The continuing controversy regarding mammography screening for breast cancer surfaced again last week with a new study in the British Medical Journal (BMJ) that found “one in three breast cancers detected in a population offered organized screening is overdiagnosed.”

This number is quite startling. Three years ago, another study found that the number of overdiagnosed breast cancers was one in six.

The implication, of course, is that one-third of women diagnosed with breast cancer may be receiving unnecessary treatment for the condition.

How cancer becomes overdiagnosed
An overdiagnosis of cancer occurs when the abnormalities that are found are of the kind that would never cause any symptoms or other problems during the person’s lifetime. In some cases, the cancer is so slow-growing that the person dies of something else first. Indeed, autopsy studies of women aged 40 to 54 who died from non breast-cancer-related causes found that 37 percent had invasive or non-invasive breast cancer at the time of their deaths — and half of those cancers would have been visible on mammograms.

Other research has suggested that up to 30 percent of cancers detected by mammograms may naturally disappear (spontaneously regress, in medical lingo) without treatment.

For the BMJ study, researchers from the Nordic Cochrane Centre in Copenhagen, Denmark, analyzed data for seven years both before and after public (free) breast cancer screening programs were started for women aged 50 to 69 in the United Kingdom, Canada, Australia, Sweden and Norway. After the screening programs got underway, there was a jump in cancer diagnoses. On the surface, this outcome would seem to be a good thing. But the data showed a more complicated story: For every woman whose life was saved by the mammography screening, 10 women were overdiagnosed and underwent unnecessary treatment.

A personal choice
These findings don’t mean women aged 50 and older should stop having screening mammograms. But they do complicate the decision-making process. As H. Gilbert Welch, MD, a professor of medicine at the Dartmouth Institute for Health Policy and Clinical Research, wrote in an editorial that accompanied the BMJ study: