PSA Worthless for Prostate Cancer Screening?
'PSA Era Over,' Test's Pioneer Now Says
20 Years and 1,317 Prostates
Stamey's latest findings -- reported in the October issue of the Journal of Urology -- come from a study of 1,317 prostate glands removed by the same surgeon between 1983 and 2003. All of the patients had PSA tests. All had biopsies that showed signs of cancer.
In prostates removed during the earliest five years of the study, PSA levels predicted cancer severity. That is, the higher the patient's PSA, the larger and more aggressive the patient's prostate cancer.
But in the last five years of the study, PSA levels had little if anything to do with cancer severity. All PSA levels predicted, Stamey and colleagues found, was an enlarged prostate -- and all men's prostates enlarge as they age.
"In the first five years, we had a correlation of 60% between high PSA levels and severe cancer -- so the PSA test wasn't perfect even in the early years," Stamey say. "But my paper shows it is a straight curve down to a 2% correlation in the last five years. We show PSA is driven by benign enlargement of the prostate."
What's going on? Urologist H. Ballentine Carter, MD, a researcher at the Johns Hopkins Brady Urological Institute in Baltimore, explains.
"At the beginning of the PSA era, we were pulling out of the population patients with larger tumors, where there was better correlation between PSA and cancer volume," Carter tells WebMD. "Dr. Stamey's contention is that as the PSA era has gone on, we have culled those large tumors. Now we are detecting very, very small tumors where there is less correlation between PSA and cancer."
Lots of Prostate Cancers, Small Chances of Death
Prostate cancer, Stamey says, is very common. He points to autopsy data showing that while many men have prostate cancer, few die of it.
"Prostate cancer is a paper tiger in terms of the number of men who die from it," Stamey says.
PSA tests send many men to their urologists for biopsies. And these biopsies often find cancer. The question is whether these really are dangerous cancers, says Howard Parnes, MD, chief of the prostate and urological cancer research group at the National Cancer Institute.
"Sixteen percent of men in the U.S. will be diagnosed with prostate cancer," Parnes tells WebMD. "About 86% of men diagnosed with prostate cancer end up with definitive treatment, either surgery or radiation. But only 3% of these men are destined to die. ... What is scary is that if we look hard for it, if we test men every year, and if we biopsy men with a PSA over 4, we can find prostate cancer in about 25% of men."
Stamey agrees that if you look hard for prostate cancer, you'll often find it. But that alone doesn't tell doctors or patients who is at risk of dying.
"When a man submits to biopsy, he might as well be resigned to a positive result," Stamey says. "But if we don't counteract this with knowing how low the death rate is, we overestimate the significance of a positive biopsy."