Menopausal hormone therapy and breast cancer phenotype: Does dose matter?

Background: Duration and type of menopausal hormone therapy (HT) has been associated with increased breast cancer risk and the development of estrogen receptor (ER)-positive tumors. The effect of HT dose on breast cancer tumor characteristics remains undefined. We sought to determine if HT dosing regimens influence breast cancer phenotype. Methods: We conducted a retrospective review of incident female breast cancers occurring in the year 2003 listed in the Kaiser Permanente Northern California Cancer Registry. Type of HT, dose, number of tablets dispensed, tumor phenotype, stage, grade, and histology were obtained from electronic records for women aged >= 50 years who had more than 1 year of uninterrupted pharmacy data (n = 1701). A dose index of HT exposure was created and odds ratios were used to determine if tumor phenotype varied between exposure groups. These results were compared with a previously published analysis of HT duration on tumor phenotype conducted with the same dataset. Results: The cumulative effect of estrogen and progesterone hormone therapy as calculated by factoring both dose and duration of HT use prior to breast cancer diagnosis did not reveal any new associations that were not previously identified by analysis of HT duration of exposure alone. Low-dose-index combination-HT users were less likely to have tumors with an ER-positive phenotype. An overall trend developed in which low- and high-dose-index exposed women had the lowest rates of ER- and progesterone receptor (PR) -positive tumors. Conclusion: Duration of use is an adequate surrogate for determining overall exposure to HT when considering the effect of HT on breast cancer phenotype.