Tumor markers are chemical substances that are produced by some cancer cells. Most of these substances can normally be found in the bloodstream in small amounts even when cancer is not present.
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Not all cancers produce tumor markers. Conversely, having a high level of a marker does not always mean that a cancer is present. Because of this, tumor markers are more often used to monitor the progress of a cancer than to detect it initially.
The most common tumor markers and the type of cancer they signal include:
- Carcinoembryonic antigen (CEA), for colon cancer
- CA-125, for ovarian cancer and sometimes cancer of the uterus
- Prostate-specific antigen (PSA), for prostate cancer
- Alpha-fetoprotein (AFP) and human chorionic gonadotropin (HCG), for testicular cancer in men and certain types of reproductive system cancers in women
- Estrogen receptors/progesterone receptors and HER-2/neu, for breast cancer
CEA 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.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 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 and HCG 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 (especially ovarian cancer and gestational trophoblastic disease, mainly choriocarcinoma)
- some liver cancers
Not all cancers of these types produce AFP or HCG. If the tumor in a given patient is producing one or both of them, then 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.
Estrogen receptors/progesterone receptors are tested in the breast tumors from women and men with breast cancer. Breast cancers that contain estrogen receptors are often referred to as "ER positive," while those with progesterone receptors are "PR positive." About 70% of breast cancers test positive for one of these markers. These cancers tend to have a better prognosis than cancers without these receptors and are much more likely to respond to hormonal therapy such as tamoxifen (i.e., Nolvadex) or aromatase inhibitors.
HER-2/neu (c-erbB-2) is a marker that is tested in the breast tumors from women and men with breast cancer. About 30% of breast cancers test positive for HER-2/neu. Its main use is as a predictor of prognosis. These cancers are more likely to respond to a newer type of therapy known as trastuzumab (i.e., Herceptin).
Many other tumor markers are currently being studies. This list, of just a few, is adapted from a comprehensive list provided by the American Cancer Society.
Beta-2-microglobulin (B2M) which is elevated in multiple myeloma, chronic lymphocytic leukemia (CLL), and some lymphomas. The level of this marker may also be elevated in some non-cancerous conditions, such as chronic kidney, or renal, disease. B2M is useful in helping to determine prognosis in some of these cancers as patients with higher levels of B2M usually have a poorer prognosis.
Bladder tumor antigen (BTA) is present in the urine of many patients with bladder cancer. However, it is also found in some non-cancerous conditions as well. It is primarily used along with NMP22 to test patients for recurrent cancer.
CA 15-3 is used mainly to monitor patients with breast cancer. Levels usually drop following effective treatment. CA 15-3 can also be seen in other cancers as well as with hepatitis and benign breast conditions.
CA 27.29 is another marker used to follow patients with breast cancer during or after treatment. Although it is a newer test than CA 15-3, it does not appear to be any better in detecting either early or advanced disease. However, it may be less likely to be positive in people without cancer. Like CA 15-3, it can be elevated in other cancers and in some non-cancerous conditions.
CA 72-4 is a newer tumor marker being studied in ovarian cancer and cancers arising in the gastrointestinal tract, especially stomach cancer. There is no evidence that it is better than current tumor markers.
CA 19-9 was first developed to detect colorectal cancer, however, it is more sensitive to pancreatic cancer. However, it will not usually detect very early disease and is best used for following patients with cancer of the pancreas.
Calcitonin is a hormone produced by the parafollicular C cells in the thyroid gland. In medullary thyroid carcinoma (MTC), blood levels of this hormone are elevated. This is one of the few tumor markers that can actually be used to screen for early cancer and is most useful in the family members of a patient with MTC.
Chromogranin A, or CgA, is produced by neuroendocrine tumors, which include carcinoid tumors, neuroblastoma, and small cell lung cancer. It is also high in people with carcinoid tumors or other neuroendocrine tumors. It is probably the most sensitive tumor marker for carcinoid tumors.
Neuron-specific enolase (NSE), like chromogranin A, is a marker for neuroendocrine tumors such as small cell lung cancer, neuroblastoma, and carcinoid tumors. It is not used as a screening test.
S-100 and TA-90 are proteins found in most melanoma cells. Tissue samples of suspected melanomas are often tested for this marker to help in diagnosis.
Cancer: Principle and Practice of Oncology. (1997) 5th Edition DeVita, V.T. (ed). J.B. Lippincott: Philadelphia. Pp. 1160, 1337, 1506, 2120.