Statistical issues of quality control in organised breast cancer screening

European guidelines for breast-cancer screening recommend an integrated approach of mammography screening with subsequent assessment and biopsy, if required, in one screening unit under permanent quality control, for which target values are released. Although the calculation of the respective rates (e.g. for participation, assessment, biopsy, or cancer detection) appears trivial, the statistical assessment of their compatibility with the target values is less obvious. This is especially true if subjects with a positive diagnostic result leave the screening-assessment chain prematurely, and information about further diagnostic results outside the organised screening is lacking.

Statistical models for the basic situation, in which complete information about the screening and assessment outcome is available, as well as for when information is incomplete, are presented. The statistical methods for obtaining the confidence limits, statistical tests and sample sizes needed to obtain a desired power of tests for the process parameters of interest are also given.

The sample-size calculations indicate that large numbers of enrolled subjects are required to obtain reasonably narrow confidence limits, and that incomplete information about the outcome of diagnostic procedures among screening positives considerably worsens the feasibility of quality control.

Although the methodology is specified for breast-cancer screening, it should be adaptable easily to other screening issues.