A Canadian study published in February reignited the years-long debate over breast cancer screening methods, arguing fairly strongly against mammography, or the use of X-rays to detect tumors. The study’s authors say mammograms have no survival benefit relative to physical breast exams and in fact lead to significant over-diagnosis. In their words: “The data suggest that the value of mammography screening should be reassessed.” Predictably, not everyone agreed with them.
The debate over mammograms centers on two questions. First, how many lives are saved by mammography? Or more specifically, how many tumors are detected early enough with this technology but would be missed by a physical exam until it was too late? Second, to what extent do mammograms increase over-diagnosis? Some small tumors will never be fatal. They will grow so slowly that within a normal lifetime, they will not cause illness. There is no reason to treat these tumors with chemotherapy or radiation since there is no survival benefit to doing so, and such treatments are unpleasant and carry their own risks. Mammograms increase the rate of over-diagnosis since they detect smaller tumors. The key question is: Do the possible survival benefits outweigh this over-diagnosis risk?
Every year, about 300,000 women are diagnosed with breast cancer and 40,000 die from it. Among the strongest predictors of survival is early detection: The smaller the tumor is when it’s identified, the better the woman’s odds of survival. This makes breast cancer screening — looking for tumors in women who otherwise have no symptoms — a central component of prevention. The simplest way to screen for tumors is a physical exam. But such an exam can only identify tumors that are large enough and close enough to the surface to be palpable. Because of these limitations, starting in the 1960s women and their doctors turned increasingly to mammograms, which use X-rays to find tumors at an even earlier stage of development.
The American Cancer Society recommends that women over 40 get a mammogram every year. Up until 2009, this was also the recommendation of the U.S. Preventive Services Task Force, an independent panel of experts appointed by the federal Department of Health and Human Services. But the task force changed its recommendation, saying that women between the ages of 50 to 74 should get a mammogram every two years. These conflicting recommendations from national organizations have muddied the waters for women around an incredibly common procedure — 67 percent of women over 40 have had a mammogram in the last year.
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