By Ulysses Torassa
San Francisco Chronicle
In a finding sure to add fuel to the debate over screening for prostate cancer, a new study shows that the common blood test, as currently used, misses anywhere from two-thirds to more than three-quarters of the cancers it is designed to detect.
The problem is that the warning level of the test, called the prostate-specific antigen (PSA) test, is set too high. But experts said it's unlikely the threshold will be lowered because it would lead to a drastic increase in invasive biopsies and overtreatment of men who are not really in danger from the cancer, which usually grows very slowly and mostly strikes older men.
Already, about 221,000 new prostate cancers are discovered each year, and doctors must have complicated conversations with these patients about the risks and benefits of getting treatment compared to simply watching the cancer to see if it becomes aggressive.
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About half of American men older than 50 take the test each year, which measures the level of PSA, a substance in the blood that is elevated when the prostate gland becomes enlarged, inflamed or contains cancer. A patient typically will undergo a biopsy to confirm the findings.
Researchers analyzed data from 6,700 men who underwent PSA screening in St. Louis, and 700 who subsequently had a biopsy to check for cancer. They used computer models comparing the standard cutoff of 4.0 nanograms per milliliter versus a lower threshold of 2.5 nanograms. According to their estimates, the higher cutoff missed 82 percent of cancers in men younger than 60 and 65 percent of cancers in men older than 60.
"We're saying that if you're using 4 as your cutoff, you might want to think again," said Dr. Rinaa Punglia, a radiation oncologist at Brigham and Women's Hospital in Boston and the lead author of the study being published today in the New England Journal of Medicine. At the same time, she said, "It may be that the test isn't good enough, period."
