In the United States, some researchers credit screening with a death-rate reduction of 15 percent — which holds steady even when screening is reduced to every other year. Gilbert Welch, a professor of medicine at the Dartmouth Institute for Health Policy and Clinical Practice and co-author of last November’s New England Journal of Medicine study of screening-induced overtreatment, estimates that only 3 to 13 percent of women whose cancer was detected by mammograms actually benefited from the test.
If Welch is right, the test helps between 4,000 and 18,000 women annually. Not an insignificant number, particularly if one of them is you, yet perhaps less than expected given the 138,000 whose cancer has been diagnosed each year through screening. Why didn’t early detection work for more of them? Mammograms, it turns out, are not so great at detecting the most lethal forms of disease — like triple negative — at a treatable phase. Aggressive tumors progress too quickly, often cropping up between mammograms. Even catching them “early,” while they are still small, can be too late: they have already metastasized. That may explain why there has been no decrease in the incidence of metastatic cancer since the introduction of screening.
At the other end of the spectrum, mammography readily finds tumors that could be equally treatable if found later by a woman or her doctor; it also finds those that are so slow-moving they might never metastasize. As improbable as it sounds, studies have suggested that about a quarter of screening-detected cancers might have gone away on their own. For an individual woman in her 50s, then, annual mammograms may catch breast cancer, but they reduce the risk of dying of the disease over the next 10 years by only .07 percent — from .53 percent to .46 percent. Reductions for women in their 40s are even smaller, from .35 percent to .3 percent.
If screening’s benefits have been overstated, its potential harms are little discussed. According to a survey of randomized clinical trials involving 600,000 women around the world, for every 2,000 women screened annually over 10 years, one life is prolonged but 10 healthy women are given diagnoses of breast cancer and unnecessarily treated, often with therapies that themselves have life-threatening side effects.
