A cancer develops in the body when cells become abnormal and continue to multiply without stopping. There are many different types of cancer and almost any part of the body can be affected. Often, medicines are developed to treat specific types of cancer. For example, tamoxifen and anastrozole are used to treat breast cancer, goserelin and cyproterone are used to treat prostate cancer. Chemotherapy medicines are cytotoxic, which means they kill cancer cells or stop them growing. They are often used along with radiotherapy to reduce the size of a cancer before surgery. Other medicines, called monoclonal antibodies, help the immune system fight cancer cells.