Radiological review of interval cancers in an Australian mammographic screening programme

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Objectives—To determine the proportion and features of invasive interval cancers that could be considered detectable at the time of the previous screen and the proportion of cases that could be classified as true intervals, false negatives, minimal signs, or radiographically occult lesions. Setting—BreastScreen Victoria, the Victorian component of the BreastScreen Australia mammography screening programme. Methods—Two separate review methodologies were adopted. Firstly a blinded review of interval, screen detected, and normal cases was undertaken, followed by a confirmation exercise to determine the proportion of invasive interval cancers that could be considered detectable at the time of the previous screen. Secondly, an unblinded review was performed to classify interval cases as true interval, false negative, minimal signs, or radiographically occult. Results—From the blinded review, it was estimated that 38% of interval cases may be considered “potentially detectable” at the time of screening. Comparison of the characteristics of interval and screen detected cases indicates that interval cases are more likely to be smaller, equivocal, ill defined masses. In the unblinded exercise, 41% of interval cases were classified as false negatives and a further 16% as minimal signs, 33% true intervals, and 10% radiographically occult. Of the interval cancers considered potentially detectable at screening, 97% were classified as false negatives in the unblinded review. Conclusions—This study highlights the importance of adopting staged review methods with both blinded and unblinded components. The blinded review and confirmation exercise allows the determination of the proportion of interval cases that could be considered potentially detectable at screening. The unblinded review provides an active important opportunity for professional development and review and a mechanism to link into the blinded review through further classification of interval cases. (J Med Screen 2000;7:184‐189)