By Dr. Dipesh Uprety
& Dr. Howard (Jack) West
What is a biomarker, and why do we test for them?
A biomarker for cancer is a measurable finding that is an indicator of the state of the disease. Biomarker testing can include specific genetic mutations in the DNA, proteins on the cancer cells, or potentially lab values from the blood. Biomarkers can be predictive of whether the patient’s cancer will respond to a specific therapy, such as a mutation in the epidermal growth factor receptor (EGFR) in someone with advanced lung cancer predicting a high probability of it responding well to a targeted therapy that blocks the EGFR protein. A cancer biomarker may also be prognostic, indicating the likely course or outcome of cancer regardless of treatments given. For example, identifying human papillomavirus (HPV) in head and neck cancer is associated with a favorable outcome for these cancers, regardless of the treatment given.
What are the sources of biomarkers?
Depending on the pattern of where cancer is present, biomarkers can potentially be tested from many different sources:
- Tumor tissue from a biopsy can provide DNA from cancer and proteins on cancer cells
- Blood samples can carry circulating cancer cells as well as proteins and DNA released from cancer cells
- Fluids that accumulate outside of the heart or lungs or inside the abdomen can also include circulating cancer cells and proteins and cancer DNA
- For a limited number of cancers, levels of proteins and other chemicals in the urine may offer information about the status of the cancer
When are biomarkers tested?
Biomarkers have a clear role in some cancers but not in others. Biomarkers are most commonly used for patients with more advanced cancers. However, the role of biomarkers and treatments guided by the results of biomarker testing evolves, and biomarker testing is increasingly becoming valuable in the earlier stages of some cancers. For example, testing for hormone receptors for estrogen and progesterone and the protein ERBB2 (also known as HER2) help define the best management even for early-stage breast cancer. In recent years, biomarkers predictive of response to targeted therapies or immunotherapy have also become important for guiding additional treatments for early-stage lung cancer that can be surgically removed, which can improve outcomes.
Importantly, biomarkers are often tested when cancer is first diagnosed. Still, one crucial role of some biomarkers is to offer feedback over time on how well or poorly the current treatment controls the cancer. For example, most prostate cancer cells produce the protein prostate-specific antigen (PSA), which is measurable in blood. When PSA levels become undetectable after prostate cancer surgery or radiation, or if the PSA declines on hormone therapy, this indicates effective control over the cancer; a rising PSA throughout repeated blood draws in a man with known prostate cancer typically indicates that the cancer is progressing. Some other cancers similarly use other biomarkers to reflect the cancer status.