Whether there really is a measurable benefit from PSA screening for younger men won’t be known for a few more years, after data from two major clinical trials studying the test are reported.

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How can it be that finding prostate cancer early doesn’t help save lives? For starters, a large percentage of prostate cancers aren’t deadly. They are slow growing and unlikely to result in any symptoms before the end of a man’s natural life expectancy. By some estimates, as many as 44 percent of the men who are treated for prostate cancer as a result of PSA testing didn’t need to be. Had they been left alone, they would have died of something else and never known they had cancer.

“Screening tests don’t only pick up life-threatening cancers, they pick up tumors that look identical to traditional tumors, but they don’t have the same biologic behavior,” said Dr. Barry Kramer, associate director for disease prevention at the National Institutes of Health. “Some are so slow growing they never would have caused medical problems in the person’s natural life span.”

In the case of PSA testing, the Preventive Services Task Force, an expert panel that makes recommendations about preventive care for healthy people, said there was not enough evidence to recommend for or against screening of younger men, although they urged doctors to advise men of all the risks and benefits of screening. But they did conclude that 75 is the age at which the risks clearly begin to outweigh the benefits, and the disease, if detected, would most likely not have a meaningful effect on life expectancy.

Another problem with determining the value of screening is that it results in “lead time bias.” For instance, someone diagnosed with lung cancer at the age of 65 may die at 67 and be remembered as a two-year survivor. If the same man had been diagnosed at 57 through screening and died at the age of 67, he would be known as a 10-year survivor. That sounds a lot better, but the reality is that diagnosis and treatment didn’t prolong his life. He died at 67 either way.

“Even a harmful screening test could appear on the surface as a helpful test,” Dr. Kramer said. “Because you measure survival from the date of diagnosis, even if the person dies of the same cause on the same day they would have without screening, it looks like survival was longer.”

Any screening test can lead to false positives, followed by invasive and risky tests. Large numbers of people often end up being poked, prodded and tested only to discover they’re fine.