RESUMEN
BACKGROUND: Although a number of studies have evaluated the associations between use of postmenopausal hormone therapy (HT) and mammographic density, few have assessed the effects of the medications containing estradiol (E2) plus norethisterone acetate (NETA). In particular, there are few data on the effects of the low-dose E2/NETA regimen. METHODS: We included data from 724 women, aged 50-70 years, residing in south-east Norway, who participated in a cross-sectional study conducted within the Norwegian Breast Cancer Screening Program. We assessed mammographic density using a previously validated computer-assisted method. RESULTS: After adjusting for age at screening, number of children and body mass index, women who currently used HT had 6.0% higher percent mammographic density than never-users, p < 0.0001. Women who used either low- or high-dose continuous combined E2/NETA regimens had 7.7% (p < 0.0001) and 8.8% (p < 0.0001) higher percent mammographic density than never-users, respectively. CONCLUSION: Our study suggests that the effect of E2/NETA regimens on mammographic density could be at least as detrimental to the breast tissue as several other estrogen + progestin regimens. Our results suggest that both low- and high-dose E2/NETA influence mammographic density, but there were some indications in our analyses that the effect of low-dose E2/NETA could be slightly lower than that of the older high-dose regimen.