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, The Associated Press reports.

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 fresh 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.

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.

Researchers used federal surveys on mammography and cancer registry statistics from 1976 through 2008 to track how many cancers were found early, while still confined to the breast, versus later, when they had spread to lymph nodes or more widely.

The scientists assumed that the actual amount of disease — how many true cases exist — did not change or grew only a little during those three decades. Yet they found a big difference in the number and stage of cases discovered over time, as mammograms came into wide use.

Mammograms more than doubled the number of early-stage cancers detected — from 112 to 234 cases per 100,000 women. But late-stage cancers dropped just 8 percent, from 102 to 94 cases per 100,000 women.

The imbalance suggests a lot of overdiagnosis from mammograms, which now account for 60 percent of cases that are found, researchers say.

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.