CTC enumeration and characterization: Moving toward personalized medicine

The primary cause of tumor-related death in breast cancer (BC) is still represented by distant metastasization. The dissemination of tumor cells from the primary tumor to distant sites through bloodstream cannot be early detected by standard imaging methods. The enumeration of circulating tumor cells (CTCs) represents an effective prognostic and predictive biomarker, which is able to monitor efficacy of adjuvant therapies, detect early development of (micro)metastases and at last, assess therapeutic responses of advanced disease earlier than traditional imaging methods. Moreover, since repeated tissue biopsies are invasive, costly and not always feasible, the assessment of tumor characteristics on CTCs, by a peripheral blood sample as a 'liquid biopsy', represents an attractive opportunity. The implementation of molecular and genomic characterization of CTCs could contribute to improve the treatment selection and thus, to move toward more personalized treatments. This review describes the current state of the art on CTC detection strategies, the evidence to demonstrate their clinical validity, and their potential impact for both future clinical trial design and, decision-making process in our daily practice.