But that means that widespread screening for prostate and breast cancer has resulted in huge numbers of patients suffering the side effects of unnecessary medicine. A study published last October in BMJ (formerly the British Medical Journal) compared P.S.A. testing rates in the region around Seattle and in Connecticut between 1987 and 1997. Men in Seattle were 5.39 times as likely to get a P.S.A. test as men in Connecticut and 5 times as likely to undergo surgical removal of the prostate. Yet there was no significant difference in the death rate from prostate cancer between the two regions. Prostate removal is a delicate surgical procedure that renders many patients impotent, incontinent or both. In a study of patients at Harvard University hospitals, where you might expect the best outcomes, the majority of men were impotent a year after their surgery.
It is less clear that mammograms are not helpful. But there is still little evidence that they are. Seven different clinical trials conducted since the 1960's, involving well over 100,000 women in the United States, Canada and Europe, have tried to determine the benefits of mammography. The most recent analyses of those trials have found critical flaws in their design, leaving an increasing number of cancer experts to conclude that there is no clear evidence to suggest that mammography is saving lives.
The same cycle is coming into play for detecting kidney cancer. More and more tumors are being spotted on the kidneys by the full-body CT scan, a new technology that is being heavily marketed by large academic medical centers and hundreds of freestanding clinics around the country. But as with breast or prostate tumors, many of these tumors may turn out to be, on closer inspection, simply benign lumps that appeared to be cancer at first. Unfortunately, closer inspection usually means a surgical procedure. Baker cites a study that indicates that 85 percent of kidney spots that show up on a scan will turn out to be false-positives. He also explains that a 40-to-60-year-old man who undergoes a full-body scan has a 1 in 20,000 chance of having malignant kidney cancer at the time. When doctors start looking for disease in healthy people, Baker says, ''the odds are in favor of making mischief rather than a cure.''
Take the DEXA, or dual-energy X-ray absorptiometry scan, which is one of the screening tests that when used properly can help prevent disease. Doctors routinely use the DEXA on women 65 and older to diagnose osteoporosis, or thinning bones, one of several risk factors for fractures in the elderly. When the DEXA scan first came into use a decade ago, a panel of international experts convened to decide the cutoff point for diagnosing osteoporosis, drawing the line at bone density that was a certain level below that of the average 25-year-old woman. The panel's standard has helped doctors to identify women at highest risk for fractures and treat them with drugs like estrogen and Fosamax, which slow bone loss.
Doctors get into trouble when they try to stem osteoporosis in younger and healthier women. At the same time that the expert panel set the standards for osteoporosis, it also invented a new term, ''osteopenia,'' for women who had suffered a little bone loss but not enough to put them in the category of osteoporosis. The panel hoped that women would be screened once soon after menopause and that a diagnosis of osteopenia would serve as a warning sign for them to stop smoking and step up their efforts to get enough calcium and exercise, primary prevention that can slow the loss of bone. Instead, a diagnosis of osteopenia has led many doctors to prescribe Fosamax and other drugs immediately rather than waiting for osteoporosis.
''Doctors and their patients don't even know what osteopenia means,'' says Susan Love, a breast-cancer surgeon and frequent critic of many postmenopausal treatments. There is no evidence to suggest that earlier treatment will be any more effective in arresting fractures than waiting for true osteoporosis. In fact, some animal studies now under way are suggesting quite the opposite, that long-term use of the drugs may cause bones to become more brittle, Love says, ''and we are prescribing these drugs to women who might or might not suffer a fracture 15 or 20 years down the road.''
But the most troubling possibility involves procedures performed in the name of prevention, whose potential risks far outweigh any possible benefits. The carotid endarterectomy, for example, is a kind of Roto-Rooter surgery for the arteries that supply blood to the brain. In use since the 1950's, carotid endarterectomy is performed on about 150,000 Americans a year to avert strokes that are caused when a blood clot forms in one of the two carotid arteries in the neck and blocks the flow of blood when it reaches a narrower arterial branch in the brain. Carotid endarterectomy is the most common vascular surgery performed in the United States, and according to the National Stroke Association's Web site, the procedure reduces the risk of stroke ''by as much as 55 percent.''