Solid tumors have long been known constantly to shed many biomolecules such as DNA, RNA and proteins into the blood and other body fluids. These biomolecules can circulate in the blood free of cells or by binding to proteins or lipids. Circulating tumor DNA (ctDNA) is tumor-derived cell-free DNA (cfDNA) and is often confused with non-tumor derived cfDNA. Recent advances in laboratory techniques enable better capture and analysis of trace amounts of circulating materials. Liquid biopsy is a minimally invasive method and allows analyses of circulating tumor cells released from peripheral tumors and/or metastatic tumors and nucleic acids in the cellfree circulation, in particular ctDNA, microRNA and extracellular RNA. The fact that ctDNA completely reflects the tumor genome has made it a powerful clinical and research tool in liquid biopsy, and a number of studies have been conducted on the diagnostic, predictive and / or prognostic use of ctDNA in cancer over the last few years. There are studies confirming the clinical validity of ctDNA for detecting tumor heterogeneity and resistance mutation, identifying candidates for targeted therapies, disease monitoring and therapeutic response assessment, early detection of recurrence, monitoring tumor burden, and risk classification. In this chapter, we have summarized the roles of ctDNA in clinical practice for diagnosis, treatment choices and responses to therapy in various solid cancers.
Keywords: Breast cancer, Cell-free DNA, Circulating tumor cells, Circulating tumor DNA, Colorectal cancer, Head and neck cancer squamous cell carcinoma, Liquid biopsy, Non-small cell lung cancer, Prostate cancer gastrointestinal stromal tumor, Solid tumors.