What is breast cancer?
Breast cancer is a malignant (cancer) tumor that starts in the 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.
The normal breast
To understand breast cancer, it helps to know something about the normal parts of the breasts, as shown in the picture below.
A woman's breast is made up of glands (called lobules) that make breast milk, 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 of the breast
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 can 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 (benign breast lumps)
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 the combination of cysts and fibrosis (what is sometimes called fibrocystic changes). Cysts are fluid-filled sacs. Fibrosis is the formation of scar-like tissue. These changes can cause breast swelling and pain. These symptoms are often worse just before a woman's period is about to start. Fibrocystic changes can also cause the breasts to feel lumpy, and sometimes there is a clear or slightly cloudy nipple discharge. For more on benign breast changes, please see our document, Non-cancerous Breast Conditions.
Breast cancer terms
It can be hard to know the meaning of 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 an early stage of cancer, when it is still confined to 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). They have not spread into deeper tissues in the breast or to other organs in the body. Because of this, ductal carcinoma in situ is sometimes called non-invasive or pre-invasive breast cancer.
When cancer cells are confined to the lobules it is called lobular carcinoma in situ. This is not actually a true cancer or pre-cancer.
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 and are not discussed further in this document.
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 a type of non-invasive breast cancer. DCIS means that the cancer cells are only in the ducts. They have not grown through the walls of the ducts into the tissue of the breast and so cannot spread to lymph nodes or other organs. Nearly all women with cancer at this stage can be cured. Mammograms find many cases of DCIS.
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) and then spreads through the wall of the lobules. It can then spread (metastasize) to other parts of the body. About 1 in 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 may make 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 early. It 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: 09/04/2012
Last Revised: 02/22/2013