Hormones are naturally-occurring substances in the body that stimulate the growth of hormone-sensitive tissues, such as the breast or prostate gland. When cancer occurs in breast or prostate tissue, its growth and spread may be caused by the body’s own hormones. Therefore, drugs that block hormone production or change the way hormones work, or removal of organs that secrete hormones such as the ovaries or testicles, are ways of fighting cancer. Hormone therapy, similar to chemotherapy, is a systemic treatment because it may affect cancer cells throughout the body.