Two models for radiological reviewing of interval cancers

Objectives-To compare two different review methods of examining how many of our interval cancers could be regarded as missed cases (overlooked and mis interpreted owing to observer's error). Setting-A mass screening programme in Stockholm 1989-91, performed at five independent screening units. 107 846 women attended for screening (70.6% of those invited), and 207 women with interval breast cancers were identified . Interval cancers from two of the units, 104 cases, are reviewed in this s tudy. Methods-Screening examinations preceding the interval cancer diagnoses were reviewed both mixed with other screening images in a ratio 1:8 and non-mix ed. Both internal reviewers (from the two units responsible for the screeni ng mammograms) and external reviewers (from the other units) took part in t he study. Results-The proportion regarded as missed cases varied between 7% and 34%, depending on what review method was used, and on the number of reviewers in cluded to identify a case as missed. Mixed reviewing reduced the number ide ntified as missed cases by 50% compared with non-mixed reviewing. Whether t he reviewer was internal or external made no difference to the results. Conclusions-Comparing the rate of missed cases from different studies may b e misleading unless the same review method is used. No difference in detect ion rate could be shown whether the radiologist reviewed images from his/he r own screening unit or not. Most of our interval cancers were not regarded as missed cases by either of the two methods.