LBCA asked our Scientific Advisory Board (SAB) and other ILC researchers to provide answers to some of the frequently asked questions (FAQ’s) that come up about lobular breast cancer. The answer to the below question is from Dr. Rebecca Shatsky, medical oncologist specializing in the treatment of breast cancer at the University of California, San Diego.
Q: What are breast cancer tumor markers?
A: Tumor markers are substances – usually proteins, that are either made by cancer cells or by the body in response to cancer. These proteins are normally found in the body in low levels in people without cancer, but in some patients with advanced cancer they are expressed at much higher levels. When cells in the body – especially cancer cells, break down these substances are released into the blood and we can use special laboratory tests to detect them. Tumor marker blood tests can be a complementary tool to help follow the response to therapy in advanced/metastatic breast cancer. If there are tumor markers in the blood but the amount found decreases after a period of time when a patient is receiving a drug treatment this can, though not always, be an indicator that the treatment is effective. The three most common tumor markers that are followed in breast cancer are the Carcinoembryonic antigen (CEA), Cancer Antigen (CA) 27.29 and Cancer antigen (CA) 15-3.
While testing for and tracking breast cancer tumor markers can be helpful in the treatment of some patients with advanced breast cancer, it is not always the case for a number of reasons. Not all breast cancers express tumor markers and levels of expression from one patient to another can vary greatly. and none of the markers described below are 100% sensitive or specific for breast cancer and they can be elevated in other forms of cancer or in a variety of benign conditions. Therefore, it is important to always interpret tumor markers with caution and never use them as the sole piece of information to determine response to therapy in advanced breast cancer, Furthermore, there is not always concordance between the trend of tumor markers in the blood and the response to metastatic breast cancer therapy as seen on imaging. Imaging is still the gold standard for evaluating the response to therapy, however in lobular breast cancer it can be difficult to see the extent of breast cancer on scans so tumor marker trending (that is, tracking how the tumor markers change over time during treatment) may be more useful in this disease than in other breast cancer subtypes.
Importantly, tumor markers should not be used to diagnose breast cancer or to diagnose relapse of breast cancer as they are not reliable for these purposes. When interpreting breast cancer tumor marker values in advanced breast cancer, the most important thing to consider is the overall direction and trend over time (i.e. whether they are increasing or decreasing over time). The absolute numerical value of the tumor marker is not necessarily important and does not correlate to a specific burden of disease. Tumor markers in one individual cannot be compared to tumor marker numerical values in another patient because no two cancers are identical, and each will have different levels of expression if the individual’s cancer expresses them at all. All three tumor markers described below can be elevated in some benign conditions such as endometriosis, sarcoidosis, lupus, pelvic inflammatory disease and liver disease, which is why they should never be used alone for diagnosis of breast cancer.
CA 15-3
CA 15-3 is a protein antigen derived from MUC1, which is a transmembrane glycoprotein that acts as a protective barrier in normal epithelial cells. MUC1 can be overexpressed in a variety of different cancers that come from epithelial cells (called adenocarcinomas).
CA 27.29
CA 27.29 is another test designed to detect overexpression of the MUC1 glycoprotein. The difference between the CA 27.29 test and the CA 15-3 test is that they test for different parts of the MUC1 protein and the tests are designed slightly differently. (CA 27.29 uses one monoclonal antibody in its assay and CA 15-3 utilizes two monoclonal antibodies).
CEA
CEA is a protein that is found in high levels in developing babies, but in very low levels in adults. A variety of different cancers overexpress this protein in addition to breast cancer, including colorectal cancer and lung cancer, making this the least specific test for breast cancer, however just like CA 15-3 and CA 27.29, the trend in this level over time may be useful additional information when following treatment of advanced breast cancer.
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