Organized Breast Cancer Screening Programs in Canada

1999 and 2000 Report

Post-Screen Cancers

Organized screening aims to ensure that a high proportion of asymptomatic women with breast cancer are identified by the screening process. Although it is highly sensitive in detecting even small tumours, mammography screening will not detect all breast cancers present at the time of screening. Some cancers, termed “post-screen cancers”, may be missed at screening or diagnosis, or may develop in the interval between screens (sometimes called “interval cancers”). Others may occur in women who do not return for subsequent screening (sometimes called “non-compliant cancers”). Post-screen cancers that are diagnosed in the interval between biennial screens need to be closely monitored because they are indicators of the sensitivity of screening and the appropriateness of the screening interval^17,18. A high detection rate
of post-screen cancers in the 24 months after a screen represents a negative outcome for a screening program. 

Provincial screening programs that track post-screen cancers link with their provincial cancer registries at least every six months to identify cancers detected outside of the program in previously screened women. As an element of the quality control process, when post-screen cancers are detected, radiologists (and, in some cases, technologists) review the previous screening film to arrive at a final decision regarding whether the cancers were newly developed in the interval between screens, were missed at screening or were missed at diagnosis. In cases of disagreement, resolutions are made either through consensus or by a majority decision by readers. 

Post-screen cancers that are diagnosed in the interval between biennial screens need to be closely monitored because they are indicators of the sensitivity of screening and the appropriateness of the screening interval.

According to the Canadian performance targets (Table 2), fewer than six post-screen detected invasive cancers per 10,000 person-years should be detected within 12 months from screening, and fewer than 12 per 10,000 person-years should be detected within 24 months from screening. While these targets were met or nearly met (Table 8), the figures must be interpreted cautiously for a number of reasons. Comparisons of post-screen cancer rates among programs require complete and up-to-date breast cancer registration and the assurance that post-screen cancers are counted in the same way. Better linkages with cancer registries will result in higher post-screen cancer rates because of higher levels of case ascertainment. In Canada, post-screen cancer rates may also be affected by the amount of screening delivered outside of screening programs, the performance of CBE and breast self-examination between screening episodes, and differences in the classification of the end of a screening episode in the event of a screening abnormality. 

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