Serum Estradiol (E2) Levels in Breast Cancer Patients During Endocrine Therapy by High Performance Liquid Chromatography-tandem Mass Spectrometry (LC-MS /MS)

Inclusion Criteria: * Over 18 years old; * Histologically confirmed breast cancer; * Patients with early breast cancer (including natural menopause and artificial menopause using LHRHa), or patients with advanced breast cancer (including natural menopause and artificial menopause using LHRHa) who use AIs (letrozole, anastrozole or exemestane) as adjuvant endocrine therapy, First-line CDK4/6 inhibitors (Palbociclib, Dalpiciclib, Abemaciclib or Ribociclib) +AI endocrine therapy, second-line CDK4/6 inhibitors (Palbociclib, Dalpiciclib, Abemaciclib or Ribociclib) + fluvestrant (FUL) endocrine therapy; * Patients with needs for hormone testing; * Standard treatment, complete clinical data, including complete efficacy evaluation and follow-up data. Exclusion Criteria: * Breast cancer patients using TAM endocrine therapy; * In advanced patients, AIs exceeded first-line use (that is, in advanced disease, one endocrine therapy regimen had been used and failed), and FUL exceeded second-line use (that is, more than one endocrine therapy regimen had been used and failed)