Circulating Tumor Cells as Predictive Biomarkers and Living Biospecimens for Cancer Monitoring and Tumor Biology Profiling

Background: Circulating tumor cells (CTCs), as intact viable cancer cells shed into the bloodstream, offer unique opportunities for early cancer detection and real-time tumor monitoring. This review evaluates the clinical relevance of CTCs across various tumor types, emphasizing their predictive value and potential as dynamic biospecimens. Methods: A comprehensive literature search was performed across peer-reviewed databases to identify studies examining CTCs as predictive biomarkers and functional biospecimens. Both clinical trials and translational research addressing CTC-based detection, enumeration, and phenotyping were included. Results: Key findings demonstrate that CTC detection can precede traditional biomarkers and imaging modalities, offering earlier insights into disease progression and therapeutic response. In high-risk hepatitis patients, CTCs preceded HCC diagnosis by months; in NSCLC, in vivo CTC capture identified patients with early recurrence; and in breast cancer, CTC-based melatonin receptor (MT1) profiling revealed subtype-dependent receptor patterns that suggest timing and biological context likely influence CTC-based biomarker interpretation. The article also explores the integration of CTC phenotyping with receptor profiling, circadian biology, and host-response markers to refine risk stratification and personalize oncologic care. Conclusions: A translational roadmap is proposed to enhance the clinical adoption of CTC-based strategies through standardized methodologies and multimodal biomarker integration.