Beyond the Brain: Circulating Tumor Cells as a Tool for Diagnosis and Monitoring

Glioblastoma (GBM) is an aggressive brain tumor characterized by extensive heterogeneity and inevitable recurrence. Although extracranial metastases are rare, circulating tumor cells (CTCs) are consistently detectable in patients with GBM, challenging the idea of GBM as a strictly localized disease and suggesting CTCs as a promising liquid biopsy tool.

We analyzed genomic and phenotypic features of bulk CTCs (BCTCs) and single CTCs in 27 patients with GBM. Whole-exome sequencing of BCTCs and matched tumors confirmed tumor origin and assessed shared mutational and copy number alteration (CNA) profiles. Single CTCs isolated with DEPArray NxT underwent low-pass whole-genome sequencing. CTCs counts were compared across primary GBM, recurrent GBM, healthy donors, and non-GBM conditions. Single-cell RNA-seq from primary GBM cell lines was used to validate observations.

BCTCs showed genomic alterations overlapping with those of matched tumors. CTCs were significantly more abundant in primary GBM than in recurrent disease or healthy controls, enabling the definition of a diagnostic cutoff of 4 CTCs per 5 mL of blood. CTCs were absent in the small cohort of radionecrosis cases analyzed, suggesting a potential role for CTCs in distinguishing neoplastic from non-neoplastic lesions, although further validation in larger cohorts is required. Single-cell analysis revealed marked heterogeneity, including CTCs lacking detectable CNAs. Similar CNA-negative populations appeared in scRNA-seq data sets, suggesting possible origin from the tumor microenvironment rather than fully malignant clones.

CTCs are tumor-derived and heterogeneous biomarkers with strong diagnostic potential in GBM. The identified cutoff improves distinction between GBM and non-neoplastic diseases, supporting CTCs as a useful liquid biopsy tool for detection and monitoring. Further studies should clarify the biologic roles of distinct CTCs subpopulations.