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. This information is only about breast cancer in women.


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 that can make breast milk (lobules), small tubes that carry milk from the lobules to the nipple (ducts), fatty and connective tissue, blood vessels, and lymph vessels. Most breast cancers begin in the cells that line the ducts. Fewer breast cancers start in the cells lining the lobules. Cancers can also start in cells of the other tissues in the breast. These are called sarcomas and lymphomas and are not really thought of as breast cancers.
                                          


The lymph system of the breast

The lymph system is one of the main ways breast cancer spreads. Normally, lymph nodes are small, bean-shaped tissues that contain a certain kind of immune system cells (cells that fight infections). Lymph nodes are connected by vessels (like small veins) that carry a clear fluid called lymph instead of blood.

Most of the lymph vessels of the breast drain into:

• Lymph nodes under the arm (axillary nodes). 
• Lymph nodes around the collar bone (supraclavicular and infraclavicular lymph nodes) 
• Lymph nodes inside the chest near the breast bone (internal mammary lymph nodes) 

Breast cancer cells can travel in lymphatic vessels and begin to grow in lymph nodes. If cancer cells spread to lymph nodes, there is a greater chance that the cells have also 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 or will come back later. Since cancer in lymph nodes doesn’t always cause the nodes to get larger, doctors often remove one or more lymph nodes to check for cancer spread. Your treatment plan will depend on whether or not cancer is found in the lymph nodes.