A case–control study to estimate the impact on breast cancer death of the breast screening programme in Wales

We set up a case–control study to estimate the effect of service screening, as provided by the National Health Service breast screening programme, on breast cancer mortality in Wales. The basic design was a case–control study with 1:2 matching using data recorded by Breast Test Wales. The cases are deaths from breast cancer in women aged 50–75 years, diagnosed after the instigation of screening in 1991 and at age 50 or over. The controls are women who have not died of breast cancer, matched by risk set and year of birth. We recruited 436 cases and 750 controls.

The odds ratio for risk of death from breast cancer for women who have attended at least one routine screen compared with those never screened was 0.62 (confidence interval: 0.47–0.81), which suggests a reduction in risk of 38%, and no smaller than 19%. After correcting for anticonservative and conservative biases, the estimated mortality reduction was 25% (odds ratio = 0.75; 95% confidence interval: 0.49, 1.14; P = 0.09).

This study suggests that the Breast Test Wales screening programme is achieving a 25–38% reduction in breast cancer mortality in women attending for screening, consistent with the results of the randomized controlled trials of mammographic screening.