Detection of Circulating Tumor DNA in Solid Tumors

Cancer is characterized by sequential and progressive genetic and epigenetic alterations in key proto-oncogenes and tumor suppressor genes, which ultimately lead to tumor development. Advances in the technology of analysis of molecular mechanisms have increased the efficiency of clinical management of cancer patients. Recent years have witnessed a progressive development in technologies that enable the detection of specific molecular abnormalities associated with various types of solid tumors in body fluids, a process that is globally known as “liquid biopsy”. Liquid biopsy is largely based on the circulating free DNA (cfDNA) present in the plasma of healthy individuals and derived either from cell apoptosis or from the active secretion of microvesicles mediated by white blood cells (WBCs). The plasma of cancer patients contains DNA, which is referred to as circulating tumor DNA (ctDNA) and is released by the tumor cells in the form of DNA fragments of various sizes bearing the various types of genetic abnormalities specific to the tumors from which were derived. Sequencing studies conducted with several thousands of cancer patients have revealed that ctDNA accounts for only a fraction of the total DNA, and the size of this fraction varies in relation to tumor burden, tumor site, tumor subtypes, and several other biological properties of the tumor cells. Therefore, the levels of ctDNA are extremely low in several early-stage tumors, requiring highly sensitive methods for the detection of genetic alterations occurring at an extremely low allelic ratio. Several studies on advanced solid tumors corroborate the view that in comparison to the standard tissue-based DNA analysis, ctDNA analysis may represent a better approach, which is more convenient, readily accessible, and less invasive. Therefore, ctDNA analysis may contribute to improving the stratification of the patients, supplementing the monitoring of the therapeutic response to treatment, and in certain cases, guiding the therapeutic interventions. Another challenging objective of ctDNA analysis is the monitoring of minimal residual disease (MRD). With rapid achievements in the study of ctDNA, an increasing number of trials that are currently under progress have incorporated liquid biopsy in their design. The majority of these studies are observational or cross-sectional in design, and in several instances, involve the comparison of ctDNA analysis to the conventional investigation approaches as the primary objective.