The imbalance suggests a lot of overdiagnosis from mammograms, which now account for 60 percent of cases that are found, Bleyer said. If screening were working, there should be one less patient diagnosed with late-stage cancer for every additional patient whose cancer was found at an earlier stage, he explained.
"Instead, we're diagnosing a lot of something else — not cancer" in that early stage, Bleyer said. "And the worst cancer is still going on, just like it always was."
Researchers also looked at death rates for breast cancer, which declined 28 percent during that time in women 40 and older — the group targeted for screening. Mortality dropped even more — 41 percent — in women under 40, who presumably were not getting mammograms.
"We are left to conclude, as others have, that the good news in breast cancer — decreasing mortality — must largely be the result of improved treatment, not screening," the authors write.
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