prostate cancer, the PSA – or prostate specific antigen – blood test, has a high level of false positives. These false positives force men to undergo biopsies, a painful procedure in which samples of prostate tissue are taken for analysis.
The PSA can’t distinguish between aggressive disease, which can be fatal, and slow-moving forms of cancer that men can safely live with for years. So many men undergo unnecessary treatment, which can include surgery to cut out the prostate.
Two long-term studies – one from Europe and the other from the US – in the New England Journal of Medicine last month found the PSA test prevented few cancer deaths and led to unnecessary treatments for many men. The European study of 182,000 men found that for every life saved by the test, 48 men were informed they had prostate cancer and received unneeded treatments for the disease. The American study of 77,000 men found no reduction in prostate cancer deaths.
The PCA3 gene is an average of 65 times more common in prostate cancer cells than in normal cells, making it a good biomarker of disease. Men with prostate cancer shed genetic material containing PCA3 in their urine. As a result, PCA3 scores tend to correlate with tumor volume but not with the size of the prostate itself. That is one advantage over the PSA, which can be elevated in men with normal, but enlarged prostates.
PCA3 scores tend to be higher in men with aggressive cancers than in those with slower-moving disease.
One study presented by European researchers in Chicago this week tracked 154 men who received biopsies after a positive PSA test. Men with high PCA3 scores faced a 70 percent chance of finding cancer on biopsy. The odds fell to 19 percent in men with low PCA3 scores.
The researchers concluded that the PCA3 test could be used to assess the need for biopsies in men with suspicious PSA scores. But Gen-Probe believes it is more likely that the test will be used to interpret negative biopsy findings in men with positive PSA scores. In such cases, a high PCA3 score might suggest a second biopsy was needed, whereas a low PCA3 score might indicate a low risk of aggressive cancer — confirming the negative biopsy.
Gen-Probe spokesman Michael Watts said that each year in the US, 700,000 to 800,000 men with positive PSA scores receive negative biopsy results. But he said as many as 10 million men could be candidates for Gen-Probe’s test because a large percentage of men with conflicting results are screened in subsequent years.
Gen-Probe plans to begin a clinical trial that could lead to FDA approval in the third quarter. The trial will enroll 500 men with negative biopsies to see if the test can predict the results of a subsequent biopsy. It will take about a year to complete.