Breast cancer forms in the cells of the breast and can affect men and women. It is second only to skin cancer in the number of new cases diagnosed each year in women, and it is the second-leading cause of cancer death in American women. According to the National Cancer Institute, older age, early menstruation, a positive personal or family history of breast cancer, hormone use, prior radiation therapy and an older age at first childbirth all increase the risk of developing breast cancer.