A new study suggests that routine mammograms, long pitched to women as lifesaving tests, may also be causing substantial harm.

The study estimates that as many as 1 in 4 cancers detected over a decade by routine mammograms are cancers that won’t grow or spread, cause symptoms, or lead to death.

Instead, these “overdiagnosed” cancers are treated with surgery, powerful drugs, and radiation, all when the cancer wouldn’t have made a woman sick in the first place.

“We are curing people who don’t need to be cured,” says Otis W. Brawley, MD, chief medical officer of the American Cancer Society.

Brawley reviewed the study for WebMD but was not involved in the research.

He says doctors have known for some time that not all breast cancers are dangerous. But he says it’s been difficult to figure out how many breast cancers are being treated when they probably don’t need to be.

In part, that’s because there aren’t any tests that can distinguish between cancers that are harmful and those that may not otherwise affect a person's health. And when doctors find cancer, they treat it, of course. Not doing so would be unethical. So there’s never been a population of women who were diagnosed with cancer but then left untreated so doctors could see what would naturally happen to those cancers over time.

“This is one of the best studies ever designed to try to figure that out,” Brawley says.

A Visual Guide to Breast Cancer
Estimating Overdiagnosis in Breast Cancer

The study included nearly 40,000 women diagnosed with invasive breast cancers in Norway before and after 1996, when the country began offering regular mammograms to residents in some regions.

By comparing the cancers diagnosed in women who were screened vs. those who were not, researchers estimated that 15% to 25% of breast cancers found by routine mammograms were overdiagnosed.

Put another way, researchers say that for every 2,500 women aged 50 to 69 offered mammograms for 10 years, just one additional life would be saved, 20 women would be diagnosed with cancers that needed treatment, and six to 10 women would be overdiagnosed.

“What women have been told before is, ‘You look for cancer and we’ll save you,’” says researcher Mette Kalager, MD, a breast cancer surgeon and an epidemiologist with the Harvard School of Public Health. “That’s not the whole story. You will be saved without screening as well.”

“I think we have to inform women about the downside or harm of mammography screening,” she says.

The study is published in the Annals of Internal Medicine.

The number of women who are overdiagnosed in the U.S. is likely to be higher, experts say, because women in this country often start getting mammograms in their 40s, rather than in their 50s as women in Norway do, and Americans are generally screened more often, every year instead of every two years.