Effects of tibolone and continuous combined hormone replacement therapy on mammographic breast density

Objective: Our purpose was to compare the effects of tibolone, continuous combined hormone replacement therapy, and placebo on mammographic breast density. Study design: A prospective, randomized, double-blind placebo-controlled study was performed. A total of 166 postmenopausal women were equally randomized to receive tibolone 2.5 mg, estradiol 2 mg/norethisterone acetate 1 mg (E2/NETA), or placebo. Mammograms were performed at baseline and after 6 months of treatment. Mammographic density was quantified according to the Wolfe classification and by the percentage area of the breast that had a dense pattern. Results: An increase in mammographic density was much more common among women receiving continuous combined hormone replacement therapy (46%-50%) than among those receiving tibolone (2%-6%) and placebo (0%) treatment. The difference between E2/NETA and placebo was highly significant (P< .001). Treatment with tibolone did not differ from that with placebo. The relative risk of an increase in breast density for E2/NETA versus tibolone was found to be 8.3 (95% CI 2.7-25.0). Conclusion: An increase in mammographic density should be regarded as an unwanted side effect of hormone replacement therapy. In contrast to estrogen/progestogen treatment, tibolone seems to exert little stimulation of breast tissue. (Am J Obstet Gynecol 2002;186:717-22.)