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 UK. ‘‘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.

--more--"