Hormone Therapy for Breast Cancer (Fact Sheet)
Hormone therapy slows or stops the growth of hormone-sensitive tumors by blocking the body's ability to produce hormones or by interfering with effects of hormones on breast cancer cells.
What are hormones?
Hormones are substances that function as chemical messengers in the body. They affect the actions of cells and tissues at various locations in the body, often reaching their targets through the bloodstream.
The hormones estrogen and progesterone are produced by the ovaries in premenopausal women and by some other tissues, including fat and skin, in both premenopausal and postmenopausal women and men. Estrogen promotes the development and maintenance of female sex characteristics and the growth of long bones. Progesterone plays a role in the menstrual cycle and pregnancy.
Estrogen and progesterone also promote the growth of some breast cancers, which are called hormone-sensitive (or hormone-dependent) breast cancers. Hormone-sensitive breast cancer cells contain proteins called hormone receptors that become activated when hormones bind to them. The activated receptors cause changes in the expression of specific genes, which can stimulate cell growth.
What is hormone therapy?
Hormone therapy (also called hormonal therapy, hormone treatment, or endocrine therapy) slows or stops the growth of hormone-sensitive tumors by blocking the body's ability to produce hormones or by interfering with effects of hormones on breast cancer cells. Tumors that are hormone insensitive do not have hormone receptors and do not respond to hormone therapy.
To determine whether breast cancer cells contain hormone receptors, doctors test samples of tumor tissue that have been removed by surgery. If the tumor cells contain estrogen receptors, the cancer is called estrogen receptor positive (ER positive), estrogen sensitive, or estrogen responsive. Similarly, if the tumor cells contain progesterone receptors, the cancer is called progesterone receptor positive (PR or PgR positive). Approximately 80% of breast cancers are ER positive.1 Most ER-positive breast cancers are also PR positive. Breast tumors that contain estrogen and/or progesterone receptors are sometimes called hormone receptorpositive (HR positive).
Breast cancers that lack estrogen receptors are called estrogen receptor negative (ER negative). These tumors are estrogen insensitive, meaning that they do not use estrogen to grow. Breast tumors that lack progesterone receptors are called progesterone receptor negative (PR or PgR negative). Breast tumors that lack both estrogen and progesterone receptors are sometimes called hormone receptor negative (HR negative).
Hormone therapy for breast cancer should not be confused with menopausal hormone therapy (MHT)—treatment with estrogen alone or in combination with progesterone to help relieve symptoms of menopause. These two types of therapy produce opposite effects: hormone therapy for breast cancer blocks the growth of HR-positive breast cancer, whereas MHT can stimulate the growth of HR-positive breast cancer. For this reason, when a woman taking MHT is diagnosed with HR-positive breast cancer she is usually asked to stop that therapy.