Extended follow-up: anastrozole better than placebo to prevent breast cancer in high-risk women (IBIS-II)

Reference

Cuzick J, Sestak I, Forbes JF, et al, for the IBIS-II investigators. Use of anastrozole for breast cancer prevention (IBIS-II): long-term results of a randomised controlled trial. Lancet 2020;395(10218):117-122.

Synopsis

In the second International Breast Cancer Intervention Study (IBIS-II), after 5 years of treatment, 1% of women who received the aromatase inhibitor anastrozole had invasive breast cancer compared with 3% of women treated with placebo (number needed to treat = 63 for 5 years; 95% CI 41 - 135). This study reports masked longer-term follow-up after completion of the initial 5 years of treatment. The researchers included postmenopausal women at increased risk of breast cancer compared with the general population. Since age is an important risk factor, the researchers used age-defined risk thresholds. During the subsequent follow-up (median total 131 months), an additional 1.5% of anastrozole-treated women developed invasive breast cancer compared with 2.7% of placebo-treated women (number needed to treat [NNT] = 83; 95% CI 47 - 315). So, the overall rate of invasive breast cancer for the entire duration of the study (12 years) was 4.4% and 8.5%, respectively (NNT = 25; 18 - 40). The overall mortality rate was the same in both groups of women and the rate of breast cancer mortality was also similar. The researchers observed no difference in the rate of fractures or myocardial infarction between treatment groups. Although the rate of major adverse effects, such as venous thromboembolic events or cerebrovascular events, was higher in anastrozole-treated women, the number of events was small and did not reach statistical significance.