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

Inclusion Criteria: * Female gender * Estrogen receptor positive (ER+) breast cancer * HER2 negative breast cancer * Post-menopausal by greater than 5 years * No previous hormonal replacement therapy * Low to intermediate histologic grade * ECOG Performance status of 0 of 1 * Adequate hematological, renal and hepatic function is required * Ability to take oral medication * Patient must have adequate tissue for diagnosis, biomarkers and Ki67 assays Exclusion Criteria: * Pre-menopausal women * Locally advanced or metastatic breast cancer * Current, previous or planning for pre-operative treatment with chemotherapy, hormone therapy including corticosteroids, radiation therapy for malignancy or other condition * Known hypersensitivity or intolerance to estradiol * Ischemic changes on baseline electrocardiogram * Symptomatic but untreated cholelithiasis * History of deep vein thrombosis, pulmonary embolism, stroke, acute myocardial infarction, congestive cardiac failure, untreated hypertension or known inherited hypercoagulable disorder * Undiagnosed abnormal vaginal bleeding or prior history of endometrial cancer * Untreated metabolic disturbances (glucose \> 15.0 mmol/L and triglycerides \> 400 mg/dL) * Current treatment with drugs known to be moderate or strong inhibitors of inducers of isoenzyme CYP3A4 * The time between study enrolment and definitive breast surgery is not sufficient for administration of at least 7 days of estradiol