"It's a milestone moment, a kind of changing of the guard," said Sally Curtin, who was the lead author of the report. She said the change reflected a reduction in deaths from leukemia, rather than an increase in deaths from brain cancer.

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Overall, the report showed, cancer death rates among children and adolescents dropped 20 percent between 1999 and 2014, continuing a long-term trend.

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The number of brain-cancer deaths first exceeded those from leukemia in 2011, Curtin said, but it wasn't until 2014 that the gap was large enough that statisticians concluded it was "a true finding."

The decrease in deaths from leukemia, once universally lethal, is a result of the enormous strides oncologists have made in recent decades in developing effective chemotherapy regimens and finding the best ways to use radiation and bone-marrow transplants, said Elizabeth Ward, senior vice president for intramural research at the American Cancer Society.

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By contrast, she said "brain cancers are generally very hard to treat," partly because surgeons have to be careful not to damage healthy tissue during operations and partly because of the blood-brain barrier, which prevents some drugs from getting into the brain.

Ann Kingston, director of research and science policy at the National Brain Tumor Society, said the brain-cancer death rates were "not acceptable," adding that she hoped that the increased use of molecular profiling of tumors, which can identify which drugs tumors are likely to respond to, and the development of other targeted therapies would lead to progress.

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Both Ward and Kingston noted that cancer therapies — surgery, radiation or chemotherapy — can often lead to long-term cognitive and developmental problems for children who survive the disease. They called on researchers to find ways to mitigate the harms of the treatments.

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Besides brain cancer and leukemia, other pediatric malignancies commonly involve bones, the thyroid and other endocrine glands and soft tissue.

The report comes as experts are pressing for stepped-up effort against pediatric cancer. For example, a blue-ribbon committee of the National Cancer Institute recently proposed focusing on "fusion oncoproteins," the drivers of many childhood cancers and setting up clinical trials to give children access to new immunotherapy drugs.