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Some doctors and public-health experts have stirred controversy in recent years by arguing that aggressive breast-cancer screening does more harm than good. Many health professionals disagree, but perhaps the most compelling critics of the less-is-more philosophy are the many breast cancer survivors who claim that a routine mammogram saved their lives.

These survivor anecdotes — which often turn up in the media — may be causing the public to overestimate the true value of mammograms, a new study suggests. In an analysis published this week in the Archives of Internal Medicine, researchers at Dartmouth estimate that only about 1 in 8 women whose breast cancer was identified during a routine mammogram actually owe their lives to the screening.

This estimate doesn’t mean that women shouldn’t bother to get mammograms, but it does suggest that overly frequent screening can result in overtreatment, the authors say. Many tumors identified during routine mammograms would have grown very slowly (if at all) and would never have threatened a woman’s life, says Dr. H. Gilbert Welch, one of the coauthors of the analysis.

“A certain fraction of women are being treated for a disease that was never going to bother them,” says Dr. Welch, a professor of medicine at the Dartmouth Institute for Health Policy and Clinical Practice, in Hanover, N.H.

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The U.S. Preventive Services Task Force (USPSTF), a government advisory panel that issues guidelines on cancer screening, recommends that women start having mammograms every other year when they turn 50, while women who have risk factors for breast cancer should talk with their doctor about starting screening earlier. (Prior to 2009, the panel recommended annual mammograms for all women beginning at age 40.) A large study published last week found that biennial screening resulted in fewer false alarms and unnecessary biopsies than annual screening.

The results of the new analysis should reassure women with average breast-cancer risk that it’s safe to wait until age 50 — or later — to begin mammograms, Dr. Welch says. However, he adds, women who would prefer to undergo screening earlier or more frequently should do so if it will make them more comfortable. “It’s generally a really close call, and how patients feel really matters,” he says.

To investigate the value of mammography in terms of lives saved, Dr. Welch and his colleague, Brittney Frankel, used breast-cancer data from the National Cancer Institute and the Centers for Disease Control and Prevention to calculate a woman’s likelihood of developing breast cancer over a 10-year period, the likelihood that the cancer would have been detected by mammography, and a woman’s risk of dying from breast cancer over a 20-year period.

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Using the relevant data for a hypothetical 50-year-old woman who received a breast cancer diagnosis following a routine mammogram, the researchers estimate that the probability that the mammogram will be responsible for saving her life is about 13%. And even that estimate may be high, they say: mammograms prevent fewer deaths today than they did 20 or 30 years ago (thanks to better treatment that has made early detection less crucial), so the actual probability could be as low as 3%.

“Women need to understand their trade-offs here,” Dr. Welch says. “The reason to be screened isn’t because you’ve heard a lot of survivor stories. Some of those women have not benefited [from screening].”

Dr. Timothy Wilt, a researcher at the Minneapolis Veterans Affairs Center who specializes in chronic disease outcomes, says that breast-cancer diagnosis and treatment can even be harmful in some cases. Radiation treatment, for instance, can cause disfigurement, accelerate blockage of arteries in the heart, and increase the risk of secondary cancers, he says.

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“Looking harder and treating more doesn’t always help people live longer and live better,” says Dr. Wilt, who coauthored an editorial accompanying the study and is a member of the USPSTF. The push toward less screening “is not about rationing care,” he adds. “It’s about making good health care decisions to help patients live better and live longer.”

Dr. Laura Esserman, the director of the breast care center at the University of California, San Francisco, says that doctors and women should shift their focus away from breast-cancer screening and toward prevention. For instance, Esserman says, women can reduce their risk of breast cancer by maintaining a healthy weight, exercising regularly, and minimizing their alcohol consumption.

“Mammography has some value,” she says, “but I think it’s pretty clear that the public has overvalued it and a lot of people who push it have overvalued it.”