Human breast cancer cell. Scanning electron micrograph (SEM) of cancer cell. Breast cancer is a malignant tumor that has developed from a cell on the site of the breast. A malignant tumor has the potential to shed its cells and spread through the bloostream or lymphatic system to other parts of the body. Many types of tumors may develop in the tissues of ducts of the mammary gland, most of them benign. Magnification: 5400x.