Breast cancer in postmenopausal women with and without hormone replacement therapy: preliminary results of the MISSION study.

Objectives: To determine breast cancer prevalence in postmenopausal women with or without hormone replacement therapy (HRT).

Patients And Methods: This was a historical-prospective study with random patient selection. Patients were recruited between January 2004 and February 2005, and allocated to two groups: the 'treated group' comprising postmenopausal women on HRT regimens commonly prescribed in France or who had stopped <5 years previously; or the 'untreated group' of postmenopausal women who had never received HRT or stopped >5 years previously.

Results: In total 6755 patients fulfilled all inclusion criteria: 3383 in the treated group and 3372 in the untreated group. The treated group was younger and less overweight than the untreated group (p < 0.0001). The treated group had fewer late menopauses (p = 0.0002) and fewer first-degree family histories of breast cancer (p < 0.0001). Mean HRT duration was 7.9 years; 30.5% of women were treated for >or=10 years. Breast cancer prevalence was 1.01% for the treated group and 6.21% for the untreated group. The European standardized incidence rate of breast cancer in the untreated group was significantly higher than in the French reference population (comparative incidence figure ratio (CIFR) = 2.24; 95% confidence interval (CI) = 1.50-3.36). In contrast, the European standardized incidence rate in the treated group on the studied HRT regimens (excluding some progestins) was not significantly different from that of the reference population (CIFR = 1.04; 95% CI = 0.35-3.15).

Conclusion: The differences between the two groups may be due to French gynecologists' treatment strategy of avoiding prescribing HRT to high-risk women, and also perhaps to a specific 'studied HRT' effect.