Despite the use of effective loco-regional and systemic treatment approaches, many women with early-stage breast cancer still relapse and die of their disease. The early detection of small micrometastatic lesions that are currently undetectable by imaging procedures may potentially increase the chances to prevent the development of incurable metastatic disease. In patients with advanced disease, biological features of the tumour can change between the primary lesion and the metastases. However, in routine clinical practice, due to the difficulty of performing a biopsy of the recurrent lesions, treatment choices are often based on the biological features of the primary tumour. Moreover, one sample from a single metastatic lesion at a specific time point cannot capture the spatial and temporal intra-tumour heterogeneity of metastatic disease. Circulating-based biomarkers (i.e. ‘liquid biopsy’) such as circulating tumour cells and circulating tumour DNA are non-invasive biomarkers that have been developed to overcome the limitations of currently available tools in both the early and metastatic settings. In this manuscript, we review the current state of the art on circulating tumour cells and circulating tumour DNA in breast cancer focusing on their potential clinical utility as prognostic biomarkers, as tools to detect minimal residual disease, and to guide and monitor response and resistance to administered anticancer therapies.

(BELG J MED ONCOL 2017;11(8):357–363)