A panel in Britain confirms the life-saving nature of breast cancer screening but says it comes at a cost.

The review finds that for every life saved, roughly three other women are overdiagnosed, meaning they are unnecessarily treated for a cancer that would never have threatened their lives.

The expert panel was commissioned by Cancer Research U.K. and Britain's department of health and analyzed evidence from 11 trials in Canada, Sweden, the U.K. and the U.S.

Scientists say the British program saves about 1,300 women every year from dying of breast cancer while about 4,000 women are overdiagnosed. This is different from another screening problem: false alarms. The study did not look at the false alarm rate.

Each year, more than 300,000 women aged 50 to 52 are offered a mammogram through the British program.

The review is published online today in the Lancet journal.

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