Healthy cells divide in a controlled way, but abnormal cells keep on dividing and dividing. This forms a lump called a tumour. Some tumours are benign, or harmless, and often don't need treatment. But malignant or cancerous tumours can spread. They are dangerous because they might invade nearby parts of the body and stop them working properly.
Breast cancer can develop in one of several different breast areas.
The interior of the female breast is divided into about 20 sections called lobes. Each lobe is subdivided into a collection of lobules, which are structures that contain milk-producing glands. These glands secrete milk into a complex system of tiny ducts. The ducts carry the milk through the breast and converge in a collecting chamber located just below the nipple. The most common form of breast cancer is Infiltrating ductal carcinoma, an invasive cancer that penetrates the wall of a duct.