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.
The study offers 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 doctors 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 end up getting treatments such as surgery and chemotherapy that they don't need.
Men have heard a similar message about PSA tests to screen for slow-growing prostate cancer, but the issue is relatively new to the debate over breast cancer screening.
The study 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. Results are in today's New England Journal of Medicine.
Breast cancer is the leading type of cancer and leading cause of cancer deaths in women worldwide. Nearly 1.4 million new cases are diagnosed each year.
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 existed — 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, 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."