New research from the Harvard School of Public Health suggests that routine mammography screenings may lead to a significant amount of overdiagnosis of invasive breast cancer. Generally viewed as an important tool in detecting breast cancer, mammography screening was found to overdiagnose between 15 to 25 percent of breast cancer cases.

The research was based on data gathered from nearly 40,000 women with invasive breast cancer in Norway. Researchers compared the number of breast cancer cases reported in women who had been offered screening with those who were not offered screening.

“The implications are that you really have to think through whether you want to go to the screening or not,” said Mette Kalager, the study’s lead author and a visiting scientist at the School of Public Health.

Kalager, who comes to Harvard from her post as a researcher at the Telemark Hospital in Norway, said that the positive effects of mammography screenings may also be overestimated, as many women are unaware that the harms may actually outweigh the benefits.

Based off the study, Kalager estimated that for every 2,500 women, only one death from breast cancer will be prevented whereas six to ten women will be overdiagnosed and treated with surgery, radiation therapy, and possibly chemotherapy without any benefit.

“There clearly is a benefit for a certain age group [women aged 50-69], but there are also certain harms which I think is true of any type of screening process,” said Rulla M. Tamimi, senior author of the study and an associate professor at the Harvard Medical School. “We haven’t done a good job with educating women about what the harms are.”

Tamimi, a School of Public Health graduate, said that though mammography has existed for a number of years, there have not been many improvements in terms of breast cancer screening.

“Ultimately, better tools are needed to reliably identify which breast cancer will be fatal without treatment and which can be safely observed over time without intervention,” read an editorial published alongside the study.

The study and editorial—which was written by Suzanne W. Fletcher, professor at the Medical School, and Joann G. Elmore, a professor of Epidemiology at the University of Washington School of Medicine—were published in the April 3 issue of the Annals of Internal Medicine.

In the editorial, Fletcher and Elmore noted that two factors may suggest that estimates of overdiagnosis from Norway may not necessarily generalize to the United States. They also noted that U.S. radiologists are more likely than their European counterparts to report abnormalities found on mammograms and that U.S. women generally start annual mammography screening at age 40, while Norwegian women start biannual screening at age 50.

“We have an ethical responsibility to alert women to this phenomenon,” Fletcher and Elmore said.

—Staff writer Cynthia W. Shih can be reached at cshih@college.harvard.edu.