What the Dog Saw [85]
The situation with mammography is different. The way in which we ordinarily speak about calcium and lumps is clear and unambiguous. But the picture demonstrates how blurry those seemingly distinct categories actually are. Joann Elmore, a physician and epidemiologist at the University of Washington Harborview Medical Center, once asked ten board-certified radiologists to look at 150 mammograms — of which 27 had come from women who developed breast cancer, and 123 from women who were known to be healthy. One radiologist caught 85 percent of the cancers the first time around. Another caught only 37 percent. One looked at the same X-rays and saw suspicious masses in 78 percent of the cases. Another doctor saw “focal asymmetric density” in half of the cancer cases; yet another saw no “focal asymmetric density” at all. There was one particularly perplexing mammogram that three radiologists thought was normal, two thought was abnormal but probably benign, four couldn’t make up their minds about, and one was convinced was cancer. (The patient was fine.) Some of these differences are a matter of skill, and there is good evidence that with more rigorous training and experience radiologists can become better at reading breast X-rays. But so much of what can be seen on an X-ray falls into a gray area that interpreting a mammogram is also, in part, a matter of temperament. Some radiologists see something ambiguous and are comfortable calling it normal. Others see something ambiguous and get suspicious.
Does that mean radiologists ought to be as suspicious as possible? You might think so, but caution simply creates another kind of problem. The radiologist in the Elmore study who caught the most cancers also recommended immediate workups — a biopsy, an ultrasound, or additional X-rays — on 64 percent of the women who didn’t have cancer. In the real world, a radiologist who needlessly subjected such an extraordinary percentage of healthy patients to the time, expense, anxiety, and discomfort of biopsies and further testing would find himself seriously out of step with his profession. Mammography is not a form of medical treatment, where doctors are justified in going to heroic lengths on behalf of their patients. Mammography is a form of medical screening: it is supposed to exclude the healthy, so that more time and attention can be given to the sick. If screening doesn’t screen, it ceases to be useful.
Gilbert Welch, a medical-outcomes expert at Dartmouth Medical School, has pointed out that, given current breast-cancer mortality rates, nine out of every thousand sixty-year-old women will die of breast cancer in the next ten years. If every one of those women had a mammogram every year, that number would fall to six. The radiologist seeing those thousand women, in other words, would read ten thousand X-rays over a decade in order to save three lives — and that’s using the most generous possible estimate of mammography’s effectiveness. The reason a radiologist is required to assume that the overwhelming number of ambiguous things are normal, in other words, is that the overwhelming number of ambiguous things really are normal. Radiologists are, in this sense, a lot like baggage screeners at airports. The chances are that the dark mass in the middle of the suitcase isn’t a bomb, because you’ve seen a thousand dark masses like it in suitcases before, and none of those were bombs — and if you flagged every suitcase with something ambiguous in it, no one would ever make his flight. But that doesn’t mean, of course, that it isn’t a bomb. All you have to go