Overdiagnosis in cancer screening: the need for a standardized denominator.

It is widely accepted that overdiagnosis is a major harm of screening, but its extent is still topic of controversy. This is partly the result of incomparable overdiagnosis estimates in scientific literature, as a variety of denominators are used to calculate the percentage of overdiagnosis in cancer screening. We propose to use the following denominator to calculate the percentage of overdiagnosis: 'all cancers detected during the screening period, both interval and screen-detected, in participants of a screening programme'. This denominator is more appropriate than existing denominators because it presents overdiagnosis as a real percentage, is unaffected by attendance percentages, is applicable to all observational study designs, and can be easily recalculated to absolute numbers. This denominator can be widely applied and increases comparability between overdiagnosis estimates, which is needed to correctly present the balance between the benefits and harms of screening.