Cancer tumor markers are simply defined as substances that are produced at abnormal levels in the bodies of people with cancer. These markers are usually proteins and can be detected in body fluids like blood or urine or in a biopsied tissue sample. Cancer tumor markers may refer to substances that are directly responsible for the tumor’s growth or metastasis, or they may refer to substances that are produced by the body in response to the cancer.
Tumor markers were originally proposed as a way to detect certain types of cancer prior to the onset of cancer-associated symptoms. It was thought that the discovery of cancer tumor markers would lead to simple blood or urine tests that could be used as diagnostic tools for a variety of cancers. Researchers and doctors were very excited by this prospect because the ability to detect cancer in the early stages can dramatically improve outcome.
Unfortunately, this has not panned out as originally hoped. Many of the proposed cancer tumor markers are produced at low levels in people that do not have the associated cancer because many conditions other than cancer can result in the production of most of these markers. In addition, not all people with a specific cancer will produce cancer tumor markers. This is most often the case in the early cancer stages. Due to the fact that most of these markers lack specificity, they are rarely used alone as a diagnostic tool.
Despite these drawbacks, several cancer tumor markers are considered possible screening tools. Probably the best known of these is prostate specific antigen (PSA), which can be used to screen for prostate cancer in men. Although many men with high PSA levels often turn out not to have prostate cancer, the test does detect cancer in many men in the early stages that otherwise would have gone unnoticed.
The CA-125 marker is sometimes used as a tool to screen women for ovarian cancer. This marker is considered less sensitive than PSA, however, and has a fairly high rate of both false positives and false negatives. The main problem with this marker is that over-production of CA-125 may result from gynecological problems that have nothing to do with ovarian cancer.
Cancer tumor markers are best used in individuals to monitor cancer treatments or to predict the recurrence, or relapse, of cancer. This is because this method measures a cancer tumor marker in the same person over a specific period of time. In these cases, a sudden drop or a sudden elevation of a specific marker is likely to represent a favorable response to treatment or the risk of cancer relapse, respectively.