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

The investigators intend to conduct a prospective cohort study to detect E2 levels during endocrine therapy in Chinese breast cancer patients using LC-MS/MS methods, and to determine the proportion of patients with premenopausal E2 levels (OFR or E2 insufficiency) during AIs or FUL endocrine therapy. To further explore the correlation between serum E2 levels and the efficacy of endocrine therapy in Chinese patients, so as to ensure the safety and efficacy of endocrine therapy for patients, and pave the way for further providing more accurate individualized treatment programs.