Use of anastrozole for breast cancer prevention (IBIS-II): Long-term results of a randomized controlled trial

The Lancet Dec 19, 2019

Cuzick J, Sestak I, Forbes JF, et al. - Considering two large clinical trials suggesting an achievement of a decreased rate of breast cancer development in high-risk women in the initial 5 years of follow-up after use of aromatase inhibitors (MAP.3 and International Breast Cancer Intervention Study II [IBIS-II]), researchers here delineated blinded long-term follow-up results for the IBIS-II trial, that investigated the efficacy of anastrozole for preventing breast cancer (both invasive and ductal carcinoma in situ) in the posttreatment period via comparing anastrozole with placebo. Postmenopausal women (n = 3,864) at increased risk of developing breast cancer were randomly assigned (1:1) to receive either anastrozole (1 mg per day, oral) or matching placebo daily for 5 years from Feb 2, 2003, to Jan 31, 2012. They observed a 49% reduction in breast cancer for anastrozole (85 vs 165 cases) after a median follow-up of 131 months. A larger reduction was noted in the first 5 years (35 vs 89), however, the reduction remained significant after 5 years (50 vs 76 new cases), and not significantly different from the first 5 years. A reduction by 54% was observed for invasive estrogen receptor-positive breast cancer, with a continued significant effect in the period after treatment. Ductal carcinoma in situ reduced by 59%, particularly in participants known to be estrogen receptor-positive. Findings thereby suggest that in breast cancer, a significant continual reduction occurs with anastrozole in the posttreatment follow-up period, with no evidence of new late side-effects.
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