aorta always looks like the aorta,” Dershaw said. “So when there’s a lump in the middle of that, it’s clearly abnormal. Looking at a mammogram is conceptually different from looking at images elsewhere in the body. Everything else has anatomy—anatomy that essentially looks the same from one person to the next. But we don’t have that kind of standardized information on the breast. The most difficult decision I think anybody needs to make when we’re confronted with a patient is: Is this person normal? And we have to decide that without a pattern that is reasonably stable from individual to individual, and sometimes even without a pattern that is the same from the left side to the right.”
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.
3.
Dershaw picked up a new X-ray and put it on the light box. It belonged to a forty-eight-year-old woman. Mammograms indicate density in the breast: the denser the tissue is, the more the X-rays are absorbed, creating the variations in black and white that make up the picture. Fat hardly absorbs the beam at all, so it shows up as black. Breast tissue, particularly the thick breast tissue of younger women, shows up on an X-ray as shades of light gray or white. This woman’s breasts consisted of fat at the back of the breast and more dense, glandular tissue toward the front, so the X-ray was entirely black, with what looked like a large, white, dense cloud behind the nipple. Clearly visible, in the black, fatty portion of the left breast, was a white spot. “Now, that looks like a cancer, that little smudgy, irregular, infiltrative thing,” Dershaw said. “It’s about five millimeters across.” He looked at the X-ray for a moment. This was mammography at its best: a clear picture of a problem that needed to be fixed. Then he took a pen and pointed to the thick cloud just to the right of the tumor. The cloud and the tumor were exactly the same color. “That cancer only shows up because it’s in the fatty part of the breast,” he said. “If you take that cancer and put it in the dense part of the breast, you’d never see it, because the whiteness of the mass is the same as the whiteness of normal tissue. If the tumor was over there, it could be four times as big and we still wouldn’t see it.”
What’s more, mammography is especially likely to miss the tumors that do the most harm. A team led by the research pathologist Peggy Porter analyzed 429 breast cancers that had been diagnosed over five years at the Group Health Cooperative of Puget Sound. Of those, 279 were picked up by mammography, and the bulk of them were detected very early, at what is called Stage One. (Cancer is classified into four stages, according to how far the tumor has spread from its original position.) Most of the tumors were small, less than two centimeters. Pathologists grade a tumor’s aggression according to such measures as the “mitotic count”—the rate at which the cells are dividing—and the screen-detected tumors were graded “low” in almost 70 percent of the cases. These were the kinds of cancers that could probably be treated successfully. “Most tumors develop very, very slowly, and those tend to lay down calcium deposits—and what mammograms are doing is picking up those calcifications,” Leslie Laufman, a hematologist-oncologist in Ohio, who served on a recent National Institutes of Health breast- cancer advisory panel, said. “Almost by definition, mammograms are picking up slow-growing tumors.”
A hundred and fifty cancers in Porter’s study, however, were missed by mammography. Some of these were tumors the mammogram couldn’t see—that were, for instance, hiding in the dense part of the breast. The majority, though, simply didn’t exist at the time of the mammogram. These cancers were found in women who had had regular mammograms, and who were legitimately told that they showed no sign of cancer on their last visit. In the interval between X-rays, however, either they or their doctor had manually discovered a lump in their breast, and these “interval” cancers were twice as likely to be in Stage Three and three times as likely to have high mitotic counts; 28 percent had spread to the lymph nodes, as opposed to 18 percent of the screen-detected cancers. These tumors were so aggressive that they had gone from undetectable to detectable in the interval between two mammograms.
The problem of interval tumors explains why the overwhelming majority of breast-cancer experts insist that women in the critical fifty-to-sixty-nine age group get regular mammograms. In Porter’s study, the women were X- rayed at intervals as great as every three years, and that created a window large enough for interval cancers to emerge. Interval cancers also explain why many breast-cancer experts believe that mammograms must be supplemented by regular and thorough clinical breast exams. (
