## 3 Circulating Tumor Cells--Relevance to Skin Cancers
### From CTCs to Other Important Components Obtained via Liquid Biopsy
CtDNA has a short half-life of around 2 h in peripheral circulation, an advantage when trying to do dynamic, real-time monitoring of tumors. Although different ctDNA levels were observed in primary versus metastas cancers, or with respect to the examined biological fluid (blood, urine, and cerebrospinal fluid), one limitation of this potential new biomarker is the absence of well-defined cutoff values [61]. Special technical considerations must also be taken into account, as the importance of preanalytical steps from sampling to DNA extraction that are crucial for a reliable ctDNA detection was reviewed recently [62]. To date, several technical strategies are available to identify specific mutations in ctDNA, most of them being polymerase chain reaction (PCR)-based and next generation sequencing (NGS)-based approaches. We mention both targeted sequencing techniques, such as Amplicon sequencing (_AmpliSeq_) [63] and Targeted error correction sequencing (_TEC-Seq_) [64], together with nontargeted sequencing approaches including whole-exome sequencing (_WES_) [65] and whole-genome sequencing (_WGS_) [66]. Although having high sensitivity and specificity and showing promising preliminary results in cancer-related clinical trials, including melanoma research [67], these techniques are limited to the detection of specific known mutations, thus not making a breakthrough in the daily clinical practice yet. Indeed, the use of ctDNA in skin cancers monitoring is still in its infancy. As currently available techniques are not sensitive enough to be used in early MM screening, researchers have turned their attention to advanced MM monitoring and prognosis. We mention the study conducted by [PERSON] et al. [68] where ctDNA was used as a powerful tool to obtain relevant prognostic information in surgically resectable melanomas, while testing during treatment allowed the assessment of systemic therapy response and resistance mechanisms identification. Lastly, the potential powerful roles of micro RNAs (miRNAs) in cancers should not be neglected, as these are the most abundant RNA molecules found in peripheral blood circulation, in close connection to exosomes and TEPs. Regulating protein expression after transcription, miRNAs play a crucial role in the development and progression of skin cancer, as specific patterns of miRNAs in specific skin cancer types could be used as diagnostic markers [69]. The majority of the recent research has focused on the role of miRNA in malignant melanoma. Several studies have already demonstrated the impact of different miRNAs, such as miR-21, miR-125b, miR-150, miR-155, miR-205, and miR-211, in skin melanoma oncogenesis [70], while the impact of miRNAs in NMSC remains less characterized. The already existing preliminary data on the association between miRNA expression profiles and premalignant and/or malignant skin lesions [71] demonstrates the capability of phenotype- and stage-related tumor miRNAs to become reliable biomarkers in the near future. In order to integrate miRNAs into the daily clinical workflow, several advances remain mandatory. The current technologies comprising quantitative PCR (qPCR) or microarray platforms are still a challenge, with the uneconomical running costs and the requirements of highly trained personnel being currently the main limitations [72]. Improvement in pre-analytical handling of samples and the use of next-generation sequencing are thus mandatory for better potential results, as in the case of ctDNA analysis.