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. Tumor markers were first developed 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 there are no tumor marker tests that 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 — high PSA levels can be seen in men without cancer, and a normal PSA does not always mean that no cancer is present. PSA is not a perfect tumor marker. And at this time, not all doctors agree that PSA screening is 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 they are only checked in people who are known to be at high risk for certain types of cancer.

Diagnosing cancer

Usually, tumor markers are not used to diagnose cancer. In most cases, cancer can only be diagnosed by a biopsy. (This means taking out some cells from a tumor so they 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 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).

Advanced cancer

Many tumor markers are helpful in checking people with advanced cancers. These tend to be cancers that are big (there’s a lot of cancer), have spread, and/or are affecting the way your body works. Advanced cancers are harder to treat and often have worse outcomes. In many cases, the tumor marker levels can be checked to see if treatment is working. (A series of levels over time should show a steady decrease when treatment is working.)

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 a sign of a more aggressive cancer and a worse outlook for survival. Patients with these high levels 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 a protein called HER2, drugs such as trastuzumab (Herceptin^®) can be helpful in treatment. If the cancer cells have normal amounts of HER2, the drugs won’t help. In cases like these, tumor tissue is checked for HER2 before treatment is started.

Seeing how well treatment is working

One of the most important uses for tumor markers is to watch patients being treated for cancer, especially advanced cancer. If a tumor marker is available for a certain type of cancer, 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, then 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 cause the level of the tumor marker to rise 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, then 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. Tumor markers may be checked 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 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).