Current Status of Circulating Tumor Cells in Head and Neck Squamous Cell Carcinoma: A Review

Abstract Objective Circulating tumor cells (CTCs) are found in the blood of patients with cancer, including head and neck squamous cell carcinomas (HNSCCs). The aim is to review the most up‐to‐date status of CTCs for applications in patients with HNSCC. Data Sources English articles in PubMed. Review Methods All the studies on CTCs in HNSCCs in the literature were reviewed. Conclusions There is emerging information on the diagnostic and prognostic value of CTCs in HNSCCs. Evidence also highlights the advantages of various downstream analysis approaches over circulating tumor DNA (ctDNA), such as single‐CTC analysis, ex vivo, and in vivo expansion of CTCs. Multiple phenotypic surface markers (cytokeratins, EpCAM, vimentin, etc.), used for CTCs characterization using different immunoassays, could predict disease progression as well as patients' response to treatment efficacy. Immune checkpoint inhibitors' status in CTCs could also provide better insight into treatment. Clonal expansion of CTCs and single‐cell analysis of CTCs are the most emerging fields nowadays which may offer an understanding of the mechanism of tumor evolution as well as therapeutic efficacy. Although several clinical trials are ongoing, limitations still exist in the detection and characterization of CTCs. Due to the lack of a gold standard protocol, the sensitivity and specificity of CTC enumeration methods vary. Implications for Practice Prospective clinical trials are still needed before CTCs can be employed as diagnostic and prognostic markers in the clinical management of patients with HNSCC.