Tumor marker: A substance that can be detected in higher-than-normal amounts in the blood, urine, or body tissues of some patients with certain types of cancer. A tumor marker may be made by a tumor itself, or it may be made by the body as a response to the tumor. Tumor marker tests are not used alone to detect and diagnose cancer because most tumor markers can be elevated in patients who don't have a tumor, because no tumor marker is entirely specific to a particular type of cancer, and because not every cancer patient has an elevated tumor marker level, especially in the early stages of cancer, when tumor marker levels are usually still normal. Although tumor markers are typically imperfect as screening tests to detect occult (hidden) cancers, when a particular tumor has been found with a marker, the marker can be a means of monitoring the success or failure of treatment. The tumor marker level may also reflect the extent (stage) of the disease, indicate how quickly the cancer is likely to progress, and help determine the outlook. Examples of tumor markers include alpha-fetoprotein (AFP), carcinoembryonic antigen (CEA), human chorionic gonadotropin (HCG), lactate dehydrogenase (LDH), prostate specific antigen (PSA), and neuron-specific enolase (NSE).