Real-time liquid biopsy: circulating tumor cells versus circulating tumor DNA

Liquid biopsy is a new diagnostic concept, i.e., analysis of therapeutic targets and drug resistance-conferring gene mutations on circulating tumor cells and cell-free circulating tumor DNA released into the peripheral blood from metastatic deposits. Here, we discuss current challenges and future perspectives of the liquid biopsy concept in clinical oncology. We postulate that this concept will contribute to a better understanding and clinical management of drug resistance in cancer patients.