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Objective: To measure the effect of the insertion of lessdifficult malignant cases on subsequent breast cancer detection by breast imaging radiologists. Methods: The research comprises two studies. Study 1: 8 radiologists read 2 sets of images each consisting of 40 mammographic cases. Set A contained four abnormal cases, and Set B contained six abnormal cases, including two priming cases (less difficult malignancies) placed at intervals of three and five subsequent cases before a subtle cancer. Study 2: 16 radiologists read a third condition of the same cases, known as Set C, containing six abnormal cases and two priming cases immediately preceding the subtle cancer cases. The readers were asked to localize malignancies and give confidence ratings on decisions. Results: Although not significant, a decrease in performance was observed in Set B compared with in Set A.There was a significant increase in the receiver operating characteristic (ROC) area under the curve (z 5 22.532; p 5 0.0114) and location sensitivity (z 5 22.128; p 5 0.0333) between the first and second halves of Set A and a marginal improvement in jackknife free-response ROC figure of merit (z 5 21.89; p 5 0.0587) between the first and second halves of Set B. In Study 2, Set C yielded no significant differences between the two halves of the study. Conclusion: Overall findings show no evidence that priming with lower difficulty malignant cases affects the detection of higher difficulty cancers; however, performance may decrease with priming. Advances in knowledge: This research suggests that inserting additional malignant cases in screening mammography sets as an audit tool may potentially lead to a decrease in performance of experienced breast radiologists.The effectiveness of screening mammography in detecting breast cancer relies heavily on accurate observer performance; 1 however, miss rates of 10-30% have been reported. 2,3 The challenge for radiologists is to gain comprehensive experience in the appearance of abnormality and keep their recall rate within acceptable limits given the low incidence of the disease. [4][5][6] It has been proposed that the insertion of "test malignant cases" into a "real life" clinical screening mammography set may increase cancer detection rates by artificially increasing the prevalence.7 However, by specifically increasing the prevalence of abnormality, there may be inadvertent effects on the detection of subsequent "real" cases.Previous work on the impact on increasing abnormality prevalence on radiologic performance has provided diverse conclusions, with one study suggesting that varying prevalence was unlikely to alter the accuracy of the radiologists, 8 another suggesting increased diagnostic efficacy with increasing prevalence 9 and a further arguing no significant effect.10 A study focusing on prevalence expectation, whilst showing no significant impact on reporting accuracy, did show that visual search, in terms of interpretation time and the number of visual fixations per image, was significantly changed w...

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