In Britain, mammograms are usually offered to women aged 50 to 70 every three years as part of the state-funded breast-cancer screening program.
Scientists said the British program saves about 1,300 women every year from dying of breast cancer while about 4,000 women are overdiagnosed, meaning women were treated for cancers that grow too slowly to ever put their lives at risk. This is different from another screening problem: false alarms, which occur when suspicious mammograms lead to biopsies and follow-up tests to rule out cancers that were not present. The study did not look at the false-alarm rate.
"It's clear that screening saves lives," said Harpal Kumar, chief executive of Cancer Research U.K. "But some cancers will be treated that would never have caused any harm and unfortunately, we can't yet tell which cancers are harmful and which are not."
Each year, more than 300,000 women aged 50 to 52 are offered a mammogram through the British program. During the next 20 years of screening every three years, 1 percent of them will get unnecessary treatment such as chemotherapy, surgery or radiation for a breast cancer that wouldn't ever be dangerous. The review was published online Tuesday in the Lancet journal.
Some critics said the review was a step in the right direction.
"Cancer charities and public health authorities have been misleading women for the past two decades by giving too rosy a picture of the benefits," said Karsten Jorgensen, a researcher at the Nordic Cochrane Centre in Copenhagen who has previously published papers on overdiagnosis.
"It's important they have at least acknowledged screening causes substantial harms," he said, adding that countries should now re-evaluate their own breast-cancer programs.
In the U.S., a government-appointed task force of experts recommends women at average risk of cancer get mammograms every two years starting at age 50. But the American Cancer Society and other groups advise women to get annual mammograms starting at age 40.
In recent years, the British breast-screening program has been slammed for focusing on the benefits of mammograms and downplaying the risks.