Breast cancer is a malignant (cancer) tumor that starts from cells of the breast. It is found mostly in women, but men can get breast cancer, too. Here we will only talk about breast cancer in women. You can learn more about breast cancer in men in our document, Breast Cancer in Men.

Parts of the normal breast

To understand breast cancer, it helps to know something about the normal structure or parts of the breasts, as shown in the picture below.

A woman's breast is made up of glands that make breast milk (called lobules), ducts (small tubes that carry milk from the lobules to the nipple), fatty and connective tissue, blood vessels, and lymph (pronounced limf) vessels. Most breast cancers begin in the cells that line the ducts (ductal cancer), some begin in the lobules (lobular cancer), and a small number start in other tissues.

The lymph system

The lymph system is one of the main ways in which breast cancers can spread. Lymph nodes are small, bean-shaped groups of immune system cells (cells that fight 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. Breast cancer cells can enter lymphatic vessels and begin to grow in lymph nodes.

Most lymph vessels of the breast lead to lymph nodes under the arm. These are called axillary nodes. If breast cancer cells reach the underarm lymph nodes and keep on growing, they cause the nodes to swell. The doctor needs to know whether cancer cells have spread to lymph nodes because if they have, there is a higher chance that the cells have also gotten into the bloodstream and spread to other places in the body. The more lymph nodes that have cancer in them, the more likely it is that the cancer will be found in other organs, too. This could affect the treatment plan.

Breast lumps that are not cancer

Most breast lumps are benign. This means they are not cancer. Benign breast tumors are abnormal growths, but they do not spread outside of the breast and they are not life threatening. But some benign breast lumps can increase a woman's risk of getting breast cancer.

Most lumps are caused by fibrocystic changes. Cysts are fluid-filled sacs. Fibrosis is the formation of scar-like tissue. These changes can cause breast swelling and pain. They often happen just before a woman's period is about to start. The breasts may feel lumpy, and sometimes there is a clear or slightly cloudy nipple discharge. For more on fibrocystic changes and other benign breast changes, please see our document, Non-cancerous Breast Conditions.

Breast cancer terms

It can be hard to understand some of the words your doctor uses to talk about breast cancer. Here are some of the key words you might hear:

Carcinoma: This is a term used to describe a cancer that begins in the lining layer 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 cancer that starts in gland tissue (tissue that makes and secretes a substance). The ducts and lobules of the breast are gland tissues because they make breast milk, so cancers starting in these areas are often called adenocarcinomas.

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

Invasive (infiltrating) carcinoma: An invasive cancer is one that has already grown beyond the layer of cells where it started (unlike 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 many types of breast cancer, but some of them are very rare. Sometimes a breast tumor can be a mix of these types or a mixture of invasive and in situ cancer.

Ductal carcinoma in situ (DCIS): This is the most common type of non-invasive breast cancer. DCIS means that the cancer is only in the ducts. It has not spread through the walls of the ducts into the tissue of the breast. Nearly all women with cancer at this stage can be cured. Often the best way to find DCIS early is with a mammogram. If there are areas of dead or dying cancer cells (called tumor necrosis) within the biopsy sample (when tissue is taken out to be looked at in the lab), the tumor is likely to grow and spread quickly (be more aggressive).

Lobular carcinoma in situ (LCIS): This begins in the milk-making glands (lobules) but does not go through the wall of the lobules. It is not a true cancer, but having LCIS increases a woman's risk of getting cancer later. For this reason, it's important that women with LCIS make sure they have regular mammograms and doctor visits.

Invasive (or infiltrating) ductal carcinoma (IDC): This is the most common breast cancer. It starts in a milk passage (a duct), breaks through the wall of the duct, and invades the tissue of the breast. From there it may be able to spread (metastasize) to other parts of the body. It accounts for about 8 out of 10 invasive breast cancers.

Invasive (infiltrating) lobular carcinoma (ILC): This cancer starts in the milk glands (the lobules). It can spread (metastasize) to other parts of the body. About 1 out of 10 invasive breast cancers are of this type.

Inflammatory breast cancer (IBC): 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, IBC makes the skin of the breast look red and feel warm. It also makes the skin look thick and pitted, something like an orange peel. The breast may get bigger, hard, tender, or itchy.

In its early stages, inflammatory breast cancer is often mistaken for infection. Because there is no defined lump, it may not show up on a mammogram, which may make it even harder to catch it early. It usually has a higher chance of spreading and a worse outlook than invasive ductal or lobular cancer. For more details, see our document, Inflammatory Breast Cancer.

There are also many other less common types of breast cancer. You can get details about these through our toll-free number or on our Web site.

Last Medical Review: 09/29/2009
Last Revised: 09/29/2009