A Brief Review of the International Breast Cancer Intervention Study (IBIS), the Other Current Breast Cancer Prevention Trials, and Proposals for Future Trialsa

The available results from breast cancer chemoprevention trials are reviewed. Four trials using tamoxifen have been performed, of which three have reported efficacy results. A fifth trial using raloxifene has also been published. The largest tamoxifen trial shows approximately a 50% reduction in breast cancer incidence in the short term, but the two smaller trials have not found any incidence reduction. Greater agreement exists for side effects: thromboembolic disease and endometrial cancers are raised about 2- to 3-fold when tamoxifen is used for 5 years. The possible reasons for the discrepancy in breast cancer reduction are explored. A review of trial parameters does not clearly explain this difference, and a metanalysis indicates that all results are compatible with a 42% reduction in short-term incidence. Several important questions remain about the clinical implication of this result, including the effect on mortality, the appropriate risk groups for chemoprevention, and the long-term effects on incidence. Continued follow-up of these trials is crucial for resolving these issues.