Breast cancer is the growth of abnormal cells in the ducts or lobes of the breast. Breast cancer may be either:

Invasive.

This means cancer has spread from the ducts or lobes into normal breast tissue. The main invasive types are:

• Ductal carcinoma. This cancer starts in the ducts of the breast. It's the most common type of breast cancer.
• Lobular carcinoma. This cancer starts in the lobes of the breast. It's the second most common type.

Some breast cancer is a mix of ductal and lobular carcinoma. Other less common invasive types include inflammatory breast cancer and male breast cancer.

Noninvasive.

This means the abnormal cells haven't spread beyond the ducts or lobes. These cancers include:

• Ductal carcinoma in situ (DCIS). In this type, the abnormal cells are only in the ducts of the breast. (Lobular carcinoma in situ [LCIS] is not considered to be cancer.)
• Paget disease of the nipple. The abnormal cells are only in or around the nipple. This is a rare type of cancer.

After the type of cancer is known, the cancer cells are checked for estrogen receptors, progesterone receptors, and large amounts of a protein called HER2. This information helps a doctor plan the treatment.

If the cancer cells don't have these three traits, they are called "triple negative." Triple-negative breast cancer is a less common type of invasive breast cancer.