About 35 percent of women get annual mammograms from age 40 onward. But the value of those screenings has been much debated, because mammograms for people in their 40s catch relatively few cases of breast cancer, generate plenty of false positive results, and produce some cases of unnecessary treatment.
Thus, while some organizations have advocated for testing to start at age 40, in 2009 the U.S. Preventive Services Task Force recommended that women start regular mammogram screening at age 50, not age 40 -- a major preventative health policy change.
But a new study co-authored by MIT scholars identifies an important challenge in designing such guidelines: Women who start getting mammograms at age 40 may be healthier than the population of 40-year-old women as a whole -- and they have a lower incidence of breast cancer than those who do not start getting tested at that age.
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Therefore, simply changing age recommendations is not, by itself, an optimal way to make breast-cancer screening policy.
For one thing, given that women who opt in to testing in their 40s are relatively healthier, altering those age guidelines has a relatively limited impact. At the same time, if mammogram screenings reached more women from ages 40-49, those tests would likely detect more cases of breast cancer, per screening, than they currently do. This suggests that new ways of identifying at-risk women who would benefit from screening would also be useful.
"Debates over when to recommend screening are missing a key point," says MIT economist Amy Finkelstein, co-author of a new paper detailing the study's results. "There are arguments about what the costs and benefits are of screening women at a certain age, but these tend to overlook the fact that those who follow the recommendations [for early screening] differ from the rest of the population. This makes the problem more complicated. You can't just forget human behavior and human selection when designing recommended health care practices."
To picture the health disparity the researchers found, suppose all women who currently start mammogram screening at age 40 shifted their first test to age 45. Now suppose an equal-size group of women with average health had been starting screening at age 40, and also shifted their first tests to age 45 as well. Both groups would see a rise in mortality due to breast cancer, but the first group would have only about one-quarter as many deaths.
"What we find in the paper is that compared to the women who don't follow the recommendation to get a mammogram, those who do are healthier, they are less likely to have cancer, and if they do find cancer, it's likelier to be smaller and at an earlier stage," says Abby Ostriker '16, an MIT PhD candidate in economics and a co-author of the study.