Abstract
Circulating tumor cells (CTCs) represent a small but clinically relevant pool of cells from tumors that can be sampled with minimally-invasive liquid biopsy procedures. They are dynamic and poorly-defined transition state of cancer cells, offering vital insights into tumor progression and metastasis. CTC frequencies are emerging as real-time means for therapeutic monitoring and patient stratification across different malignancies. However, their detection, isolation, and characterization pose a major challenge leading to variation in their counts that limit their clinical utility. This review delves into the key parameter, CTC counts, which often correlate with clinical outcomes. Further, it highlights the significance of culturing CTCs in vitro and employing CTC-derived xenograft (CDX) models to obtain in vivo insights into tumor biology, treatment efficacy, and personalized medicine strategies. The review examines the role of CTCs as diagnostic, prognostic, and therapeutic monitoring marker for different cancers. It also discusses the opportunities in CTC-directed clinical utility and associated limitations such as CTC heterogeneity and technological variations among different isolation strategies that hinder their widespread implementation. Hence, it underscores the evolving importance of CTCs in clinical oncology.