A friend was diagnosed to have breast cancer last year. She had an operation followed by chemotherapy. Since her operation she has had a couple of follow-ups during which she underwent ultrasound and X-ray studies and a blood exam for CA 15-3. So far, all her exams are normal. My question is: What is this blood exam for CA 15-3 all about? — Gina P., Makati City.
CA 15-3 is a tumor marker for breast and other cancers including lung and ovarian. So the next question is: What are tumor markers?
Tumor markers are substances—mostly proteins and hormones—that are present in higher than normal amounts in the body of some people with certain types of cancer. They are produced either by the body itself as a response to the presence of a cancer or by the cancer cells themselves.
There are now more than 20 established tumor markers that are routinely used by physicians; many other potential ones are being studied. Tests for tumor markers utilize blood (as in the test for CA 15-3), urine or body tissues as sample.
Some tumor markers are seen in several types of cancer (e.g., CA 15-3 is elevated in many cancers including breast, lung and ovarian), a few are specific for one type of cancer.
However, regardless of their specificity, tumor markers, by themselves, are not used to diagnose cancer because most yield false positive results (i.e., their levels are sometimes elevated in people with benign tumors and conditions) as well as false negative results (i.e., their levels do not go up even if a cancer exists, especially if the malignancy is in its early stages).
At present, the procedure used to definitively diagnose cancers involves taking a sample of a suspected malignant tissue and examining it under the microscope (for solid tumors, this procedure is called a biopsy).
If they cannot diagnose cancer, what then is the value of testing for tumor markers?
Tests for tumor markers serve several purposes:
Although they, by themselves, cannot diagnose cancer, most (e.g., CA125, a tumor marker for ovarian cancer) help detect and diagnose certain cancers when used with other tests (e.g., X-ray, MRI, etc.).
At least one, Prostate Specific Antigen (PSA) is already widely accepted as a screening procedure (for prostate cancer) in men. A high PSA level suggests prostate cancer, which means the patient needs further work-up or, depending on other circumstances, further follow-up.
Many help to determine the characteristics of a cancer: how aggressive it is; how fast it grows; how far it has spread into other tissues and organs, etc. These data can help the physician in determining prognosis (i.e., ultimate outcome) of the disease. Prognosis and extent of spread dictates to a large extent the regimen to be followed in treating a cancer.
They can be used as guide for treatment. For example, breast cancer patients who are positive for estrogen receptors (a tumor marker) can benefit from drugs that neutralizes the effects of estrogen while those who are negative for estrogen receptors will not. Likewise, only advance breast cancer patients who are positive for the tumor marker Her2/neu will respond to Herceptin drug treatment.
Most are useful, especially in advanced cancers, in monitoring the patient’s response to treatment. If the level of the tumor marker that is being observed goes down, the treatment is working; otherwise, dosing adjustments or the application of other treatment modalities may be in order.
They are used to detect recurrence. In your friend’s case, this is most probably the purpose for which the test for CA 15-3 is done periodically, although the test is also useful in monitoring the effectiveness of treatment in advanced breast cancer.
Address inquiries on health matters to Dr. Eduardo G. Gonzales, DLSU College of Medicine, Dasmarinas, Cavite 4114.
