First results from the International Breast Cancer Intervention Study (IBIS-I): a randomised prevention trial

Area of Science:

• Oncology
• Clinical Trials
• Preventive Medicine

Background:

• Mixed results from previous tamoxifen trials for breast cancer prevention.
• Evidence suggests tamoxifen reduces breast cancer risk, but risk-benefit balance is uncertain.
• Need for further investigation into tamoxifen's efficacy and safety in breast cancer prevention.

Purpose of the Study:

• To evaluate the efficacy and safety of tamoxifen for breast cancer prevention.
• To assess the impact of tamoxifen on breast cancer incidence in high-risk women.
• To determine the risk-benefit ratio of prophylactic tamoxifen use.

Main Methods:

• Double-blind, placebo-controlled randomized trial.
• 7152 women aged 35-70 at increased risk of breast cancer received tamoxifen (20 mg/day) or placebo for 5 years.
• Primary outcome: breast cancer frequency; intention-to-treat analysis.

Main Results:

• Tamoxifen reduced breast cancer risk by 32% (p=0.013) after median 50-month follow-up.
• Endometrial cancer non-significantly increased (11 vs 5).
• Thromboembolic events significantly increased with tamoxifen (OR 2.5, p=0.001); all-cause mortality also significantly higher (25 vs 11, p=0.028).

Conclusions:

• Prophylactic tamoxifen significantly reduces breast cancer risk.
• Increased risks of thromboembolic events and endometrial cancer necessitate careful consideration.
• The overall risk-benefit ratio for tamoxifen in breast cancer prevention remains unclear, requiring continued follow-up.