The study was paid for by the study authors' universities.
"This study is important because what it really highlights is that the biology of the cancer is what we need to understand" in order to know which ones to treat and how, said Dr. Julia A. Smith, director of breast cancer screening at NYU Langone Medical Center in New York. Doctors already are debating whether DCIS, a type of early tumor confined to a milk duct, should even be called cancer, she said.
Another expert, Dr. Linda Vahdat, director of the breast cancer research program at Weill Cornell Medical College in New York, said the study's leaders made many assumptions to reach a conclusion about overdiagnosis that "may or may not be correct."
"I don't think it will change how we view screening mammography," she said.
A government-appointed task force that gives screening advice calls for mammograms every other year starting at age 50 and stopping at 75. The American Cancer Society recommends them every year starting at age 40.
Dr. Len Lichtenfeld, the cancer society's deputy chief medical officer, said the study should not be taken as "a referendum on mammography," and noted that other high-quality studies have affirmed its value. Still, he said overdiagnosis is a problem, and it's not possible to tell an individual woman whether her cancer needs treated.
"Our technology has brought us to the place where we can find a lot of cancer. Our science has to bring us to the point where we can define what treatment people really need," he said.
Marilynn Marchione can be followed at http://twitter.com/MMarchioneAP
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