Classifying interval cancers.
No definitive way of classifying interval breast cancers (cancers presenting between screening rounds) has been determined, yet their number and classification forms one of the quality assurance (QA) standards for the National Health Service Breast Screening Programme (NHSBSP). This study was undertaken to identify how different screening centres undertake this process, and to compare the classifications obtained when a test set of mammograms was reviewed using three different methods. A questionnaire was sent to the 17 Regional UK breast screening QA centres. Twelve (80%) of the 15 centres completing the questionnaire had a formal method for reviewing interval cancers. Five of these (33%) attempted to simulate screening by mixing the interval cancers with other screening films. In 11 (73%) centres, a group (size range 3-14) of radiologists was involved. In a simulated film viewing exercise we assessed whether different methods of classification would alter the number of interval cancers classified as false negative (where an abnormality suspicious of malignancy can be identified on review of the original screening films). Six radiologists reviewed a set of 50 interval cancers by three different methods: independent reading of the interval cancers mixed with screening mammograms; independent reading of the interval cancers on their own; and finally a consensus opinion of the interval cancers alone using the films taken at diagnosis. No discussion or review of these cases had taken place prior to the study. The number of interval cancers classified as false negative increased by 10% when they were reviewed in isolation compared to review among 'normal' screening films. The false negative rate varied widely (4% to 56% P < 0.01) depending on the criteria required to fulfil a false negative interval cancer classification--whether only one, a majority, or all of the radiologists were required to see the abnormality on the original screening films. In order for inter-unit and regional comparisons to be useful, a standard review method of mixing the interval cancer films with normal screening films and using a consensus opinion by a minimum of three external reviewers is suggested.