How are tumor markers used?
Tumor markers can be helpful in a number of ways.
Screening and early detection of cancer
Screening refers to looking for cancer in people who have no symptoms of the disease. Early detection is finding cancer at an early stage, when it’s less likely to have spread and is easier to treat. The first goal of tumor marker testing was to screen for cancer – to look for cancer in people without symptoms – but very few markers have been shown to be helpful in this way.
A perfect tumor marker would be one that could be used as a cancer screening blood test for all people. The tumor marker would only be found in people with cancer. It would tell doctors the type of cancer, how much cancer there is, and which treatment would work best. At this time no tumor marker tests work like this.
Today, the most widely used tumor marker is the prostate-specific antigen (PSA) blood test. The PSA test is used to screen men for prostate cancer. Men with prostate cancer usually have high PSA levels. But it’s not always clear what the test results mean — men without cancer can have high PSA levels, and a normal PSA level does not always mean that no cancer is present. PSA is not a perfect tumor marker. And now, many doctors agree that PSA screening is not right for all men.
Right now, no other tumor marker is used to help screen for cancer in the general population. Some of the markers used now can help find cancer at an early stage, but because of their limitations, they are only checked in people who are known to be at high risk for certain types of cancer.
Usually, tumor markers can’t be used alone to diagnose cancer. In most cases, cancer can only be diagnosed by a biopsy. (This means taking out a piece of the tumor so it can be checked for cancer by looking at the cells under a microscope.) Still, tumor markers can help figure out if cancer is a possibility. And if a cancer is already widespread when it’s found, tumor markers can sometimes help figure out where it started.
For instance, let’s say a woman has cancer throughout her pelvis and belly (abdomen). A high level of the tumor marker CA 125 will strongly suggest ovarian cancer, even if it isn’t clear after surgery that the cancer started in the ovary. This can be important because treatment can then be aimed at ovarian cancer.
Alpha fetoprotein (AFP) is another example of a tumor marker that may be used to help diagnose cancer. The level of AFP can go up with some liver diseases, but when it reaches a certain high level in someone with a liver tumor, doctors can be fairly sure that the tumor is liver cancer (a biopsy will still be needed, though).
Determining the outlook (prognosis) for certain cancers
Some types of cancer grow and spread faster than others. But even within a cancer type, for instance testicular cancer, some cancers grow and spread more quickly or may be more or less responsive to certain treatments. Sometimes the level of a tumor marker can help predict the behavior and outlook for certain cancers. In testicular cancer, very high levels of the tumor markers HCG (human chorionic gonadotropin) or AFP are signs of a more aggressive cancer and a worse outlook for survival. Patients with high levels of these markers may be given more aggressive cancer treatments.
Seeing if certain treatments are likely to work
Certain markers found on cancer cells can be used to help predict if a certain treatment is likely to work. For example, in breast and stomach cancers, if the cells have too much of HER2 protein, drugs such as trastuzumab (Herceptin®) can be helpful in treatment. In cases like these, tumor tissue is checked for HER2 before treatment is started. If the cancer cells have normal amounts of HER2, the drugs won’t help.
Seeing how well treatment is working
One of the most important uses for tumor markers is to monitor (watch) patients being treated for cancer, especially advanced cancer. If a certain type of cancer has a tumor marker, the level of the marker may be able to be used to see if the treatment is working, instead of doing other tests like x-rays, CT scans, or bone scans.
If levels of the tumor marker in the blood go down, it’s almost always a sign that the treatment is working. On the other hand, if the marker level goes up, this could mean the cancer is not responding and the treatment may need to be changed. (One exception is if the cancer is very sensitive to a certain chemotherapy treatment. In this case, the chemo can cause many cancer cells to die and release large amounts of the marker into the blood, which will raise the level of the tumor marker for a short time.)
Looking for recurrent cancer
Tumor markers are also used to look for cancer that might have come back (recurred) after treatment. Certain tumor markers may be useful once treatment is done and there is no sign of cancer in the body.
For instance, people who have been treated for colorectal cancer often have their blood tested for levels of the tumor marker CEA. This tumor marker is often checked before any treatment (including surgery) is done. If it’s high, it will be checked again during and after treatment. It should go down as treatment progresses, and be at a normal level after treatment. Then, the level may be checked as part of follow-up, and if it starts going up again, it may be a sign that the cancer has come back (recurred), even when the person has no symptoms. Often, when a patient’s CEA level goes up, their doctor will order imaging tests to see if a new tumor can be found early.
When are tumor markers checked?
Whether or not tumor markers are checked regularly depends on the type of cancer a person has and what kind of tumor marker is being measured.
Tumor markers that are measured in the blood or other body fluids may be tested at diagnosis; before, during, and after treatment; and then regularly for many years to see if the cancer has come back. During treatment, changes in tumor marker levels can be a sign of whether treatment is working.
Tumor marker levels of this type can change over time. The changes are important, which is why results of a series of level tests often mean more than a single result. If at all possible it’s best to compare results from tests done at the same lab, and always be sure that the results are of the same value, such as ng/mL (nanograms per milliliter) or U/mL (units/milliliter).
Testing of the tumor markers on the cancer itself is often done only at diagnosis. Unless the cancer comes back or doesn’t go away, there may nothing left to measure after treatment.
Last Medical Review: 10/30/2013
Last Revised: 10/30/2013