Tamoxifen Prevents Breast Cancer: Worth a Chat With Patients

Lidia Schapira, MD


February 23, 2015

Tamoxifen for Prevention of Breast Cancer: Extended Long-term Follow-up of the IBIS-I Breast Cancer Prevention Trial

Cuzick J, Sestak I, Cawthorn S, et al
Lancet Oncol. 2015;16:67-75

Study Summary

IBIS-I was a randomized controlled trial that enrolled premenopausal and postmenopausal women 35-70 years of age between 1992 and 2001, who were considered to be at an increased risk of developing breast cancer, and randomly assigned them to receive tamoxifen or placebo for 5 years. The study enrolled over 7000 women. This publication reports the results after a median follow-up of 16 years.

There were 601 breast cancers reported, 251 (7.0% of the 3579 patients) in the tamoxifen group and 350 (9.8% of the 3575 patients) in the placebo group. The risk of developing breast cancer was similar in both groups between years 0-10 and after 10 years. The greatest reduction in risk was seen in invasive ER-positive breast cancer and noninvasive cancer (ductal carcinoma in situ), but no effect was noted for ER-negative breast cancer.

The results show that tamoxifen offers a very long period of protection after treatment cessation, lending further support to its use as an agent to reduce breast cancer risk in selected populations.


Four previous randomized controlled trials[1,2,3,4] have shown that tamoxifen can reduce the risk ER-positive breast cancer in the first 10 years of follow-up. This important IBIS-I study confirms that the benefit is extended for more than 10 years, especially in reducing risk for invasive ER-positive cancers. The authors calculate that 22 women need to be treated for 5 years to prevent one breast cancer in the next 20 years. The profile of side effects and toxicities is identical to those reported previously.



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