Tumor markers are substances that can be found in the body when cancer is present. The classic tumor marker is a protein that can be found in the blood in at increased levels when a certain type of cancer is present, but not all tumor markers are like that. Some are found in urine or other body fluid, and others are found in tumors and other tissue. They can be products of the cancer cells themselves, or made by the body in response to cancer or other conditions. Most tumor markers are proteins, but some newer markers are genes or other substances.
There are many different tumor markers. Some are seen only in a single type of cancer, while others can be found in many types of cancer.
To test for a tumor marker, the doctor most often sends a sample of the patient's blood or urine to a lab. The marker is usually found by combining the blood or urine with man-made antibodies that react with the tumor marker protein. Sometimes a piece of the tumor itself is tested for tumor markers.
Tumor markers alone are rarely enough to show that cancer is present. Most tumor markers can be made by normal cells as well as by cancer cells. Sometimes, non-cancerous diseases can also cause levels of certain tumor markers to be higher than normal. And not every person with cancer may have higher levels of a tumor marker.
This is why only a few tumor markers are commonly used by most doctors. When a doctor looks at the level of a certain tumor marker, he or she will consider it along with the patient's history and physical exam and other lab tests or imaging tests.
In recent years, doctors have begun to develop newer types of tumor markers. With advances in technology, levels of certain genetic materials (DNA or RNA) can now be measured. It's been hard to identify single substances that provide useful information, but doctors are now beginning to look at patterns of genes or proteins in the blood. These new fields of genomics and proteomics are discussed in the section, "What's new in tumor marker research?"
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