**2. Detection of specific particles by liquid biopsy**

As previously mentioned, in liquid biopsy, circulating tumor cells (CTC), cell-free nucleic acids, exosomes, or tumor-educated platelets (TEPs) can be acquired and studied for different purposes [9, 10].

Circulating tumor cells are cells which detaches from primary tumor and deposits in a patient's blood. The described ability of CTC is particularly important to fully understand metastatic process; therefore a large scientific research field on this topic has evolved. It is believed that CTC can survive in the bloodstream because of the undergoing epithelial-mesenchymal transition (EMT). In this process tumor cells gain plasticity and motility which allows to extravasate from primary tumor into blood and intravasate into distant tissues as well. Detection of CTC can give valuable clinical information about patient's medical status—CTC can be used as a prognostic marker in different carcinomas including malignant thyroid tumors. Analysis of these cells can serve in treatment process as well—response to different pharmaceutical drugs in individual patient can be analyzed [9, 10].

Circulating cell-free nucleic acids (cfDNA, cfRNA, and cfmiRNA) discharge from apoptotic and necrotic tumor cells into the bloodstream. cfDNA originates not only form tumor cells but also from non-tumor cells after exercise, trauma, or inflammation [10]. Extraction of circulating tumor DNA (ctDNA) alone is a difficult process; therefore mutation in cfDNA particles is being searched and indicates the presence of tumor. A lot of mutation has been analyzed in different types of tumors, for example, epidermal growth factor receptor mutation in nonsmall cell lung cancer. At present, it is the only liquid biopsy test that has granted and has FDA approval [9, 11].

By liquid biopsy, cfRNA can be analyzed but this particle is not as stable as cfDNA, therefore it is harder to investigate. Another circulating cell-free nucleic acid—microRNAs—is a new and more stable tumor marker in the blood. Exosomes are microvesicles (40–150 nm) that are released in the blood from tumors and normal cells as well. Exosomes contain proteins, DNA, RNA, miRNA, lipids, and metabolites. Tumor-educated platelets (TEPs) are anucleated cell fragments that can be educated by the transfer of tumor-associated particles, mostly RNA [9, 10].
