Tumor marker
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Tumor markers are substances found in the blood, urine or body tissues that can be elevated in cancer. There are many different tumor markers. They are used in oncology to help determine the presence of cancer. An elevated level of a tumor marker can indicate cancer, however there can often also be other causes of the elevation.
Tumor markers can be produced directly by the tumor or by non-tumor cells as a response to the presence of a tumor.
Tumor markers can be used in screening programs, such as screening for elevated levels of prostate specific antigen to indicate possible prostate cancer. Tumor markers are also used to monitor patients for cancer return.
Tumor markers can be classified in two groups: Cancer-specific markers and tissue-specific markers.
[edit] Cancer-specific markers
Cancer-specific markers are related to the presence of cancerous tissue. Because there is a large overlap between the many different tumor types and the markers produced these markers tend to be unhelpful in making a diagnosis. 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.
An example of a cancer-specific marker, 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 cancer case, meaning that an elevated level does 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.
[edit] Tissue-specific markers
Tissue-specific markers are related to specific tissues which 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), AFP-L3 - (a lectin-reactive AFP) and Thyroglobulin. For example, if man has an elevated PSA, a search for prostate cancer will be undertaken. If an individual has an elevated level of beta-HCG, AFP or AFP-L3%, a search for a testicular or liver cancer, respectively, will be made.
An example of a tissue-specific marker is PSA, or Prostate specific antigen, which is produced by the normal prostate. It is a protein enzyme called a serine protease that usually acts as an 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. An accurate way to tell if an elevated PSA level results from cancer is to biopsy the prostate.
