Tumor markers are substances, often proteins, that are produced by the cancer tissue itself or sometimes by the body in response to cancer growth. Because some of these substances can be detected in body samples such as blood, urine, and tissue, these markers may be used, along with other tests and procedures, to help detect and diagnose some types of cancer, predict and monitor a person's response to certain treatments, and detect recurrence.
More recently, the idea of what constitutes a tumor marker has broadened. Newer types of tests have been developed that look for changes in genetic material (DNA, RNA), rather than proteins, in patient samples. The genetic changes have been found to be associated with certain cancers and can be used as tumor markers to help determine prognosis, guide targeted treatment, and/or detect cancers early on. Moreover, advances in technology have led to tests that can evaluate several genetic markers or panels of markers at the same time, providing expanded information about characteristics of a tumor. Examples of these are included, along with more traditional tumor markers, in the table within this article.
While quite a few tumor markers are available and have been found to be clinically useful, others are available but not ordered frequently because they have been found to be less sensitive and/or specific. Still others are currently used only in research settings and continue to be evaluated in clinical trials. With ongoing research, and as the field continues to evolve, it is likely that more tumor markers with greater effectiveness will come on the market in the coming years, eventually replacing less useful ones.
While tumor marker tests can provide very useful information, they do have limitations:
- Many tumor markers may also be elevated in persons with diseases other than cancer.
- Some tumor markers are specific for a particular type of cancer, while others are seen in several different types of cancer.
- Not every person with a particular type of cancer will have an elevated level of the corresponding tumor marker.
- Not every cancer has a tumor marker that has been identified as associated with it.
Consequently, tumor markers alone are not diagnostic for cancer; for some types of cancer, they provide additional information that can be considered in conjunction with a patient's medical history and physical exam as well as other laboratory and/or imaging tests.
How are tumor markers used?
Tumor markers may be used for a variety of purposes. However, they are not typically used alone. Depending on the type of cancer, they may be used in conjunction with a tissue biopsy or a bone marrow or blood smear examination, for example, and/or with other tumor markers. They are not definitive but provide additional information that can be used to help:
- Screen. Because most tumor markers are not sensitive or specific enough, these tests are not well suited for screening the general population; however, a few may be used to screen people who are at high risk because they have a strong family history or specific risk factors for a particular cancer.
- Diagnose. In a person who has symptoms, tumor markers may be used to help detect the presence of cancer and help differentiate it from other conditions with similar symptoms.
- Stage. If a person does have cancer, tumor marker elevations can be used to help determine whether the cancer has spread (metastasized) to other tissues and organs and to what extent.
- Determine prognosis. Some tumor markers can be used to help determine how aggressive a cancer is likely to be.
- Guide choice of treatment. A few tumor markers provide information about which treatments might be effective against a person's cancer. This is a growing area of research. For more information, see the article Genetic Tests for Targeted Cancer Therapy.
- Monitor success of treatment and detect recurrence. Tumor markers can be used to monitor the effectiveness of treatment, especially in advanced cancers. If the marker level drops, the treatment is working; if it stays elevated, adjustments are needed. (The information must be used with care, however, since other conditions can sometimes cause tumor markers to rise or fall.) One of the most important uses for tumor markers, along with guiding treatment, is to monitor for cancer recurrence. If a tumor marker is elevated before treatment, low after treatment, and then begins to rise over time, then it is likely that the cancer is returning. (If it remains elevated after surgery, then chances are that not all of the cancer was removed.)
Examples of tumor markers that are used in each of these ways are provided in the table below.