Decline in breast cancer mortality: how much is attributable to screening?

When estimating the decline in breast cancer mortality attributable to screening, the challenge is to provide valid comparison groups and to distinguish the screening effect from other effects. In Funen, Denmark, multidisciplinary breast cancer management teams started before screening was introduced; both activities came later in the rest of Denmark. Because Denmark had national protocols for breast cancer treatment, but hardly any opportunistic screening, Funen formed a "natural experiment", providing valid comparison groups and enabling the separation of the effect of screening from other factors.

Using Poisson regression we compared the observed breast cancer mortality rate in Funen after implementation of screening with the expected rate without screening. The latter was estimated from breast cancer mortality in the rest of Denmark controlled for historical differences between Funen/rest of Denmark. As multidisciplinary teams were introduced gradually in the rest of Denmark from 1994, the screening effect was slightly underestimated.

Over 14 years, women targeted by screening in Funen experienced a 22% (95% confidence interval 11%-32%) reduction in breast cancer mortality associated with screening (a reduction in breast cancer mortality rate from 61 to 47 per 100,000). The estimated reduction for participants corrected for selection bias was 28% (13%-41%). Excluding deaths in breast cancer cases diagnosed after end of screening, these numbers became 26% and 31%, respectively.

There is additional benefit in reducing breast cancer mortality from the early detection of breast cancer through mammographic screening over and above the benefits arising from improvements in treatment alone.