The past few decades the identification of cancer biomarkers has become a major project in cancer research.
A tumor marker is anything present in or produced by cancer cells or other cells of the body in response to cancer or certain benign (noncancerous) conditions that provides information about a cancer, such as how aggressive it is, whether it can be treated with a targeted therapy, or whether it is responding to treatment.
Tumor markers have traditionally been proteins or other substances that are made by both normal and cancer cells but at higher amounts by cancer cells. These can be found in the blood, urine, stool, tumors, or other tissues or bodily fluids of some patients with cancer. Increasingly, however, genomic markers such as tumor gene mutations, patterns of tumor gene expression, and nongenetic changes in tumor DNA, are being used as tumor markers.
Many different tumor markers have been characterized and are in clinical use. Some are associated with only one type of cancer, whereas others are associated with multiple different cancer types. No “universal” tumor marker has been found that can reveal the presence of any type of cancer.
There are two main types of tumor markers that have different uses in cancer care: circulating tumor markers and tumor tissue markers.
Circulating tumor markers can be found in the blood, urine, stool, or other bodily fluids of some patients with cancer. Circulating tumor markers are used to:
- estimate prognosis
- detect cancer that remains after treatment (residual disease) or that has returned after treatment
- assess the response to treatment
- monitor whether a cancer has become resistant to treatment
Although an elevated level of a circulating tumor marker may suggest the presence of cancer, this alone is not enough to diagnose cancer. For example, noncancerous conditions can sometimes cause the levels of certain tumor markers to increase. In addition, not everyone with a particular type of cancer will have a higher level of a tumor marker associated with that cancer. Therefore, measurements of circulating tumor markers are usually combined with the results of other tests, such as biopsies or imaging, to diagnose cancer.
Tumor markers may also be measured periodically during cancer therapy. For example, a decrease in the level of a circulating tumor marker may indicate that the cancer is responding to treatment, whereas an increasing or unchanged level may indicate that the cancer is not responding.
Circulating tumor markers may also be measured after treatment has ended to check for recurrence (the return of cancer).
Tumor tissue markers are found in the actual tumors themselves, typically in a sample of the tumor that is removed during a biopsy. Tumor tissue markers are used to:
- diagnose, stage, and/or classify cancer
- estimate prognosis
- select an appropriate treatment (eg, treatment with a targeted therapy)
In some types of cancer, the level of a tumor marker reflects the stage (extent) of the disease and/or the patient’s prognosis (likely outcome or course of disease).
Tumor markers may be measured before treatment to help doctors plan the appropriate therapy. Some tumor tissue markers are targets for specific targeted therapies. However, not all targets of targeted therapies are tumor markers that are tested in patients.
Examples of commonly used tumor tissue markers include estrogen receptor and progesterone receptor (breast cancer), used to determine whether treatment with hormone therapy and some targeted therapies is appropriate; EGFR gene mutation analysis (non-small cell lung cancer), to help determine treatment and estimate prognosis; and PD-L1 (many cancer types), and Tumor Mutational Burden (TMB) to determine whether treatment with a type of targeted therapy called an immune checkpoint inhibitor is appropriate.
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