Overview

Breast cancer is the abnormal growth of the cells that line the ducts and lobes of the breast.

When breast cancer has spread outside the ducts or lobes into normal breast tissue, it is said to be invasive. The main types of invasive breast cancer are:

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

Some breast cancer is a mixture of both ductal and lobular carcinoma.

There are also some less common types of invasive breast cancer, such as inflammatory breast cancer and male breast cancer.

When abnormal cells in the lining of a duct or lobe of the breast haven't spread, they are said to be non-invasive, or "in situ" (say "in-SY-too"). These include:

• Ductal carcinoma in situ (DCIS). This is a precancer that may progress to breast cancer. The abnormal cells are only in the ducts of the breast.
• Lobular carcinoma in situ (LCIS). Because LCIS cells don't spread, it's not considered to be cancer. But having LCIS increases a person's risk of developing breast cancer later in life.

Other factors

Some breast cancer cells have:

• Hormone receptors for estrogen and progesterone. Some breast cancers need the hormones estrogen or progesterone (or both) to grow. These cancer cells have "receptors" on their surfaces. Receptors are like doorways to let hormones in. These types of cancer are called estrogen-receptor-positive (ER+) and progesterone-receptor-positive (PR+) breast cancer.
• Large amounts of a protein called HER-2. And some breast cancers also have a large amount of a protein called human epidermal growth factor receptor 2 (HER-2 or HER-2/neu). These breast cancers tend to grow faster and spread more quickly than breast cancers without as much HER-2. So, HER-2 positive (HER-2+) breast cancers are usually treated with a targeted medicine (such as trastuzumab) and chemotherapy.

If the breast cancer cells don't have estrogen receptors, progesterone receptors, or large amounts of HER-2 protein, they are said to be triple-negative. Triple-negative breast cancer is a less common type of breast cancer. It tends to grow faster and spread more quickly and is harder to treat than breast cancers that have hormone receptors. Medicines that target hormone receptors or HER-2 won't help with triple-negative cancer, but chemotherapy can help.