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.

Non-invasive.

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.