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This is ALL THINGS CONSIDERED from NPR News. I'm Michele Norris.

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And I'm Melissa Block.

News now about a blood test for prostate cancer called a PSA. Most American men over 50 have had one done. Two big studies out today were supposed to settle a long controversy about whether regular PSA tests reduce a man's chance of dying from prostate cancer. The New England Journal of Medicine today published the studies online, but as NPR's Richard Knox reports, the debate goes on.

RICHARD KNOX: The studies are about as large as they come: 77,000 American men, 162,000 Europeans. Half had regular PSA tests. The question: Would they have less risk of dying from prostate cancer? Seven to 10 years later, the answer is maybe.

Dr. MICHAEL BARRY (Researcher, Massachusetts General Hospital): This appears to be a controversy that just won't go away.

KNOX: Dr. Michael Barry has been wrestling with the PSA controversy for the better part of two decades.

Dr. BARRY: Many people hoped, including me, that we might get some definitive recommendations one way or the other. You should definitely be screened, you should definitely not be screened. We're left somewhere in between. But that may just be the way it is.

KNOX: In the American study, there was no difference in prostate cancer deaths between men who got PSAs and those who didn't. In Europe, there were 20 percent fewer deaths, a difference experts consider real, but not large.

Barry, a researcher at the Massachusetts General Hospital, says there's no doubt PSA tests can signal hidden prostate cancer.

Dr. BARRY: The real question is not whether prostate cancer screening is effective. It's whether it does more good than harm.

KNOX: That is, if doctors find prostate cancer early, is it more curable? And if so, does that happen often enough to justify possible harm? Many men suffer serious side effects of surgery, radiation and drugs who wouldn't otherwise die from prostate cancer.

Dr. Gerald Andriole of Washington University explains.

Dr. GERALD ANDRIOLE (Washington University): When we find prostate cancer, we don't know whether it's a killer cancer or what has been termed a toothless lion.

KNOX: The tumors he calls toothless lions are so slow-growing, that often a man will die of something else first. Barry says that it helps to compare PSA testing with mammography. Both tests reduce the risk of death by about the same amount. But there's one big difference: Many more men have to be treated to get that reduction in deaths. That's because fewer breast cancers are toothless lions. They're more likely to be lethal.

Dr. BARRY: And breast cancer, we said you had to treat about 10 women to prevent one death. In prostate cancer, it's about 50 men to prevent one death.

KNOX: That gap could be narrowed if scientists could devise a test that identifies which prostate tumors are dangerous. They're working on that. Meanwhile, Andriole suggests an approach that may spare more men from unnecessary cancer treatment. First, he says, older men who have other health problems don't need to get a regular PSA test at all. Something else is likely to kill them first. But men in their 50s and 60s, all those baby boomers entering their prostate cancer-prone years, pose a trickier problem.

Dr. ANDRIOLE: So for those patients, I would say, listen. We need to know whether you're at risk for prostate cancer.

KNOX: Like many American doctors, he offers them regular rectal exams and PSAs. If those show a possible problem, he'll talk with a patient about whether to do a biopsy. But if the biopsy indicates cancer, Andriole doesn't necessarily recommend treatment right away. Instead, he monitors patients closely with rectal exams, more frequent PSA tests and biopsies to get a sense of how fast the cancer's growing. It's called active surveillance, and it's a growing trend.

Experts don't expect doctors to stop ordering PSA tests. After all, most older male physicians get the test themselves. What may change is how they interpret the PSA tests in light of the studies released today.

Richard Knox, NPR News.

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