Liquid biopsy in breast cancer: current biomarker platforms and clinical applications

Breast cancer remains one of the leading causes of cancer-related mortality worldwide, and despite considerable advances in therapeutic strategies, the absence of reliable, minimally invasive diagnostic tools continues to limit early detection and real-time disease monitoring. Liquid biopsy has emerged as a transformative paradigm in oncology, offering the capacity to integrate tumor biology through the analysis of tumor-derived materials that circulate in peripheral blood and other biofluids. This review provides a comprehensive overview of the key analytes employed in liquid biopsy, including circulating tumor DNA (ctDNA), circulating tumor cells (CTCs), microRNAs (miRNAs), extracellular vesicles (EVs), and protein biomarkers with particular emphasis on their translational relevance in breast cancer. Drawing on landmark clinical trials and foundational molecular studies, we highlight how these biomarkers capture spatial and temporal tumor heterogeneity in ways that conventional tissue biopsy cannot. Finally, we address the technical, clinical, and ethical challenges that still impede widespread adoption of liquid biopsy, and we outline a forward-looking research agenda oriented toward multi-omics integration and point-of-care diagnostics. Taken together, this review underscores the potential of liquid biopsy to complement existing diagnostic standards and improve how we screen for, monitor, and treat breast cancer across disease stages.