Cancer patients usually receive therapies according to their primary tumor's molecular traits. These characteristics may change during the molecular evolution of distant metastases as the leading cause of cancer deaths. Primary tumor tissue, if accessible at all, does not always provide enough information to stratify individual patients to the most promising therapy. Re-analysis of metastatic lesions by needle biopsy is possible but invasive, and limited by the known intra-patient heterogeneity of individual lesions. These hurdles might be overcome by analyzing tumor cells or tumor cell products in blood samples (liquid biopsy), which in principle might reflect all subclones present at that specific time point and allow sequential monitoring of disease evolution. Liquid biopsies inform on circulating tumor cells as well as tumor-derived cell-free nucleic acids, exosomes and platelets. Here, we introduce the different approaches of blood-based liquid biopsies and discuss the clinical applications in oncology.
