Prognostic Evaluation of Changing Endocrine Therapy in Women With Breast Cancer

Inclusion Criteria: * Written informed consent must be obtained and documented according to the local regulatory requirements prior to beginning specific protocol procedures. * Age of at least 45 and at most 55 years. * Performance status (Karnofsky-Index) \>80% * Steroid receptor (estrogen and progesterone) positive tumor (diagnosis according to hospital standard-procedures). * No clinical evidence of local recurrence or distant metastases. * Complete staging work-up within 3 months prior to registration. All patients must have (bilateral) mammography or breast MRI, chest X-ray; other tests may be performed as clinically indicated. * Patients have taken the SERMs as endocrine therapy for 2-4 years. * Patients who have had amenorrhea for at least half a year. * Life expectancy of at least 10 years, disregarding the diagnosis of cancer. * Adequate organ function including normal red and white blood count, platelets, serum creatinine, bilirubin, and transaminases within normal range of the institution. * Patients must be available for and compliant to treatment and follow-up. * Patients registered on this trial must be treated and followed up at the participating center. Exclusion Criteria: * Known hypersensitivity reaction to the investigational compounds or incorporated substances. * Hormone receptor-negative breast cancer. * Local recurrence and/or metastasis of breast cancer. * History of hysterectomy. * Pregnant or lactating patients. Patients of childbearing potential must have a negative pregnancy test (urine or serum) within 14 days prior to registration. * History of osteoporosis and/or fractures due to osteoporosis. * Prior or concomitant secondary malignancy (except non-melanomatous skin cancer or carcinoma in situ of the uterine cervix) * Any other serious medical pathology, such as congestive heart failure; unstable angina; history of myocardial infarction during the previous year; uncontrolled high risk arrhythmias * Other serious illness or medical condition that may interfere with the understanding and giving of informed consent and the conduct of the study. * Concurrent treatment with other experimental drugs or any other anti-cancer therapy. * Males.