Results of Screening

Organized breast cancer screening aims to ensure that all breast cancers are identified in asymptomatic women while minimizing the number of healthy women who experience unnecessary follow-up procedures.

Abnormal recall rates on first screen are normally high, reflecting prevalent cancers among screened women. Abnormal recall rates differed little among age groups, ranging from 10.4% to 11.5% of first screens (Table 4). For rescreens occurring less than 2.5 years from the previous screen, the abnormal recall rate was substantially lower (between 5.1% and 6.2%) (Figure 9). The lower rate may reflect either the value of having previous comparison mammograms or the likelihood that fewer cancers would develop between screens or both factors. The abnormal recall rates for rescreens occurring at least 2.5 years after the previous screen start to revert back towards the rates at first screen. This emphasizes the benefits of returning for a subsequent screen in a timely fashion.

Figure 9 Abnormal recall rate* by age group, 1997 and 1998

*Includes mammography and clinical breast examination as screening modalities. **Half of the women who were rescreened 2.5 or more years from the previous screen returned for a screen by 3.4 years. Note: Data for the New Brunswick program are incomplete and therefore do not comprehensively reflect program activity.

The rate of abnormal screens was slightly higher for first screens in comparison with standards set by other national breast screening programs (see Appendix 1), which specify that fewer than 7% to 10% of first screens should be abnormal. However, these programs use mammography as the sole modality of screening, whereas several Canadian programs also use clinical breast examination (CBE). For women aged 50 to 69, CBE alone accounted for approximately 12% to 17% of the abnormal screens and 5% to 7% of cancers detected. For women aged 50 to 69, the rate of abnormalities detected by mammography alone are within standards set by the UK and Australian programs for first screen, and within the UK standard for rescreens. In general, Canadian recall rates just slightly exceed the standards set by Sweden and Europe.