Breast cancer occurs when cells in the breast region become malignant and develop into clumps called tumors. These malignant tumors can spread to surrounding areas of the breast and eventually to other tissues in the body, if left untreated.

According to the American Cancer Society, most breast cancers begin in the ductal areas (i.e., the tubes where milk passes to the nipple), although the cancer can start in any of the breast areas (i.e., lobules (produces milk), or lymph nodes).

Classifications of Breast Cancer

  • Noninvasive (in situ) breast cancer. In situ breast cancer refers to cancer in which the cells have remained within their place of origin — they haven’t spread to breast tissue around the duct or lobule. The most common type of noninvasive breast cancer is ductal carcinoma in situ (DCIS), which is confined to the lining of the milk ducts. The abnormal cells haven’t spread through the duct walls into surrounding breast tissue. Doctors sometimes refer to this type of cancer as stage 0 cancer.[Endnote]
  • Invasive breast cancer. Invasive (infiltrating) breast cancers spread outside the membrane that lines a duct or lobule, invading the surrounding tissues. The cancer cells can then travel to other parts of your body, such as the lymph nodes. If your breast cancer is stage I, II, III or IV, you have invasive breast cancer.[Endnote]

Hormone Status of Breast Cancer

The growth of many breast cancers is produced by female hormones, estrogen and progesterone. Women with estrogen receptor (ER)–positive and progesterone receptor (PR)–positive tumors have a better prognosis compared to women with ER-negative and PR-negative tumors, with the worse outcome. The hormone type will help determine how your cancer develops and the types of possible treatment available.

  • Estrogen receptor (ER) positive. This type of breast cancer relies on estrogen to help it grow.
  • Progesterone receptor (PR) positive. This type of breast cancer relies on progesterone to help it grow.
  • HER-2. The Human Epidermal Growth Factor Receptor 2, is a gene that helps control how cells grow, divide, and repair themselves.
  • Hormone receptor (HR) negative. This type of cancer doesn’t have hormone receptors, so it doesn’t need hormones to help it grow[Endnote]. The most common growth factors expressed in ER/PR-negative and HER2-negative breast canceris the epidermal growth factor receptor, or EGFR.[Endnote]

Areas of the Breast Where the Cancer Grows

The type of tissue where your breast cancer arises determines how the cancer behaves and what treatments are most effective. Parts of the breast where cancer begins include:

  • Milk ducts. Ductal carcinoma is breast cancer that starts in the milk ducts (i.e., how the milk travels to the nipples) and is the most commonly diagnosed type of breast cancer.
  • Milk-producing lobules. Lobular carcinoma starts in the lobules of the breast, where breast milk is produced. The lobules are connected to the ducts, which carry breast milk to the nipple.
  • Connective tissues. Rare breast cancer types like phyllodes tumor and angiosarcoma begin in the connective tissue that’s made up of muscles, fat and blood vessels. Cancer that begins in the connective tissue is called sarcoma.