Impact of changing from annual to biennial mammographic screening on breast cancer outcomes in women aged 50-79 in British Columbia

Objectives The objective of this study was to compare breast cancer outcomes among women subject to different policies on mammography screening frequency. Setting Data were obtained for women participating in the Screening Mammography Programme of British Columbia (SMPBC) for 1988–2005. The SMPBC changed its policy for women aged 50–79 years from annual to biennial mammography in 1997, but retained an annual recommendation for women aged 40–49 years. Methods Breast cancer outcomes were compared for women participating in the programme before and after 1997 for two groups: ages 40–49 and 50–79 years. Results There were data on 658,151 women. Comparing pre-1997 and post-1997, the median interscreen interval increased by 11.1 months in women 50–79 but by only 0.3 months in women aged 40–49. Excluding those detected at initial screen, 6291 breast cancers were identified. Comparing pre-1997 and post-1997: the relative rates (RR) of screen detected cancer increased in women aged 40–49 (RR = 1.32) and the rate of invasive cancers ≥20 mm at diagnosis decreased (RR = 0.83); the rate of cancers with axillary node involvement increased in women aged 50–79 (RR = 1.23). Cancer survival improved after 1997 for women diagnosed at ages 40–49 (hazard ratio = 0.62), but was unchanged for women aged 50–79. Breast cancer mortality rates did not change between the periods in either age group. Conclusion The proximal cancer outcomes considered (staging and survival) improved in women aged 40–49 but this was offset in women aged 50–79 associated with the change in screen frequency. These changes did not result in alterations in breast cancer mortality rates in either age group.