Is a dense mammographic parenchymal pattern a contraindication to hormonal replacement therapy?

The aim of the study was to find out whether the effect of hormonal replacement therapy (HRT) is modified by the mammographic parenchymal patterns on the risk of breast cancer. Subjects were 4163 Finnish women aged 40-47 years at entry who were invited to breast cancer screening every second year from 1982 to 1990. Mammographic parenchymal patterns (Wolfe's classification) were recorded at each screening round. The information, on use of HRT, was recorded from 1984. The follow-up ended in 1993 and up until that time 68 new breast cancers were diagnosed. A Poisson regression model was used in the analysis of the data. Use of HRT was not related to the risk of breast cancer (RR = 0.7, 95% CI 0.4-1.4), whereas mammographic parenchymal pattern was statistically significantly associated with risk of breast cancer. The age-adjusted relative risk of breast cancer among women with P2 versus N1 pattern was 2.5 (95% CI 1.3-4.8) and with DY versus N1 pattern 4.9 (951% CI 1.6-15.1). Women using HRT and with DY pattern were at substantially increased risk of breast cancer (RR = 11.6, 95% CI 2.5-53.6) compared with women not using HRT and with N1 pattern. There was an increased risk of breast cancer among women with DY mammographic parenchymal pattern who used HRT, which was consistent with a synergistic joint effect.