LONDON -- Children who get several CT scans have a slightly higher chance of brain cancer and leukemia in later life, though the risk is small and probably outweighed by the need to get the test, researchers reported.

CT scans have risen rapidly since their introduction 30 years ago. In children, they're used to evaluate head, neck or spine injuries or neurological disorders.

International researchers studied nearly 180,000 patients younger than 22 who had a CT scan in British hospitals between 1985 and 2002 and followed those patients until 2008. They found 74 of them were diagnosed later with leukemia, while 135 had brain tumors.

The number of scans, mostly of the head, were not measured, but scientists looked at data measuring radiation doses from them. That's because the amount of radiation received by body parts such as the brain and bone marrow depends on the age and size of the patient.

Those who got leukemia or brain tumors were compared with a group of people who got a very low dose of radiation to the same parts of their bodies.

"CT scans are very useful, but they also have relatively high doses of radiation, when compared to X-rays," said Mark Pearce of Newcastle University, the study's lead author. He said CT scans were warranted in most situations, but more needed to be done to reduce the amount of radiation.

The study was paid for by the U.S. National Cancer Institute and the British Department of Health. It was published online in the journal Lancet.

In the U.K., laws already require radiation from medical scans be kept as low as possible. In the United States, the government is pushing manufacturers to design new scanners to minimize radiation exposure for the youngest patients.

"If an imaging scan is warranted, the immediate benefits outweigh what is still a very small long-term risk," Dr. Marta Schulman, chairwoman of the Radiology Pediatric Imaging Commission of the American College of Radiology, said. "Parents should certainly discuss risk with their provider, but not refuse care that may save and extend their child's life."

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