Higher Dose Preoperative taMOxifen in Premenopausal bREast Cancer Patients

Inclusion Criteria: 1. Histopathologically and immunohistochemically confirmed ER+ and HER2- Premenopausal BC patients 2. Tumor size \>0.5cm on USG 3. Stage I-IIIA BC and planned curative surgery 4. ECOG 0-2 5. Patients with adequate bone marrow function \- Hemoglobin \> 10 g/dL, Plt \> 100,000/mm3 6. Patients with adequate kidney function \- serum Cr ≤ 1.4 mg/dL 7. Patients with adequate liver function * Bilirubin: ≤ 1.5 times of upper normal limit * AST/ALT: ≤ 1.5 times of upper normal limit * Alkaline phosphatase: ≤ 1.8 times of upper normal limit 8. Patients who decided to voluntarily participate in this trial with written informed consent 9. Premenopausal women : women who has not removed both ovaries, women who had menses in recent 1 year and FSH level is less than 30mIU/ml Exclusion Criteria: 1. Previous history of ipsilateral invasive breast cancer, in situ lesion 2. Previous history of chemotherapy or endocrine therapy on contralateral BC for the past 2 years 3. Patients who has distant metastasis 4. Patients who is pregnant or breastfeeding 5. Hormon receptor negative BC 6. Her-2 positive BC 7. Diagnosed pituitary adenoma 8. Women who has endometriosis, unknown vaginal bleeding 9. Inability to understand and willingness to sign a written informed consent 10. Patients with endometriosis or unexplained vaginal bleeding 11. Patients with a history of bleeding constitution, coagulopathy, or thromboembolism 12. Patients who have administered a CYP3A inhibitor or inducer, CYP2D6 inhibitor, etc. within 4 weeks prior to randomization