Hormones are chemicals produced by glands in the body and circulated in the bloodstream. Estrogen and progesterone are hormones that affect the way some cancers grow. If tests show that the cancer cells have estrogen and progesterone receptors (molecules found in some cancer cells to which estrogen and progesterone will attach), hormone therapy is used to block the way these hormones help the cancer grow. This may be done by using drugs that block the way hormones work (Tamoxifen) or block the body's ability to produce these hormones (Femara, Arimidex) or by surgically removing organs that make hormones, such as the ovaries.

Hormone therapy can be primary therapy in combination with other types of treatment, or used to prevent the return of cancer following primary therapy such as surgery, radiation and chemotherapy.