Breast cancer arises when cells within the breast grow and multiply abnormally. This process can lead to the formation of a lump or mass of extra cells, called a tumor. In many cases the tumors are non-cancerous, or benign. In the case of malignant tumors, the cells continue dividing uncontrollably and have the capacity to invade nearby tissues and potentially spread (metastasize) to distant sites. There is also a form of "noninvasive" breast cancer called ductal carcinoma in situ (DCIS); some DCIS tumors may become invasive. According to the American Cancer Society, breast cancer is the most commonly diagnosed cancer in the United States among women, detected in more than 192,000 women each year. It is the second leading cause of cancer death among women, claiming more than 40,000 lives. (Although it is much rarer, breast cancer can also occur in men. Learn more about male breast cancer.)