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.
Tumor markers can be used for one of five purposes:
• Screening a healthy population or a high-risk population for the probable presence of cancer.
• Diagnosis of cancer or a specific type of cancer.
• Evaluating prognosis in a patient.
• Predicting potential response of a patient to therapy.
• Monitoring recovery of a patient receiving surgery, radiation, or chemotherapy.
How are tumor markers used in cancer care?
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
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)
How are tumor markers measured?
A doctor takes a sample of tumor tissue or bodily fluid and sends it to a laboratory, where various methods are used to measure the level or presence (or absence) of the tumor marker.
If the tumor marker is being used to determine whether treatment is working or whether there is a recurrence, the marker’s level will be measured in multiple samples taken at different times during and after treatment. Usually “serial measurements,” which show how the level of a marker is changing over time, are more meaningful than a single measurement.
Examples of tumor markers include prostate-specific antigen (PSA) for prostate cancer, cancer antigen 125 (CA 125) for ovarian cancer, calcitonin for medullary thyroid cancer, alpha-fetoprotein (AFP) for liver cancer and human chorionic gonadotropin (HCG) for germ cell tumors, such as testicular cancer and ovarian cancer.
Today, the most widely used tumor marker in the clinical setting is prostate-specific antigen (PSA). In addition, only a few markers that are now available have clinically useful predictive values for cancer at an early stage, and only when patients at high risk are tested. Tumor markers are not the gold standard for diagnosis of a cancer. In most cases, a suspected cancer can only be diagnosed by a biopsy.
Are tumor markers reliable indicators of cancer?
Not every tumor will cause an elevation in the tumor marker test, especially in the early stages of cancer. Physicians can use changes in tumor marker levels to follow the course of the disease, to measure the effect of treatment, and to check for recurrence.
Tumor markers are not always reliable for the following reasons:
• Most tumor markers can be made by normal cells, as well as cancer cells.
• Tumor markers can be associated with noncancerous conditions.
• Tumor markers are not always present in early stage cancers.
• People with cancer may never have elevated tumor markers.
• Even when tumor marker levels are high, they are not specific enough.