Since 1991, 160,884 women were randomised in the ratio 1:2 to intervention (annual mammography from age 40 to 41) and control arms. All breast cancers diagnosed in both arms were identified, and subjected to detailed histology review. Predicted deaths from breast cancer up to 10 years from entry were calculated using three different prognostic indices, based on 1287 cases diagnosed before 2000.
There is currently an 8% excess of invasive breast cancers in the intervention arm, but nonsignificant decreases in the rates of cancers ≥20 mm and node-positive cancers. The ratio of predicted deaths in the intervention and control arms, adjusted for the excess diagnosis in the intervention arm, ranges from 0.89 to 0.90, and is borderline significant.
This trial may result in a smaller breast cancer mortality reduction than other trials of women aged under 50, due to inclusion of all women from age 40 with possible lower sensitivity at younger ages. However, the present analysis, based on surrogate outcome measures, relies on several assumptions to overcome potential biases. Firm conclusions must await the analysis of observed mortality from breast cancer.