'breast cancer', disease characterized by the growth of malignant cells in the mammary glands. Breast cancer can strike males and females, although women are about 100 times more likely to develop the disease than men. Mostcancers in female breasts form shortly before, during, or after menopause, with three-quarters of all cases being diagnosed after age 50. Generally, the older a woman is, the greater is her likelihood of developing breast cancer. Worldwide, breast cancer is the most common cancer among women, and in North America and Western Europe, where life spans are longer, the incidence is highest. In addition, in high-income countries, breast cancer is the leading cause of cancer death among women age 20 to 59.