Why mammograms are not perfect?


Category : General, Medical Rants

Frequent readers know that I admire Malcolm Gladwell. His current New Yorker article has done it again. He clearly explicates the difficulties in mammographic interpretation – THE PICTURE PROBLEM

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 a hundred and fifty mammograms – of which twenty-seven had come from women who developed breast cancer, and a hundred and twenty-three from women who were known to have been healthy. One radiologist caught eighty-five per cent of the cancers the first time around. Another caught only thirty-seven per cent. One looked at the same X-rays and saw suspicious masses in seventy-eight per cent 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 sixty-four per cent 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.

He describes the classic signal noise detection problem – Signal Detection Theory. The problem with mammograms simply put is that the signal does not differ enough from the noise. Thus, detection becomes very difficult.

Under the circumstances, it is not hard to understand why mammography draws so much controversy. The picture promises certainty, and it cannot deliver on that promise. Even after forty years of research, there remains widespread disagreement over how much benefit women in the critical fifty-to-sixty-nine age bracket receive from breast X-rays, and further disagreement about whether there is enough evidence to justify regular mammography in women under fifty and over seventy. Is there any way to resolve the disagreement? Donald Berry says that there probably isn’t—that a clinical trial that could definitively answer the question of mammography’s precise benefits would have to be so large (involving more than five hundred thousand women) and so expensive (costing billions of dollars) as to be impractical. The resulting confusion has turned radiologists who do mammograms into one of the chief targets of malpractice litigation. “The problem is that mammographers—radiology groups—do hundreds of thousands of these mammograms, giving women the illusion that these things work and they are good, and if a lump is found and in most cases if it is found early, they tell women they have the probability of a higher survival rate,” says E. Clay Parker, a Florida plaintiff’s attorney, who recently won a $5.1 million judgment against an Orlando radiologist. “But then, when it comes to defending themselves, they tell you that the reality is that it doesn’t make a difference when you find it. So you scratch your head and say, ‘Well, why do you do mammography, then?”

The answer is that mammograms do not have to be infallible to save lives. A modest estimate of mammography’s benefit is that it reduces the risk of dying from breast cancer by about ten per cent—which works out, for the average woman in her fifties, to be about three extra days of life, or, to put it another way, a health benefit on a par with wearing a helmet on a ten-hour bicycle trip. That is not a trivial benefit. Multiplied across the millions of adult women in the United States, it amounts to thousands of lives saved every year, and, in combination with a medical regimen that includes radiation, surgery, and new and promising drugs, it has helped brighten the prognosis for women with breast cancer. Mammography isn’t as a good as we’d like it to be. But we are still better off than we would be without it.

“There is increasingly an understanding among those of us who do this a lot that our efforts to sell mammography may have been over-vigorous,” Dershaw said, “and that although we didn’t intend to, the perception may have been that mammography accomplishes even more than it does.” He was looking, as he spoke, at the mammogram of the woman whose tumor would have been invisible had it been a few centimetres to the right. Did looking at an X-ray like that make him nervous? Dershaw shook his head. “You have to respect the limitations of the technology,” he said. “My job with the mammogram isn’t to find what I can’t find with a mammogram. It’s to find what I can find with a mammogram. If I’m not going to accept that, then I shouldn’t be reading mammograms.”

The article deals with more than mammography. It also addresses the difficulties in finding enemy weapons.

Read the entire article. You will better understand the signal detection dilemma.

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Gladwell on mammography
The New Yorker’s Malcolm Gladwell is out with an article on the very imperfect art/science of mammogram-reading. MedRants focuses in on the litigation implications; there’s an enormous leeway to second-guess the readings of radiologists and others wh…

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