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What Is Breast Cancer?

Updated September 27, 2010

Most of us learn about breast cancer from the news and from events and studies that get us closer to “the cure.” But in reality, researchers are looking for many cures, because breast cancer is not a single disease. Breast cancer is a group of diseases that are related because they involve the tissues of the breast.

Breast cancer occurs when normal breast cells grow and reproduce out of control, turning into cancerous (malignant) cells. These abnormal cells grow so much that they fill the ducts (ductal cancer) or the lobules (lobular cancer) of the breast. The lobules are glands that produce breast milk, and the ducts are the passageways that carry the milk from the lobules to the nipple.

Ductal cancers and lobular cancers are named by the way they look under a microscope. Ductal cancers tend to grow together in a mass, making them somewhat easier to find. Lobular cancers tend to grow in more than one area of the breast in a pattern like a sheet, making them more difficult to find. There are other special types of breast cancer, but in general doctors treat them in similar ways.

When cancer cells stay inside and fill the walls of the ducts, this is called in situ breast cancer or ductal carcinoma in situ(DCIS). Other times the cells escape and grow through the walls of the ducts and lobules, invading nearby tissues. This is called invasive or infiltrating cancer. Invasive cancers can enter the bloodstream or the lymphatic channels and possibly travel to the lymph nodes under the arm, or even to organs beyond the breasts.

When you first receive your diagnosis, you will have little information about the cancer, its behavior or whether it has traveled beyond your breast. Over the next few weeks, your doctors will do tests to learn more about the features of the cancer. Once you know more, you and your doctors can determine together the most appropriate treatment plan for you.

Read more about breast cancer basics and the providers who helped us write this page in our Guide for the Newly Diagnosed.