## 1 Introduction  
The accurate detection and diagnosis of cancer in the very early stages of its progression is highly important to increase the probability of achieving a successful treatment. Recently, liquid biopsy has emerged as a minimally invasive approach that can potentially be used in medicine for early diagnosis and monitoring of patient's health. This approach is based on the reliable detection of tumor markers circulating in biological fluids using various techniques. Recently, identification and quantification of circulating tumor cells (CTC), circulating tumor DNA (ctDNA), and exosomes have attracted particular attention for early cancer detection. CTC is one of FDA-approved liquid biopsy methods with relatively high specificity that allows performing in vitro examination by cell culturing as well as phenotypic and genotypic analysis. On the other hand, CTC's complex heterogeneity, low abundance (1-10 cells/10 mL of peripheral blood of cancer patients [1]), and difficulty to develop an enrichment technology in order to achieve a high limit of detection (LOD) present a challenge for CTC to be a single, universal, and reliable approach for cancer diagnostics in its early stages [2]. Quantification and analysis of ctDNA present in cancer patient's blood is another Food and Drug Administration (FDA)-approved liquid biopsy method that can become a supplement to conventional biopsies due to its high sensitivity and easy access to genomic information of the patient. Access to such information allows the selection of the most appropriate treatment and to monitor its effectiveness in real-time. In addition, it was shown that ctDNA levels often correlate with tumor burden [3]. However, ctDNA has a short half-life in blood (~2 h) [4] and can interfere with cell-free DNA (cfDNA) that is not related to the tumor [5]. Similar to CTC, current FDA-approved ctDNA assays have low LOD at \(\sim\)25 copies/mL [6] making its detection and ability to obtain a detailed molecular profile a difficult task. Although detection of CTC and ctDNA can become highly effective approaches for cancer diagnosis, the mentioned challenges impact the feasibility of their implementation for diagnosis especially at the initial stage of cancer. In this respect, analysis of exosomes circulating in the patient's body may be an innovative and promising method for the diagnosis of cancer and solving the most important problems of sensitivity, specificity, and reliability of currently available diagnostic methods. Exosomes are stable, membrane-bound nano-vesicles with a diameter of 20-150 nm [7; 8] and density ranging 1.15-1.19 g/mL [9], that are secreted into circulation by virtually all cell types including epithelial cells, mesenchymal cells, lymphocytes, and cancer cells [10; 11]. Exosomes are transported into the extracellular space as a result of fusion of late multivesicular bodies (MVBs) with the plasma membrane [12; 13]. As a result, exosomes entering the extracellular space can be found in any body fluid, including blood (reported concentration ranges \(10^{7}\)-\(10^{12}\) exosomes/mL of plasma or serum [14; 15; 16; 17; 18]), urine, and saliva [19]. They contain cargo such as nucleic acids (including microRNAs and other non-coding RNAs) and surface proteins inherited from the mother cell [20; 21; 22; 23; 24]. The amount and type of such membrane proteins and nucleic acids contained in exosomes are now the main focus of research and discussion [25], as exosomes play an important role in short- and long-range intercellular communication [26; 27; 28; 29]. However, delivery of a distinct type of cargo by tumor-derived exosomes to a normal cell can induce changes and cause the recipient cell to become malignant, causing cancer progression [30]. Finding tumor-derived exosomes in a patient's sample can allow for diagnosis of cancers in their earliest stage when symptoms are not yet present and dramatically increase the success rate of treatment.