Diagnosis rates for early-stage tumors and pre-invasive malignancies known as ductal carcinoma in situ (DCIS) have surged in recent years as more women get screening mammograms.

The current study offers fresh evidence linking routine screening to over-diagnosis of non-aggressive tumors because it compares outcomes over a single time period in two regions of Denmark – one that offered biennial mammography for women aged 50 to 69 and one that didn’t.

“Overdiagnosis means that healthy women get unnecessary breast cancer diagnoses,” said lead study author Dr. Karsten Juhl Jorgensen, of the Nordic Cochrane Center and Rigshospitalet in Copenhagen.

“A breast cancer diagnosis is a life-changing event for the woman and her family, with substantial implications for their quality of life,” Jorgensen added by email. “It also leads to overtreatment with surgery, radiotherapy and sometimes chemotherapy – we know these treatments have serious, sometimes lethal, consequences.”

The goal of mammograms is to detect tumors before they can be felt in a physical breast exam, catching cancer sooner when it’s easier to treat. Ideally, this should mean fewer women are diagnosed when tumors are bigger, rapidly growing, and harder to attack.

To see how well mammograms achieve this ideal, researchers examined data on more than 1.4 million Danish women aged 35 to 84 from 1980 to 2010.

Organized breast cancer screening programs began in Copenhagen in 1991, Funen in 1993 and Frederiksberg in 1994 – altogether covering about 20 percent of the population. Other regions gradually introduced screening starting in 2007.

Screening wasn’t associated with a lower incidence of advanced tumors, researchers report in the Annals of Internal Medicine.

This raises doubts about whether mammography reduces breast cancer deaths, the researchers conclude.

In addition, researchers estimate that as many as one in three breast tumors diagnosed in women who got mammograms would never have caused a noticeable health problem or led to death – and are therefore examples of overdiagnosis.

One limitation of the study is that because mammography has improved since this study began in 1980, women might get different results today, the authors note.

While the study adds compelling evidence that routine mammograms carry a risk of overdiagnosis, that doesn’t mean it’s time for screening guidelines to change, Dr. Otis Brawley, chief medical officer of the American Cancer Society, said in a phone interview.

Under current U.S. guidelines, mammograms should be optional for women aged 40 to 44, then done annually from 45 to 54. Women 55 and older should switch to mammograms every two years, continuing as long as they are in good health with a life expectancy of at least a decade.

“This isn't about saying don't get a mammogram, this is about using it most effectively to save the most lives,” said Brawley, who wrote an editorial accompanying the study.

Beyond screening, there are other preventive measures women shouldn’t overlook like eating well, getting plenty of exercise, and maintaining a healthy weight.

In the future, it may be possible to use screening results to pinpoint women who might be candidates for “watchful waiting,” the close follow-up without treatment that’s common for many men with early-stage prostate cancer, Brawley said.

“We aren't ready for that because we don't know what watchful waiting looks like for breast cancer yet,” Brawley said.

So for now, that means accepting that overdiagnosis and screening go hand in hand, along with the potential for some women to get unnecessary treatment.

“If it's small and slow-growing, I think all breast cancers ought to be treated,” Brawley added.