A measure of how often a particular event happens in one group compared to how often it happens in another group, over time. In cancer research, hazard ratios are often used in clinical trials to measure survival at any point in time in a group of trial participants who have been given a specific treatment compared to a control group given another treatment or a placebo. A hazard ratio of one means that there is no difference in survival between the two groups. A hazard ratio of greater than one or less than one means that survival was better in one of the groups.

Also called HIPAA and Kassebaum Kennedy Act. A 1996 U.S. law that allows workers and their families to keep their health insurance when they change or lose their jobs. The law also includes standards for setting up secure electronic health records and to protect the privacy of a person's health information and to keep it from being misused.

An advance medical directive in the form of a legal document that designates another person (a proxy) to make health care decisions in case a person is rendered incapable of making his or her wishes known. The health care proxy has, in essence, the same rights to request or refuse treatment that the person would have if he or she were capable of making and communicating decisions.

Also called spiral CT scan. A detailed picture of areas inside the body. The pictures are created by a computer linked to an X-ray machine that scans the body in a spiral path.

Also called trastuzumab. A monoclonal antibody that binds to HER2 (human epidermal growth factor receptor 2) and can kill HER2-positive cancer cells. Monoclonal antibodies are made in the lab and can locate and bind to substances in the body, including cancer cells. Herceptin is used to treat breast cancer that is HER2-positive and has spread after treatment with other medicines. It is also used with other anticancer medicines to treat HER2-positive breast cancer after surgery. Herceptin is also being studied in the treatment of other types of cancer.

Also called germline mutation. A gene change in a body's reproductive cell (egg or sperm) that becomes incorporated into the DNA of every cell in the body of the offspring. Hereditary mutations are passed on from parents to offspring. Some breast cancer genes are hereditary.

An intensive treatment that uses medicine to kill cancer cells, but that also destroys the bone marrow and can cause other severe side effects. High-dose chemotherapy is usually followed by bone marrow or stem cell transplantation to rebuild the bone marrow.

A type of radiation therapy that uses high-energy photons (units of light energy). High-energy photons penetrate deeply into tissues to reach tumors while giving less radiation to superficial tissues such as the skin.

Health Insurance Portability and Accountability Act, or Kassebaum Kennedy Act. A 1996 U.S. law that allows workers and their families to keep their health insurance when they change or lose their jobs. The law also includes standards for setting up secure electronic health records and to protect the privacy of a person's health information and to keep it from being misused.

The study of tissues and cells under a microscope.

An individual treated in the past and used in a comparison group when researchers analyze the results of a clinical study that had no control group. The use of a control group, or comparison group, helps researchers determine the effects of a new treatment more accurately.

Treatment that adds, blocks or removes hormones. To slow or stop the growth of certain types of breast cancer, synthetic hormones or other medicines may be given to block the body's natural hormones. Sometimes surgery is needed to remove the gland that makes a certain hormone. Also called endocrine therapy, hormonal therapy and hormone treatment.

A test to measure the amount of certain proteins, called hormone receptors, in cancer tissue. Hormones can attach to these proteins. A high level of hormone receptors may mean that hormones help the cancer grow.

In oncology, describes cancer that responds to hormone treatment.

One of many chemicals made by glands in the body. Hormones circulate in the bloodstream and control the actions of certain cells or organs. Some hormones can also be made in the lab.

A sudden, temporary onset of body warmth, flushing and sweating that is often associated with menopause.

Laws set by the U.S. Department of Health and Human Services (DHHS) to protect a person from risks in research studies that any federal agency or department has a part in. Also called 45 CFR 46, 45 Code of Federal Regulations Part 46 and Protection of Human Subjects.

A form of nutrition that is delivered into a vein. Hyperalimentation does not use the digestive system. It may be given to people who are unable to absorb nutrients through the intestinal tract because of vomiting that won't stop, severe diarrhea or intestinal disease. It may also be given to those undergoing high-dose chemotherapy or radiation and bone marrow transplantation. It is possible to give all of the protein, calories, vitamins and minerals a person needs using hyperalimentation. Also called parenteral nutrition, total parenteral nutrition and TPN.

Radiation treatment in which the total dose of radiation is divided into small doses and treatments are given more than once a day. Also called hyperfractionation and superfractionated radiation therapy.

An abnormal increase in the number of normal cells in an organ or tissue.

Also called hypofractionation. Radiation treatment in which the total dose of radiation is divided into large doses and treatments are given less than once a day.