(1) tumor marker is a substance present in or produced by a tumor or by the host, which can be used to differentiate neoplastic normal tissue, based on measurements in body fluids, secretions, cells or tissues. Markers are used in the diagnosis, staging and prognosis of cancer, provide an estimate of tumour burden and serve to monitor the effects of the therapy, detection of recurrences, the localization of tumors and screening in general populations. Tumor markers have been categorized as follows: enzymes, isoenzymes, hormones, ONCOFETAL antigens, epitopes of carbohydrates, products oncogenic and genetic modifications. There is no identified tumor marker that corresponds to the ideal profile of specificity.

2) a substance sometimes found in the blood, other body fluids or tissues. A high level of tumor marker may mean that a certain type of cancer is in the body. Examples of tumor markers include CA 125 (ovarian cancer), CA 15-3 (cancer), CEA (ovarian, lung, breast, pancreas and gastrointestinal tract cancers) and PSA (prostate cancer). Also called biomarker.

3) substances found in abnormal quantities in the blood, other body fluids or tumor tissue of patients suffering from certain types of cancer.