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Anastrozole decreases rate of breast cancer in high-risk postmenopausal women (IBIS-II)

Clinical Question:
Does anastrozole decrease the rate of breast cancer in high-risk postmenopausal women?

Bottom Line:
Postmenopausal women at high risk of developing breast cancer who take anastrozole for 5 years have a lower rate of developing invasive breast cancer during that time frame. (LOE = 1b)

Cuzick J, Sestak I, Forbes JF, et al., for the IBIS-II investigators. Anastrozole for prevention of breast cancer in high-risk postmenopausal women (IBIS-II): an international, double-blind, randomised placebo-controlled trial. Lancet 2014;383(9922):1041-1048.  [PMID:24333009]

Study Design:
Randomized controlled trial (double-blinded)

Industry + govt



These authors report outcomes from the second International Breast Cancer Intervention Study (IBIS-II), in which postmenopausal women at high risk for breast cancer were randomly assigned to receive anastrozole (1 mg daily for 5 years; n = 1920) or placebo (n = 1944) in 18 countries. The authors used a complex series of definitions to identify high-risk women: aged 40 to 44 years with an estimated 4-fold increased risk compared with the general population; aged 45 to 60 years with double the estimated risk; aged 60 to 70 years with 1.5-fold increased risk. The authors also used the Tyrer-Cuzick model and included any additional women with an estimated 10-year risk greater than 5%. They excluded women who were premenopausal, had a previous breast cancer diagnosis, or who had previously received chemoprophylaxis. The researchers and support staff were all masked to treatment allocation. The researchers evaluated the women at baseline, 6 months, 12 months, and then annually for 5 years. The researchers used intention to treat to analyze the rate of incident breast cancers. By the end of the study, 2% of women receiving anastrozole had invasive breast cancer compared with 3% of women treated with placebo (number needed to treat = 62 for 5 years; 95% CI, 38 - 155). Approximately one third of the women in the anastrozole group and one fourth in the placebo group stopped taking their medication, mainly because of "side effects" (I hate those placebo-induced side effects). There was no difference in the overall death rate (1%) in each group.


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