For years, breast cancer awareness campaigns have urged women over 40 to get a yearly mammogram. When women hesitate to comply, it's often to avoid the discomfort of having their breasts squeezed or the fear of getting called back for more tests, even if it turns out there's no cancer.
But screening poses another downside: A routine mammogram can find cancers that would never have become life-threatening, subjecting women to painful and toxic treatments they never actually needed.
A new study calculates that this is just what happens in as many as one in every three breast cancers diagnosed by a screening mammogram. That research, published July 9 in the British Medical Journal, comes on the heels of several other studies suggesting that some breast cancers found on mammograms would naturally have regressed on their own without treatment.
The studies don't mean that women should abandon mammography, most experts say. But some think it's time to reconsider the way that mammogram screening is done.
"For too long, we've taken a brain-dead approach that says the best test is the one that finds the most cancers -- but that's wrong," says Dr. H. Gilbert Welch of the Dartmouth Institute for Health Policy and Clinical Practice in Lebanon, N.H, who wrote an editorial accompanying the British Medical Journal study. "The best test is the one that finds the right cancers and nothing else."
The detection of tumors that would never have caused trouble is known in the medical trade as overdiagnosis. It's a common problem with screening tests, which, by definition, aim to detect disease in people without symptoms.
Breast cancers generally behave in one of three ways. Some grow very aggressively and metastasize (i.e. spread to other tissues) long before any mammogram can detect them. Others grow more gradually and can be successfully treated if caught early. Still others grow so slowly that they'll never cause the woman a problem.
Autopsy studies have found undetected breast cancer in about 37% of women who died of some other cause. And a study of 42,238 Norwegian women published in November calculated that 22% of symptom-free cancers found on a screening mammogram naturally regressed on their own.
The problem is that, even under a microscope, it's impossible to distinguish these different types of cancer from one another, and mammograms are better at catching the less dangerous kinds.
