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A new study out today is raising questions about the safety of a medical test that is among the most commonly used - CAT scans, also known as CT scans. The extensive international study found that children who get CT scans are at increased risk of brain cancer and leukemia. As NPR's Rob Stein reports, the findings provide the first direct evidence that the popular medical scans pose a health risk.

ROB STEIN, BYLINE: CT scans are great for diagnosing all sorts of medical problems. They create detailed images of the inside of the body. They're so good that they've become very popular. More than 80 million are being done every year now in the United States.

But the scans use a lot more radiation than a regular X-ray. And evidence has been mounting that radiation from CTs may increase the risk for cancer. But no one knew for sure. Amy Berrington helped lead the new study. She's at the National Cancer Institute.

AMY BERRINGTON: This is the first study that's looked directly at the population that had CT scans and then looked at their subsequent cancer risk.

STEIN: The researchers studied nearly 180,000 British patients who got CT scans before their 22nd birthday. They looked at this age group because they're the most sensitive to radiation.

BERRINGTON: We found that the radiation exposure from the CT scans was associated with a subsequent increased risk of both leukemia and brain tumors.

STEIN: The researchers calculate that a kid who gets two or three scans of the head before age 15 has three times the risk of brain cancer. They calculate the risk of leukemia triples after five to 10 head scans. Now, researchers stress that the risk of brain cancer and leukemia is very low, which means it's still quite low even for those who get scans.

BERRINGTON: During the follow-up period for our study, which was about 10 years, we estimated about one excess brain tumor and one leukemia per 10,000 head CT scans performed in young children.

STEIN: But Berrington says scans of other parts of the body also look like they aren't completely safe. And there's a good chance that even one CT scan poses some risk, both to adults and to children, and possibly for all sorts of cancers.

ANDREW EINSTEIN: I think it's really a landmark paper.

STEIN: That's Andrew Einstein of Columbia University Medical Center. He wrote an analysis of the study that's being published along with the paper in the journal The Lancet.

EINSTEIN: It's the first paper that's convincingly established that radiation exposure at low doses, in this case from CT scans, is associated with an increased cancer risk to patients.

STEIN: Previous research has been based mostly on what happened to survivors of the bombings at Hiroshima and Nagasaki. That's why the new study is attracting so much attention. But some experts worry that people might over-react. It's important to point out that CT scans save a lot of lives. Donald Frush is with the American College of Radiology.

DONALD FRUSH: There are clearly situations in which CT is indicated: major motor vehicle accidents where there's multiple potential organ injuries, in abdominal pain where surgery might be required for, say, a bowel obstruction, or in some patients with appendicitis.

STEIN: If we just focused on the negatives, Frush says, there are a lot of things in life we wouldn't do.

FRUSH: Just hearing the downsides of driving a car, or crossing a street or flying in an airplane, if you only discussed the risks of all of those things, no one would drive a car or walk across the street or fly in an airplane.

STEIN: Other experts agree. But many argue that a lot of CT scans, maybe as many as half, are unnecessary. Meaning doctors have to be much more selective about how they use them. David Brenner at Columbia's done a lot of research on this.

DAVID BRENNER: It's absolutely fair if your physician suggests that you or your child has a CT scan - ask that physician why, ask are there good medical reasons why the CT scan is justified.

STEIN: It could turn out the test's really not needed or that there's a different test that's safer. Meanwhile, researchers are looking for ways to cut the amount of radiation emitted by every scan.

Rob Stein, NPR News.

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