Tumor markers are chemical substances that are produced by some cancers. Most of these substances can be found in the bloodstream in small amounts even when cancer is not present. When cancers produce these substances, they are usually produced in very large amounts. The amount of the substances may increase far above a normal level as the disease worsens.
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Not all cancers produce tumor markers. Having a high level of a marker does not always mean that a cancer is present. Because of this, tumor markers are not usually used to diagnose cancer. Instead, tumor markers are often used to monitor cancer after it has been diagnosed. It is also used to monitor the treatment of certain types of cancer. The most common tumor markers and the type of cancer they signal include:
CEA, for colon cancer
CA-125, for ovarian cancer and sometimes cancer of the uterus
PSA, for prostate cancer
AFP/BCG, for testicular cancer in men and certain types of cancers of the reproductive system in women
CEA. This marker may be elevated in some colon cancers and certain other cancers. CEA levels are used to monitor people who are being treated for colon cancer. It is not used to identify people who are thought to have colon cancer. This is because many colon cancers do not produce abnormally high levels of CEA in the early stages.
CA-125. This is a tumor marker for ovarian cancer and occasionally cancer of the uterus. It is useful to have this test done when cancer is suspected. However, this test is not always reliable. Some women with ovarian cancer may not have high levels of CA-125. High levels are common in women with advanced ovarian cancer.
This tumor marker is also used to see how the body is responding to treatment. If the level of CA-125 in the blood goes down after treatment, the disease is probably responding. If the level goes up, the disease may be getting worse.
PSA. Prostate-specific antigen (PSA) is a substance that is normally produced by the prostate gland in small amounts. Men with prostate cancer may produce large amounts of this substance. The PSA level can also be slightly high in a man whose prostate gland is enlarged for reasons other than cancer. A higher-than-normal PSA level does not always indicate cancer. A normal level of PSA does not always mean that prostate cancer is not present. PSA alone is not a completely reliable tool for detecting prostate cancer in healthy men. However, when combined with a physical examination of the prostate, PSA can detect prostate cancer early. For men who have prostate cancer, PSA levels can help in seeing how the disease is responding to treatment.
AFP/BCG (alpha fetoprotein and beta chorionic gonadotropin). These two substances are normally produced by pregnant women. People with certain cancers can have extremely high levels of one or both of these markers, such as:
men with some types of testicular cancers
women with certain types of cancers of the reproductive organs
some liver cancers
Not all cancers of these types produce these markers. If the cancer does produce one or both of these substances, monitoring their level can be helpful in seeing how the disease is responding to treatment. The level of the markers will drop as the cancer is treated.
In general, tumor markers are helpful to monitor the course of a cancer. The levels can show if a particular treatment is working or if the cancer is getting worse. Tumor markers are not always reliable enough to be used to detect cancer early on.