car-sin-o-ma in sigh-too): This term is used for the early stage of cancer, when it is confined to the layer of cells where it began. Specifically in breast cancer, in situ means that the abnormal cells remain confined to ducts (ductal carcinoma in situ) or lobules (lobular carcinoma in situ). Ductal carcinoma in situ (DCIS), which is also known as intraductal carcinoma and non-invasive breast cancer, is the most common type of carcinoma in situ of the male and female breasts. Although lobular carcinoma in situ (LCIS) is sometimes classified as a type of non-invasive breast cancer, most breast specialists feel it a pre-cancerous condition rather than a true noninvasive cancer. LCIS is very uncommon in men.

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

Ductal Carcinoma In Situ (DCIS)

DCIS accounts for about 1 in 10 cases of breast carcinoma in men. In DCIS, cancer cells fill the ducts but do not invade through the walls of the ducts into the fatty tissue of the breast or spread outside the breast. 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 metastasize (or spread) to other parts of the body. IDC (alone or mixed with other types of invasive or in situ breast cancer) accounts for 80% to 90% of male breast cancers.

Invasive Lobular Carcinoma (Lobular Breast Cancer)

This type of breast cancer is very rare in men, accounting for only about 2% of breast cancers. This is because men do not usually have lobular tissue (the part of the breast that, in women, contains milk-producing glands).

Paget Disease of the Nipple

This type of breast cancer starts in the breast ducts and spreads to the skin of 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 ductal carcinoma in situ (DCIS) or with infiltrating ductal carcinoma. It accounts for about 1% of female breast cancers and a higher percentage of 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. If no lump can be felt and the biopsy shows DCIS but no invasive cancer, the prognosis (outlook for survival) is generally very good.