Liquid biopsy in breast cancer: promises and challenges.

This review aims to provide the latest updates regarding circulating tumor DNA (ctDNA) applications in breast cancer. We discuss the current indications and recent research efforts including important ongoing trials. In the metastatic setting, ctDNA has an established role for identifying druggable mutations (e.g. PIK3CA, ESR1, etc.). Various ctDNA assays used in randomized trials have been approved by regulatory authorities as companion diagnostic devices. Beyond precision medicine, emerging evidence indicates that ctDNA could be used for disease monitoring and detecting acquired resistance mechanisms. In the early setting, two types of ctDNA detection assays can be used: tumor-agnostic and tumor-informed, both having pros and cons. The clinical utility of ctDNA as a surveillance tool for patients with early breast cancer after treatment with curative intent is under investigation. Moreover, many research groups have demonstrated the potential role of ctDNA as a biomarker for assessing response to neoadjuvant systemic treatment and modification of therapeutic plan. CtDNA is currently used in metastatic breast cancer for treatment selection, and its clinical utility in early breast cancer is prospectively evaluated.