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 wit h breast cancer detected at screening and the previous round attendance (in cidence cancer). Methods-Screening mammograms, obtained before detection of the incidence ca ncers, were reviewed first mixed with other screening images (ratio 1:8) an d then non-mixed. Reviewers from the screening unit responsible for the mam mograms as well as reviewers from other units interpreted all images by bot h single and double reading. Results-The proportion detected on retrospective review varied between 5% a nd 50% depending on the review method used and the number of reviewers incl uded to classify a case as truly identified. Generally more cancers were de tected when non-mixed samples of mammograms were reviewed than when mixed s amples were reviewed (mean increase 23%) and when interpreted by double rea ding compared with single reading (mean increase 14%). Conclusions-In an experimental retrospective set up, fewer incidence cancer s were identified in mixed than in nonmixed review. Generally more incidenc e cancers were identified on review (22%) than previously reported for inte rval cancers (14%), probably reflecting differences in tumour biology and g rowth. How many women with potentially visible incidence cancers would have benefited from earlier tumour detection still needs to be evaluated.