You can build a high-tech camera capable of taking pictures in the middle of the night, in other words, but the system works only if the camera is pointed in the right place, and even then the pictures are not self- explanatory. They need to be interpreted, and the human task of interpretation is often a bigger obstacle than the technical task of picture taking. This was the lesson of the Scud hunt: pictures promise to clarify but often confuse. The Zapruder film intensified rather than dispelled the controversy surrounding John F. Kennedy’s assassination. The videotape of the beating of Rodney King led to widespread uproar about police brutality; it also served as the basis for a jury’s decision to acquit the officers charged with the assault. Perhaps nowhere have these issues been so apparent, however, as in the arena of mammography. Radiologists developed state-of-the-art X-ray cameras and used them to scan women’s breasts for tumors, reasoning that, if you can take a nearly perfect picture, you can find and destroy tumors before they go on to do serious damage. Yet there remains a great deal of confusion about the benefits of mammography. Is it possible that we place too much faith in pictures?
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The head of breast imaging at Memorial Sloan-Kettering Cancer Center, in New York City, is a physician named David Dershaw, a youthful man in his fifties, who bears a striking resemblance to the actor Kevin Spacey. One morning not long ago, he sat down in his office at the back of the Sloan-Kettering Building and tried to explain how to read a mammogram.
Dershaw began by putting an X-ray on a light box behind his desk. “Cancer shows up as one of two patterns,” he said. “You look for lumps and bumps, and you look for calcium. And, if you find it, you have to make a determination: is it acceptable, or is it a pattern that might be due to cancer?” He pointed at the X-ray. “This woman has cancer. She has these tiny little calcifications. Can you see them? Can you see how small they are?” He took out a magnifying glass and placed it over a series of white flecks; as a cancer grows, it produces calcium deposits. “That’s the stuff we are looking for,” he said.
Then Dershaw added a series of slides to the light box and began to explain all the varieties that those white flecks came in. Some calcium deposits are oval and lucent. “They’re called eggshell calcifications,” Dershaw said. “And they’re basically benign.” Another kind of calcium runs like a railway track on either side of the breast’s many blood vessels – that’s benign, too. “Then there’s calcium that’s thick and heavy and looks like popcorn,” Dershaw went on. “That’s just dead tissue. That’s benign. There’s another calcification that’s little sacs of calcium floating in liquid. It’s called ‘milk of calcium.’ That’s another kind of calcium that’s always benign.” He put a new set of slides against the light. “Then we have calcium that looks like this – irregular. All of these are of different density and different sizes and different configurations. Those are usually benign, but sometimes they are due to cancer. Remember you saw those railway tracks? This is calcium laid down inside a tube as well, but you can see that the outside of the tube is irregular. That’s cancer.” Dershaw’s explanations were beginning to be confusing. “There are certain calcifications in benign tissues that are always benign,” he said. “There are certain kinds that are always associated with cancer. But those are the ends of the spectrum, and the vast amount of calcium is somewhere in the middle. And making that differentiation, between whether the calcium is acceptable or not, is not clear- cut.”
The same is true of lumps. Some lumps are simply benign clumps of cells. You can tell they are benign because the walls of the mass look round and smooth; in a cancer, cells proliferate so wildly that the walls of the tumor tend to be ragged and to intrude into the surrounding tissue. But sometimes benign lumps resemble tumors, and sometimes tumors look a lot like benign lumps. And sometimes you have lots of masses that, taken individually, would be suspicious but are so pervasive that the reasonable conclusion is that this is just how the woman’s breast looks. “If you have a
Dershaw was saying that mammography doesn’t fit our normal expectations of pictures. In the days before the invention of photography, for instance, a horse in motion was represented in drawings and paintings according to the convention of ventre a terre, or “belly to the ground.” Horses were drawn with their front legs extended beyond their heads, and their hind legs stretched straight back, because that was the way, in the blur of movement, a horse seemed to gallop. Then, in the 1870s, came Eadweard Muybridge, with his famous sequential photographs of a galloping horse, and that was the end of ventre a terre. Now we knew how a horse galloped. The photograph promised that we would now be able to capture reality itself.
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
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 on is what it looks like on the X-ray screen – and the screen seldom gives you quite enough information.
