A retrospective observational study to assess whether hormonal treatment (HT) with transdermal estrogens alone or in combination with micronized progesterone increases breast density and to compare these changes to those of a control group of 4120 patients were not given HT. We included 150 patients whose baseline breast density was assessed with photon-counting spectral mammography and 1 year after hormone treatment. The reduction in breast density was compared using an analysis of covariance. The difference in breast density between mammographies in the HT group was -0.40 ± 5.5 and -0.85 ± 4.2 in the control group. The changes in density according to the type of HT, we found that women on treatment with estrogen alone presented a difference of 0.44 ± 5.8, and -1.35 ± 5 (p = 0.13) in women on combined treatment. After adjusting changes in density for age and average number of days between mammographies, we observed a difference of -0.36 95% confidence intervals (CI) [-1.04 to -0.31] in the women on HT and -0.71 95% CI [-1.65 to -0.21] in the control group. No increased breast density was observed in women on HT treatment, nor did we observe an increase according to HT type. The difference in breast density loss was smaller in the HT group versus the control group.
Keywords: Breast density; hormone replacement therapy; mammography; micronized progesterone; transdermal estrogens.