Breast Cancer happens when cells in the breast begin to grow out of control enabling them to invade nearby tissues or even spread through the body. Sizable collections of the out of control tissue are called tumors. Not all Brest Tumors are considered to be cancer because certain types of large cell collections cannot spread or threaten a person's life. The preceding type of tumor is called a benign tumor. The tumors that can spread throughout the body or invade nearby tissues are considered to be cancerous cells and are called malignant tumors. In theory, any of the types of tissue in the breast can form cancer cells, but usually cancer comes from either ducts or glands in the breast. Because it may take months or years for a tumor to get large enough to notice in the breast, we screen for tumors with mammograms, which can sometimes see disease before we can feel it.