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 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 develop in the interval between screens (sometimes called 'interval cancers'). Others may occur among 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^24,25. A high detection rate in the 24 months following a screen represents a negative outcome for a screening program.

At least every 6 months, provincial screening programs that track post-screen cancers link with their provincial cancer registries to identify cancers detected outside of the screening program in previously screened women. When post-screen cancers are detected, the previous screening film is reviewed by radiologists and, in some cases, technologists to arrive at a final decision, either by consensus or a majority of readers, regarding whether the cancers had newly developed in the interval between screens, or were missed at screening, or missed at diagnosis.

Because consistent classification of the end of a screening episode in the event of a screening abnormality has not yet been achieved among Canadian programs, the post-screen cancer rate in the 60 months following a normal screening examination is presented (Table 11). Women screened during 1994 and 1995 were monitored up to 60 months after their screening exam or, if it occurred sooner, until their next program screen.

Comparisons of post-screen cancer rates between provinces and countries require complete and up-to-date breast cancer registration and the assurance that post-screen cancers are counted in the same way²⁶. However, in Canada, post-screen cancer rates may also reflect the amount of screening delivered outside of screening program settings. Interim clinical breast exam and breast self-examination may also increase the rate at which post-screen cancers are detected in the interval between screening.

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