For years, women have been told that regular mammograms can help reduce their risk of dying from breast cancer by catching tumors at their earliest, most treatable stages.
But a Danish study is the latest research to challenge that assumption. Researchers followed thousands of women in Denmark over more than a decade and found that perhaps one-third of the abnormalities detected by mammograms may never cause health problems.
"Breast screening can have some very substantial harms," says Karsten Jorgensen, deputy director of the Nordic Cochrane Centre in Denmark, who led the study. "And the most important one of those is the overdiagnosis of breast cancers that would never have developed into something life-threatening."
The study adds to growing evidence that breast cancer screening can lead to unneeded treatment.
"There's a tendency in the United States to think that screening is better than it actually is," says Otis Brawley, chief medical officer at the American Cancer Society. Brawley wrote an editorial accompanying the study.
"It's important that we learn the limitations of screening so that we can apply that tool as best we possibly can to save as many lives as possible," Brawley says.
The problem, Brawley says, is that doctors can't yet tell which tumors really need to be treated and which they might be able to just monitor. For now, he says, it's important all women continue to follow mammography guidelines and get treated if they are diagnosed.
"One of my nightmares is people will read this paper," Brawley says, and "elect to not get treated."
The cancer society's guidelines say mammograms should be an option starting at age 40, and that annual mammograms should start at age 45. At age 55, women can cut back to every two years, according to the guidelines. Another group, the U.S. Preventive Services Task Force, advises that regular biennial mammograms start at age 50.
But others argue the paper should encourage women to think twice about whether they need a mammogram.
"Women should understand all of these issues and make their own decision if they want to have a mammogram," says Fran Visco of the National Breast Cancer Coalition, an advocacy group. "They should really think very carefully before getting a mammogram."