What are they?
Tumor markers are substances, usually proteins, that are produced by the body in response to cancer growth or by the cancer tissue itself and that may be detected in blood, urine, or tissue samples. Some tumor markers are specific for a particular type of cancer, while others are seen in several cancer types. Most of the well-known markers may also be elevated in non-cancerous conditions. Consequently, tumor markers alone are not diagnostic for cancer.
There are only a handful of well-established tumor markers that are routinely used by physicians. Many other potential markers are still being researched. Some markers cause great excitement when they are first discovered but, upon further investigation, prove to be no more useful than markers already in use.
The goal is to be able to screen for and diagnose cancer early, when it is the most treatable and before it has had a chance to grow and spread. So far, the only tumor marker to gain wide acceptance as a screening test is Prostate Specific Antigen (PSA) for prostate cancer in men. Even with PSA there is continued debate among experts and national organizations over the usefulness of this test for screening asymptomatic men. Other markers are either not specific enough (too many false positives, leading to expensive and unnecessary follow-up testing) or they are not elevated early enough in the disease process to be useful for screening.
Some people are at a higher risk for particular cancers because they have inherited a genetic mutation. While not considered tumor makers, there are tests that look for these mutations in order to estimate the risk of developing a particular type of cancer. BRCA1 and BRCA2 are examples of gene mutations related to an inherited risk of breast cancer and ovarian cancer. For more information, see our overview on genetic testing.