Tumor markers are specific substances found in the blood that arise from cancerous tissues.

There are two forms of tumor marker.

The first marker group are related to the presence of cancerous tissue, and unfortunately these markers tend to be very unhelpful in making a diagnosis (because there is a large overlap between the many different tumor types and the markers produced). They can, however, be useful in follow up of treated patients to describe progress of the disease before any further masses can be found clinically or by imaging. A few examples of these markers are CEA, CA19-9, CA125.

CEA, or carcinoembryonic antigen, is a blood-borne protein, first noted to be produced by tumors of the gastrointestinal system. Further investigation showed that it was produced by the occasional lung and breast case, meaning that an elevated level did not mean a bowel cancer. However, in a patient with a history of a treated bowel cancer, a rising CEA level is an early sign of bowel cancer return. This usually occurs BEFORE the site of return can be identified on imaging or examination, and so many oncologists question the wisdom of doing a blood test for CEA when the end result is bad news that terrifies the patient, but does not have much impact on treatment.

The second marker group as related to specific tissues which now have developed cancer. Generally speaking, these substances are not specifically related to the tumor, and may be present at elevated levels when no cancer is present. But unlike the previous group, elevated levels point to a specific tissue being at fault. Examples include PSA, beta-HCG - Human chorionic gonadotropin, AFP - Alpha-fetoprotein, and Thryoglobulin . So if someone has an elevated PSA, a search for PROSTATE cancer will be undertaken. If a male has an elevated level of beta-HCG or AFP, a search for a testicular cancer will be made.

PSA, or Prostate specific antigen (NOT prostate cancer specific antigen), is produced by the normal prostate gland. It is a protein enzyme called a serine protease that usually acts as a anticoagulant to keep semen liquid. Only small amounts leak into the circulation in normal circumstances. Enlarged prostates leak more substantial amounts, and cancerous prostates also leak substantial amounts. Of course the only accurate way to tell if an elevated PSA level results from cancer is to biopsy the prostate.