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A retrospective analysis of all incident cancers with their previous screening round mammograms diagnosed between April 2006 and March 2008 at the North London Breast Screening unit was performed.
Two hundred and forty cancers were reviewed. In 187 cases (77.9%) the previous round mammograms were normal (group A), in 53 cases (22.1%) an abnormality was detected (group B). Of these, five (9.4%) were classified as normal/benign, 40 (75.5%) were uncertain and eight (15.1%) were suspicious. There was no significant difference in the size of the lesions between the two groups; there was, however, a significant increase in size in the lesions in group B on the subsequent mammograms (P <0.0001). Of the lesions in group B, 25 (47%) of the cases had microcalcifications only on the previous mammograms; this is higher than previously published data of 27% and 17% [1,2] with all of these cases being subsequently diagnosed as either DCIS or IDC. Group A had less non-invasive (53, 29%) and grade 3 tumours (26, 20%) compared with group B (non-invasive = 18, 34%; grade 3 = 9, 29%) and more grade 1 tumours (51, 39% vs. 8, 26%) with a similar amount of grade 2 tumours between the groups (41% and 45%).
In 22% of incident cancer cases an abnormality is present on the previous screening round mammogram, and the most frequently overlooked lesions are microcalcifications.