Efficiency of cancer detection during routine repeat (incident) mammographic screening: two versus one view mammography

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Abstract
Objective To examine the influence of one view versus two view mammography on cancer detection and recall for further investigation of women attending incident (subsequent) screening. Setting All cancers (invasive and in situ) detected as incident cases during the second screening round (January 1994 to January 1997) at the South West London Breast Screening Service were used. This service uses two view mammography and double reading, with arbitration by a third or further readers for all screens. Methods Mammograms of cases were mixed with those of controls in a 1:2 ratio in nine test sets; each set was read independently by three film readers. Fourteen readers, each reading from one to four test sets, took part in the study. Initially, the oblique view only was read, then the craniocaudal view was read in addition. Previous films were available to the readers. Data on abnormalities noted on the films and probability of recall were recorded and analysed. Results 10 of the 14 readers obtained increased sensitivity using two views (p=0.04), for two readers there was no difference, and for two readers sensitivity decreased. The mean sensitivity increase was 6.1% (p=0.01). The overall increase in sensitivity from all readings of invasive cancers was 8.9%, with no increase seen for in situ cancers. 11 of the 14 readers obtained an increase in specificity (p=0.006), two readers showed no increase, and the specificity for one reader was decreased. The mean increase in specificity using two views was 5.7% (p=0.006). Conclusion This study showed an increase of 8.9% in sensitivity for the detection of invasive cancers when two views are used at incident screening, with a ratio of two control mammograms for every case. This is equivalent to a sample from population screening with a cancer detection rate of 333 per 1000. Such a study is considered to be likely to underestimate the benefit of two views in screening under non-test conditions where the cancer detection rate is of the order of five per 1000. The use of two view mammography for the detection of in situ cancers showed no increased benefit. A randomised controlled trial is needed to obtain a reliable estimate of the increase in cancer detection rate for incident screening in normal populations.

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