Enroll : Sep03,2018\~(Planned N= 224)
1. Inclusion criteria
* Premenopausal
* ER+
* Planned tamoxifen(TMX)
* No planned ovary function suppression(OFS)
* Regardless of ChemoTx
* Mammography(MMG) density check via Volpara\*(=Baseline MMG density, BaMD) (\*Volpara= software to check MMG density)
2. Check MMG Density via Volpara After 1yr TMX (At1yrMD) Check menopausal status after 1yr TMX(Menstruation episode or FSH \<30)
3. MMG Density Reduction(MDR =BaMD-AtMD ) at 1yr
4. MDR ≥5% -\> Keep go on TMX MDR \<5% -\> 1:1 randomization -\> Keep go on TMX vs OFS add on to TMX
5. Analysis : 5yr MDR (1'endpoint), 5yr DFS(2'endpoint), 5yr OS
6. Calculation of patients' number In previous study(\<Kim et al. Breast Can Res 2012\>) MDR (Mammography Density Reduction) was found in about 50% of all patients who received endocrine therapy.
Expected
* MDR in "TMX only" cohort -\> 6 ± 7%
* MDR in "OFS add on to TMX" -\> 10 ± 7%
* after 1yr, Significance level 5%, average MDR 4% difference, 80% power, 10% dropout rate =\> 1:1 randomization Number = 112(56:56) Total number = 224
