**2. Circulating tumour cells and related issues**

Invasion is a typical feature of malignant tumours. An epithelial malignancy, namely, carcinoma, starts its invasive growth from intraepithelial carcinoma in situ, destroys the basement membrane, infiltrates connective tissues and reaches smaller or larger blood and lymphatic vessels, entering the circulation. Thus, at some point of cancer pathogenesis, malignant cells appear in the bloodstream. This is a crucial step of carcinogenesis, leading to the metastatic spread—the hallmark of malignant process. Circulating tumour cells (CTCs) have been described by Thomas Ashworth as long ago as in 1869 [9]. Similarly, circulating foetal cells have been identified in peripheral maternal blood [3]. However, major technological advances were necessary to develop reliable tests to identify the circulating non-haematological cells.

CTCs are generally recognised as non-leukocytic nucleated cells in the bloodstream. They can be recognised by their physical properties including large size, mechanical plasticity and dielectric mobility. Physical filters, density gradient, dielectric, microfluidic or photoacoustic methods are used for physical separation. Immunophenotype by expression of different antigens also can be applied to identify CTCs; the relevant methods include cytometric highthroughput imaging and immunomagnetic and adhesion-based separation as well as negative depletion of leukocytes and CTC identification by tumour markers [5].

Considering the pathogenesis of malignant tumours, the burden of CTCs should appear and increase in parallel with advancing cancer course. Indeed, higher numbers of circulating tumour cells were observed in patients having tumours with higher pT or pN or higher pTNM stage [10]. However, CTCs were found in more than 80% of patients presenting with pT1 and/or pN0 gastric cancer [2]—an interesting finding that has a practical value regarding the possibilities of early diagnostics but also an impact on the theoretical considerations of carcinogenesis.

already confirmed cancer, ctDNA can identify either minimal residual disease after surgery with curative intent or tumour relapse. Similarly, the response to treatment can be monitored.

Introductory Chapter: Liquid Biopsy — A Promising Technology of the Future

http://dx.doi.org/10.5772/intechopen.86918

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In addition to the diagnostic, prognostic and predictive role in oncology, cfDNA analysis might be useful in other medical situations. Thus, circulating cell-free DNA can be derived from transplanted organs or from the foetus during pregnancy, serving as an early manifestation of graft rejection or reflecting genetic features on the foetus, respectively. In addition, cfDNA levels can be valuable also as a nonspecific biomarker of tissue damage in critical care

MicroRNAs (miRNAs) are small, evolutionary conserved, single-stranded, non-coding RNA molecules (approximately 22 nucleotides in length) that bind target mRNA to regulate gene expression [11, 12] at the posttranscriptional level [13]. These molecules act as large-scale molecular switches. MicroRNAs are involved in different physiological and pathological events, including apoptosis, cell proliferation and differentiation; therefore, it is not surprising to see miRNAs participating in carcinogenesis as either tumour suppressors [14, 15] or oncogenes [16]. The cardinal tumour features include cell proliferation, invasion and metastasis as well as activated angiogenesis. miRNAs regulate all the steps. In addition, up- or down regulation of certain miRNAs is associated with the biological potential of cancer, e.g. proliferation, invasivity and epithelial-mesenchymal transition or grade. miRNAs can be assessed either in tissues or in biological liquids, in the last case becoming a target for liquid biopsy. Again, the diagnostic and regulatory roles of miRNAs are not limited to oncology. Women who develop pre-eclampsia and spontaneous preterm birth are characterised by specific exosomal miRNA profile at early gestation. Considering the interplay between exosomal secretion, oxygen tension and endothelial proliferation, aberrant exosomal signalling by placental

medicine and related clinical situations, e.g. sepsis, haemodialysis and others [4].

cells is suggested to have a pathogenetic role in pregnancy complications [17].

ubiquitous, while others are characteristic for specific cells or tissues.

Exosomes represent a class of extracellular vesicles mediating intercellular communication. They consist of lipid bilayer, transmembrane and non-membrane proteins and single- and/or double-stranded DNA and RNAs, including microRNAs. A fraction of exosomal proteins are

Exosomes are collected for liquid biopsy analysis by different technological approaches, including methods targeting physical (size, density, sedimentation) or antigenic properties. After the tumour-derived exosomes have been isolated and identified, exosomal protein expression and genetic profile by exosomal RNAs or DNAs or exosomal miRNA signature can be tested. In addition to cancer-produced exosomes, those originating from immune cells

Molecular alterations can be assessed to select personalised treatment.

**4. Circulating microRNAs**

**5. Exosomes**

also can be detected and evaluated [5].

Thus, CTC detection has manifold roles in oncology. A high number of CTCs is an adverse prognostic factor, shown in breast, prostatic, colorectal, gastric, pancreatic and neuroendocrine carcinomas and sarcomas as well as non-small cell lung carcinoma [5]. However, a diagnostic role has also been confirmed, e.g. in the case of breast, prostate and colorectal carcinoma [5]. Early diagnostics by CTCs has been verified in non-small cell lung carcinoma [5] and gastric cancer [2]. CTCs can provide the information on key genetic features of cancer cells and on the epigenetic changes. Hence, prediction of the treatment response by CTC has been demonstrated in breast, prostate and colorectal cancer as well as in melanoma and non-small cell lung carcinoma. Further, changes of CTC count during treatment dynamically reflects the response to treatment paralleling the residual tumour burden [5].
