It suggests in the Guideline that the postmenopausal women with breast cancer who have taken selective estrogen receptor modulators (SERMs) therapy for 2-3 years could benefit from changing endocrine therapy to aromatase inhibitors (AIs). This is a prospective, randomized and non-inferior trial to evaluate the prognosis of changing endocrine therapy from SERMs to AIs in perimenopausal and recently postmenopausal women with early-stage hormone receptor-positive 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.