Sunday, December 6, 2009

False Positives

Last month, the Preventive Services Task Force of the United States Department of Health and Human Services issued a report recommending a significant reduction in mammographic screening for breast cancer. This recommendation was received with what can only be described as hysteria. While some of the protest was a cynical ploy by those who oppose Obama's proposed health insurance reform and some of it (very little) was a serious disagreement over the Task Force's calculation of the net benefits of mass breast cancer screenings, most criticism of the report centered on the idea that it is immoral not to screen millions of healthy women if, by not screening, there is one life lost that could have been saved. "So what," these critics argued, "if the chances of saving a life of a woman between ages 40 and 50 by screening is 1 in 1900, we should do everything we can to save that life." The problem with this sort of thinking is that it completely ignores or minimizes the costs of routine mammography for the 1899 women who undergo the test but whose lives are not saved by it.

In most of those cases, the x-ray shows no sign of cancer. These women are reassured, but they do pay a price in radiation, money, inconvenience, and discomfort. They receive no tangible benefit from the test; indeed, the radiation exposure slightly increases their chances of developing breast cancer in the future. Most women who are now being screened probably think that the psychological comfort of a "negative" mammogram is worth these risks, but this "benefit" is purchased at a cost, much of it paid by other women.

In a significant number of cases, the mammogram shows a suspicious breast lesion which is biopsied and diagnosed as benign. These are the classic "false positives." These women typically receive more radiation in the form of additional imaging and suffer additional expense and pain. Most importantly, however, they understandably suffer a good deal of anxiety during the time between the "positive" mammogram and the post-biopsy "all clear" signal. Mammography advocates typically dismiss this psychological cost as trivial, but they can't have it both ways. If the emotional reassurance provided by a true negative mammogram is a good reason to do them, then the psychic pain caused by a false positive is a reason not to do them.

The group of women most hurt by being screened are those in whom the mammogram leads to the discovery of a slow-growing cancer, one which, if undiscovered, would never have cause illness or which, at least, would not have metastasized before it reached a size at which it would have been discovered without x-rays. Some of these women will be treated unnecessarily, typically undergoing surgery, chemotherapy, and/or radiation. All of these treatments are dangerous and unpleasant. In addition, these patients are deprived by the screening test of the blissful ignorance which allows them to live their lives innocent of the existence of mutated cells within them.

The same considerations apply to those women in whom mammography finds aggressive, incurable tumors. They will die with or without mammography, but without it they will have more time in which to live normal lives,free of the knowledge they they have a life-threatening disease.

While it is impossible to tell in advance which woman is in which of these groups, it is possible for scientists to make informed judgments about the relative sizes of these groups and to consider these facts in making a cost-benefit analysis of the benefit of screening mammography for different groups. That is apparently what the Task Force tried to do. Others can contest their measurements of costs versus benefits in good faith, but many of those denouncing their recommendations do so without acknowledging that there are, in fact, serious costs to mass screening of healthy people.

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