Circulating tumor cells in liquid biopsy: progress and challenges in detection and quantification

Circulating tumor cells (CTCs) are extremely rare in peripheral blood, typically occurring at a frequency of approximately one cell per million to ten million nucleated blood cells. This rarity presents significant challenges for their detection, particularly in early-stage cancers. Despite these limitations, CTCs have emerged as valuable biomarkers due to their ability to provide real-time insights into tumor biology through non-invasive liquid biopsy approaches. Unlike cell-free DNA (cfDNA) and RNA (cfRNA), CTCs represent intact, viable cells that offer both phenotypic and genotypic information, including insights into tumor heterogeneity and metastatic potential. Recent advancements have enabled the high-purity isolation of CTCs, facilitating their use in downstream molecular analyses such as comparative genomic hybridization, polymerase chain reaction, and next-generation sequencing (NGS). These analyses support the identification of actionable genomic alterations and mechanisms of therapeutic resistance, as well as the development of personalized treatment strategies. Importantly, a combined approach incorporating CTCs, ctDNA, and cfRNA may improve early cancer detection, the monitoring of treatment response, and the assessment of minimal residual disease. This review summarizes the biological characteristics of CTCs, discusses current detection and enrichment methodologies, and explores their integration with other liquid biopsy components. We also highlight the clinical implications, existing limitations, and future directions necessary to optimize CTC-based diagnostics in precision oncology.