Variation in the effectiveness of breast screening by year of follow-up.

This meta-analysis assesses the effectiveness of breast screening by year from the start of screening for women aged 40-49 at study entry. Data from previous randomized controlled trials on breast cancer screening were combined, and cumulative and yearly breast cancer mortality rates and relative risks (RRs) were calculated for women offered screening compared to those not offered screening. At 7 years of follow-up, no reduction in breast cancer mortality from screening starting at ages 40-49 was found. At 10 years of follow-up, a nonsignificant reduction in breast cancer mortality was seen for women aged 40-49 at entry (RR = 0.93; 95% CI: 0.77-1.11). A nonsignificant excess of breast cancer mortality in those offered screening aged 40-49 was observed during the early years of follow-up in several trials. While the favorable effect of screening was observed within the first 5 years of study entry for women aged 50 or more, no similar effect was seen for women aged 40-49. The delayed effect for the 40-49 cohort may be attributable to 1) a biological difference in the effects of screening, which may be related to the onset of menopause, and 2) screening that occurred when women were aged 50 or more rather than before that age.