Breast cancer occurs when breast cells begin to grow and divide at an uncontrolled pace. Normal breast cells grow and divide as needed and then die as they age or suffer damage. In contrast, breast cancer cells continue to divide, resist cell death signals, and gain capacity to invade, eventually forming masses called tumours.
Typically, breast cancer begins either in the cells of the ducts (the channels that carry milk from the glands to the nipple) or the lobules (the groups of glands that create milk). Breast cancer that starts in the ducts is called ductal carcinoma, while breast cancer that begins in the lobules are referred to as lobular carcinoma.
Both types of carcinoma can be in situ or invasive. In situ means that the cancer is in its original place and has not invaded into the nearby tissues. Invasive means the cancer has spread from where it began into the surrounding healthy tissues.