This is a non-interventional, real-world observational study. Patients with histologically confirmed advanced breast cancer, aged ≥18 years, with expected survival \>3 months, who consent to ctDNA testing will be enrolled. A total of 400 patients will be recruited from multiple centers in China. Peripheral blood (10 mL) will be collected for ctDNA next-generation sequencing panel testing. Results will be returned to clinicians, who decide on subsequent treatment independently. Baseline data, treatment history, treatment regimens, and efficacy assessments (imaging every 8-12 weeks) will be recorded. Follow-up will continue until death or study end. Primary endpoint: overall survival (OS). Secondary endpoints: progression-free survival (PFS), objective response rate (ORR), correlation of ctDNA level/mutations/clonality with treatment response and OS, and correlation of testing timing/clinical decisions with OS. Statistical analysis will use intention-to-treat (ITT) principle, Kaplan-Meier, Cox regression, and subgroup analyses.
