A large percentage of breast cancers are referred to as being hormone receptor positive. A hormone receptor positive tumor means that the breast cancer cells have a receptor inside of them that responds to estrogen or progesterone. So, just like an appliance needs electricity to run, so do these tumors need estrogen or progesterone to grow.

For such tumors thought to be hormone receptor positive, it is known that estrogen in a patient’s blood may cause hidden cancer cells in other parts of the body to grow. 

Hormonal therapy is administered by giving a patient oral medications for at least five years that either interfere with estrogen’s ability to stimulate breast cancer cells to grow, or that decrease estrogen production, which in turn helps keep the cancer cells from growing.