Proportion of cancers detected at the first incident screen which were false negative at the prevalent screen

Radiologists, blinded to the final diagnosis, reviewed mammograms which had been reported as normal at the prevalent screen, in a group of 133 patients with cancer detected 3 years later at the first incident screen. The aim of this study was to assess whether the mammograms at the prevalent screen demonstrated the cancers picked up at the incident screen, using a similar method to that described by the Northern Regional Audit Group for classification of interval cancers. Of the total number of cancers detected at the incident round there were 30 (22.6%) which could be classified as false negative or missed at the prevalent round. Of these, 25 (83.3%) had been either diagnosed as benign or not noticed on screening, the remainder (5) had been misdiagnosed on assessment. The most common histological type was invasive ductal; 22 (88%) were either Grade 1 or 2. Of the 27 false negative cancers with documented histological lymph node assessment 25% had associated axillary node metastases. By combining the above false negative rate for incident cancers with a similarly calculated one for interval cancers, it was possible to calculate the prevalent round sensitivity for cancer detection at 79.0% for single reading and 85.5% for double reading.