Introduction
Interval cancers are those cancers which present between screening examinations in women who previously screened negative. They can be classified in a review process comparing diagnostic mammograms with those from the preceding screen. Of all intervals, some were not present at the time of screening but developed fast enough to present symptomatically (true intervals); some were not evident mammographically even at diagnosis (occult); some showed non-specific signs at screening but were not sufficiently suspicious to warrant further investigations (minimal sign) and some were evident at screening but were missed or incorrectly interpreted (false negative). The rate of false negatives in a screening programme is a measure of radiologists' performance. However, the method of review and classification will determine the proportion of cases classified as false negative.