Prospective Study to Evaluate the Clinical Utility of ctDNA Tumor Fraction (TF) for Detection of ESR1 Mutations in HR-positive HER2-negative Metastatic Breast Cancer (MBC)

Participants are prospectively assigned to undergo ctDNA testing with FoundationOne® Liquid CDx (F1LCDx) at protocol-defined clinical decision points. Participants with an indeterminate negative F1LCDx result (ctDNA TF \<1% and negative for ESR1mut) are assigned to undergo reflex tissue biopsy NGS unless deemed medically unsafe. The protocol does not mandate treatment selection; treating clinicians retain full discretion regarding therapy.All research-related procedures will be performed at the time of standard of care clinical visits. No extra visits outside of routine care are required. For…