Breast Cancer Circulating Tumor Cells: Current Clinical Applications and Future Prospects

BACKGROUND: Identification and characterization of circulating tumor markers, designated as "liquid biopsies," have greatly impacted the care of cancer patients. Although more recently referring to circulating tumor DNA (ctDNA), the term liquid biopsy initially was coined to refer to any blood-borne biomarker related to malignancy, including circulating tumor cells (CTCs) in blood. In this manuscript, we review the specific state of the art of CTCs in breast cancer. CONTENT: Liquid biopsies might play a clinical role across the entire spectrum of breast cancer, from risk assessment, prevention, screening, and treatment. CTC counts have been shown to carry clear, independent prognostic information in the latter situation. However, the clinical utility of CTCs in breast cancer remains to be determined. Nonetheless, in addition to CTC enumeration, analyses of CTCs provide tumor molecular information representing the entire, often-heterogeneous disease, relatively noninvasively and longitudinally. Technological advances have allowed the interrogation of CTC-derived information, providing renewed hope for a clinical role in disease monitoring and precision oncology. SUMMARY: This narrative review examines CTCs, their clinical validity, and current prospects of clinical utility in breast cancer with the goal of improving patient outcomes.