Searching for more reliable prostate cancer screens
Dear Doctor K: I keep hearing about problems with the PSA test. Are any new screening tests for prostate cancer in development?
Answer: Prostate-specific antigen (PSA) is a biomarker. Biomarkers are chemicals that can reveal that something abnormal is happening in the body. PSA is a chemical released by cells in the prostate gland. It is released at relatively high levels when prostate cells turn cancerous. However, PSA also is released in noncancerous prostate conditions.
The PSA test has been used for decades to detect prostate cancer before symptoms begin. However, there are two problems with the test. First, as just mentioned, high PSA levels do not always indicate that there is prostate cancer. The phrase doctors use to describe this imperfection is that the test result can be “falsely positive.”
Second, the PSA levels measured by the test can sometimes be normal even when a person does have prostate cancer. The phrase doctors use to describe this imperfection is that the test result can be “falsely negative.”
The third problem with the test is that even when it accurately detects prostate cancer, it doesn’t distinguish aggressive cancer from nonaggressive cancer. As I’ve said before, even though it seems odd: Cancer is not always bad for your health. There are types of cancer that can cause no symptoms, that grow slowly (if at all) and that are unlikely to spread. There are types of cancer that you will never know you had. You will die with these cancers, but you won’t die from them.
If the PSA test detects a prostate cancer that will not grow and spread, then detecting it can lead to invasive diagnostic tests that can have harmful side effects of their own, and to overtreatment of a cancer that will never cause any harm. The diagnosis and treatment will be worse than the disease.
Scientists are trying to find new and potentially better biomarkers for prostate cancer. They’re looking for biomarkers that have fewer falsely positive and falsely negative results, and that distinguish between aggressive and nonaggressive prostate cancers. Here are two of the most promising options:
Genetic tests of prostate
tissue. Cells in the prostate gland turn cancerous because of changes in their genes. Certain changes in genes lead to aggressive cancers. These genetic changes can be identified in biopsy specimens of the cancer. Such tests do not influence decisions about how the cancer should be treated; they just help distinguish between cancers that should be treated more or less immediately versus those nonaggressive cancers that just need to be closely monitored to make sure they don’t start growing.
Circulating tumor cells. Cancer spreads when tumor cells break away, get swept up into the bloodstream and start to grow in other parts of the body. A new “liquid biopsy” uses a simple blood test to measure circulating tumor cells and their telltale markers. This test isn’t yet part of standard care. But it could someday reduce the need for follow-up biopsies and help judge whether treatment is working. As for the PSA test, I think it is worth doing in men who are at increased risk for getting prostate cancer, such as those who have had relatives with the disease.