Mammograms have done surprisingly little to catch deadly breast cancers before they spread, a big U.S. study finds.

At the same time, more than a million women have been treated for cancers that never would have threatened their lives, researchers estimate.

Up to one-third of breast cancers, or 50,000 to 70,000 cases a year, don’t need treatment, the study suggests.

It’s the most detailed look yet at overtreatment of breast cancer, and it adds new evidence that screening is not as helpful as many women believe. Mammograms are still worthwhile, because they do catch some deadly cancers and save lives, doctors stress. And some of them disagree with conclusions the new study reached.

But it spotlights a reality that is tough for many Americans to accept: Some abnormalities that doctors call “cancer” are not a health threat or truly malignant. There is no good way to tell which ones are, so many women wind up getting treatments like surgery and chemotherapy that they don’t really need.

“We’re coming to learn that some cancers — many cancers, depending on the organ — weren’t destined to cause death,” said Dr. Barnett Kramer, a National Cancer Institute screening expert.

Kramer had no role in the study, which was led by Dr. H. Gilbert Welch of Dartmouth Medical School and Dr. Archie Bleyer of St. Charles Health System and Oregon Health & Science University.

Breast cancer is the leading type of cancer and cause of cancer deaths in women worldwide. Nearly 1.4 million new cases are diagnosed each year. Other countries screen less aggressively than the U.S. does. In Britain, for example, mammograms are usually offered only every three years and a recent review there found similar signs of overtreatment.

The dogma has been that screening finds cancer early, when it’s most curable. But screening is only worthwhile if it finds cancers destined to cause death, and if treating them early improves survival versus treating when or if they cause symptoms.

Mammograms also are an imperfect screening tool — they often give false alarms, spurring biopsies and other tests that ultimately show no cancer was present.

The new study looks at a different risk: Overdiagnosis, or finding cancer that is present but does not need treatment.

“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.

 

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