Level 2: Mid-level evidence
A randomized trial conducted in England, Wales, and Scotland evaluated mammography screening in 160,921 women aged 39-41 years (Lancet. 2006;368:2053-2060). Women were randomized to annual mammography screening to age 48 years vs. usual medical care. At mean follow-up of 10.7 years, overall mortality was 1.66 per 1,000 women-years with annual mammography, compared with 1.72 per 1,000 women-years with usual care (rate ratio 0.97). Breast cancer mortality was 0.18 vs. 0.22 per 1,000 women-years (rate ratio 0.83, NNT 2,512 but not statistically significant). The discussion section notes nine other trials and results for women enrolled before age 50 years (though some benefit may occur from continued screening after age 50 years). A meta-analysis of seven of these trials plus the cited trial found a statistically significant reduction in breast cancer mortality.