Circulating tumor cells in solid tumors: biological adaptation, detection technologies, and clinical implications

Circulating tumor cells (CTCs) are rare tumor-derived cells released from primary or metastatic lesions into the bloodstream and represent key intermediates in hematogenous dissemination. As intact cellular components of liquid biopsy, CTCs provide opportunities to investigate tumor heterogeneity, metastatic evolution, therapeutic resistance, and dynamic molecular alterations that may not be fully captured by tissue biopsy or circulating cell-free analytes. This review summarizes current advances in the biological mechanisms, detection technologies, and clinical applications of CTCs in solid tumors, with particular emphasis on their adaptive responses within the circulation. CTCs exhibit substantial phenotypic and metabolic plasticity, including epithelial–mesenchymal plasticity, altered energy metabolism, and enhanced resistance to oxidative and mechanical stress. Their interactions with platelets, neutrophils, and other immune components can further promote survival, immune evasion, and metastatic competence, particularly through the formation of homotypic and heterotypic CTC clusters. Technological advances in antigen-independent enrichment, microfluidic isolation, single-cell multi-omics, and artificial intelligence-assisted analysis are progressively extending CTC assessment beyond enumeration toward molecular and functional characterization. Emerging clinical evidence supports the potential value of CTCs for prognostic stratification, treatment monitoring, and biomarker assessment in breast, lung, prostate, colorectal, and head and neck cancers, although clinical utility varies considerably among tumor types and platforms. Further standardization and prospective validation, together with integration of metabolic, immune, and multi-omic CTC features, will be essential to establish their role in precision oncology.