A Study to See Whether Estrogen Can Slow the Growth of Some ER Positive Breast Cancers

Breast cancer is a heterogeneous disease that includes two ER+ genetic subtypes (luminal A and luminal B) that differ in their response to treatment. Results from the Women's Health Initiative Trial showed that estrogen treated hysterectomized women with no prior history of breast cancer had a significant and persistent decrease in breast cancer incidence when compared to placebo treated participants. This implies that some ER+ breast cancers are in fact growth inhibited by estrogens and are not growth promoted. The hypothesis of this study is that some ER+ breast cancers (luminal A) are actually sensitive (growth inhibited) by estrogen. Objectives: 1. To assess changes in breast cancer proliferation after a 7-14 day trial of estradiol in newly diagnosed estrogen receptor positive post-menopausal breast cancer patients prior to surgery. 2. Exploratory analysis of biologic correlates with comparison to available genotyping tests. This is an open-label single group assignment pilot study for safety/efficacy and exploratory biologic correlates