CHICAGO – An ambitious study by 10 of the world’s top breast cancer statisticians has confirmed that screening mammograms can help prevent deaths from the disease. But the benefit may be smaller than women have been led to believe.

The percentage of American women who die of breast cancer has been declining steadily since 1990, prompting advocates of screening to proclaim that earlier detection through special X-rays called mammograms is saving lives. But skeptics have argued that the credit belongs to improved post-surgical treatments that have come into widespread use – mainly combination chemotherapy and tamoxifen, a hormonal medication.

The statisticians were convened by the National Cancer Institute to try to determine how much of the improvement is due to mammography and how much to treatment.

In today’s issue of the New England Journal of Medicine, the researchers who make up the Cancer Intervention and Surveillance Modeling Network, or Cisnet, reported that both screening and treatment have helped reduce the death rate from breast cancer in the United States. On average, they estimated that screening is responsible for nearly half of the decline.

“This provides one piece of additional evidence that mammography saves lives,” said Eric Feuer, chief of statistical research at the National Cancer Institute.

Using seven different mathematical models, the researchers estimated that screening, by itself, reduced deaths by somewhere between 7 and 23 percent, or an average of 15 percent. The average benefit from post-operative drug treatment was 19 percent, the researchers estimated.

Those benefits may be lower than experts had expected, said Cisnet member Donald Berry, of M.D. Anderson Cancer Center in Houston.

Clinical trials have found mammography can reduce breast cancer deaths by more than 20 percent, and some estimates top 60 percent. Likewise, studies on chemotherapy and tamoxifen have indicated that together they can reduce deaths by about one-third.

But clinical trials often overestimate the benefit of a medical intervention for the general population, Feuer said. In the real world, not every woman gets mammograms, and not every breast cancer patient obtains the recommended treatments. The Cisnet project, he said, tried to factor in how many women were actually utilizing mammography and drugs in any given year.

Dr. Leonard Berlin, chair of radiology at Rush North Shore Medical Center in Skokie, Ill., said a 15 percent mortality benefit “is not insignificant – but it’s far from the sensational figures that have been bandied about in the past. We’ve oversold (the benefit of) mammography.”

The bottom line? Berlin said women “should avail themselves of mammography and available therapies.” But how often women should get mammograms remains an open question.

“No one has shown that mortality is increased by doing it every two years, rather than every year,” he said.

Dr. Leslie Laufman, a breast cancer researcher in Columbus, Ohio, said the Cisnet analysis probably underestimated the value of drugs and overestimated the value of screening for today’s women.

“The death rate in 2000 reflects treatments that were given 10 to 15 years ago,” she said. “Since then we’ve made a lot of advances in the area of new treatments … but no great strides have been made in screening technologies.”

As treatments improve, Laufman said, screening becomes less effective. In testicular cancer, for instance, “treatments have now become so effective that we don’t need to screen.”

The journal article did not go into the costs of mammography screening. But according to a 2001 analysis in The Lancet, for every 10,000 mammograms given to apparently healthy women, at least 500 will be suspicious. Only 27 of the suspicious tests will turn out to be cancer, but doctors will have to do follow-up tests for all 500. So they will order 647 diagnostic mammograms, 358 ultrasound exams, 209 surgical biopsies, 104 needle-aspiration biopsies, and 500 additional office visits.

The vast majority of the cost of those procedures – as well as the associated anxiety – is borne by women who do not have cancer, experts note.

Berry said women thinking about going in for a screening mammogram should be aware of the high rate of false alarms and other down sides of screening, including the possibility of being diagnosed and treated for a cancer that would never have become life-threatening. Those “quite certain” risks, he said, should be weighed against the “probable” benefit of reducing their chances of dying of breast cancer.

The American Cancer Society estimates that more than 250,000 U.S. women will be diagnosed with breast cancer this year and nearly 40,000 will die of the disease.

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