Circulating tumor cells and the metastatic process: the complexity of malignancy

Despite improvements achieved in terms of early detection and therapeutic approach, metastatic breast cancer remains one of the principal worldwide causes of death. In recent years, due to the heterogeneous response of each patient to chemotherapy, clinical research highlights the needed of a personalized approach. Circulating tumor cells represent a promising tool for this purpose. Unfortunately, even if their correlation with severity, outcome and metastatic nature of the tumor it has been established, several issues, mainly concerning their characterization and isolation, need to be solved. In this review, latest knowledges on circulating tumor cells and metastatic process in breast cancer were analyzed, aiming to understand their clinical utility and validity for a prospective therapeutic scenario.