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.

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)

2023.9-2024.68 Breast cancer patients who received aromatase inhibitor (AIs) or fulvestrant (FUL) endocrine therapy in Sun Yat-sen University Cancer Center and need to test hormone levels.

Not yet decided