Implications of the Canadian National Breast Screening Study

摘要

The Canadian National Breast Screening Study (CNBSS) [1] was initiated in 1980 and, recently, we published the results of our 25-year follow-up of all 89,835 participants, who when enrolled were aged 40-59 years [1]. They were randomly assigned with informed consent to mammography or control arms. In the mammography arm, two-view mammography was used together with breast examinations annually. In the control arm there was no mammography screening, but a single breast examination for women aged 40-49 years followed by usual care, and annual breast examinations for women aged 50-59 years. All women were taught breast self-examination (BSE) [1]. Screening was for up to 5 years. We found no evidence that mammography screening reduced breast cancer mortality. However, because of the long-term follow-up, we were able to estimate that mammography resulted in the over-diagnosis of 22% of screen-detected invasive breast cancers, 50% of the impalpable cancers detected by mammography alone. If one were to include in situ cancers, the proportion of screen-detected cancers that were overdiagnosed increases to 35%, 72% of those detected by mammography alone.