Circulating tumor cells

Breast cancer (BC) therapy has fundamentally progressed in the last 30 years with the change from radical mastectomy to recent individualized local and systemic therapy regimens. By combining the modern treatment modalities, approximately 77 % of BC patients can be cured, still leaving potential for optimization in 23 % of cases, which will develop metastatic disease due to tumor cell dissemination despite optimal treatment. It has been known since the 19th century that most of the solid cancers shed circulating tumor cells (CTCs) into the blood circulation already at a very early stage. Based on this observation, CTCs are a surrogate marker for minimal residual disease (MRD) and precursors of metastatic disease ("seed"). Current research indicates that the phenotype and genotype differ between CTCs and primary tumor, which may result in different therapeutic responses. Therefore, characterization of CTCs may be an important step for the optimization of adjuvant and metastatic systemic treatment.