What is breast cancer?

Breast cancer starts in the cells of the breast. The breast tissue covers an area larger than just the breast. It extends up to the collarbone and from the armpit across to the breastbone in the centre of the chest. The breasts sit on the chest muscles that cover the ribs.
Each breast is made of glands, ducts (thin tubes) and fatty tissue. Lobules are groups of glands that can produce milk. Milk flows from the lobules through a network of ducts to the nipple. The nipple is in the centre of a darker area of skin called the areola. Fatty tissue fills the spaces between the lobules and ducts and protects them.
A woman’s breasts may feel different at different times of her menstrual cycle, sometimes becoming lumpy just before her period. Breast tissue also changes with age. Breast tissue in younger women is mostly made of glands and milk ducts, but older women’s breasts are made up mostly of fatty tissue.

The breasts also contain lymph vessels and lymph nodes, which are part of the lymphatic system. The lymphatic system helps fight infections. Lymph vessels move lymph fluid to the lymph nodes. Lymph nodes trap bacteria, cancer cells and other harmful substances. There are groups of lymph nodes near the breast under the arm, near the collarbone and in the chest behind the breastbone.
Cancer cells may start within the ducts (this is called ductal carcinoma) or in the lobules (lobular carcinoma). Ductal carcinoma is the most common type of breast cancer.