Radiological review of incidence breast cancers

Objectives-To examine the rate of incidence cancers detectable on review of previous screening mammograms using two reviewing methods. To compare the results with a previous study of interval cancers using the same reviewing methods. Setting-Almost 50 000 women are regularly invited for service screening at Stockholm Slider Hospital. From 1989 to 1993, 119 women were identified with breast cancer detected at screening and the previous round attendance (incidence cancer). Methods-Screening mammograms, obtained before detection of the incidence cancers, were reviewed first mixed with other screening images (ratio 1:8) and then non-mixed. Reviewers from the screening unit responsible for the mammograms as well as reviewers from other units interpreted all images by both single and double reading. Results-The proportion detected on retrospective review varied between 5% and 50% depending on the review method used and the number of reviewers included to classify a case as truly identified. Generally more cancers were detected when non-mixed samples of mammograms were reviewed than when mixed samples were reviewed (mean increase 23%) and when interpreted by double reading compared with single reading (mean increase 14%). Conclusions-In an experimental retrospective set up, fewer incidence cancers were identified in mixed than in nonmixed review. Generally more incidence cancers were identified on review (22%) than previously reported for interval cancers (14%), probably reflecting differences in tumour biology and growth. How many women with potentially visible incidence cancers would have benefited from earlier tumour detection still needs to be evaluated.