Liquid Biopsy in Primary CNS Tumors: Bridging Biology, Technology, and Clinical Care

Liquid biopsy has emerged as a minimally invasive approach for the molecular characterization and longitudinal monitoring of primary central nervous system (CNS) tumors. Although extensively validated in systemic malignancies, its clinical application in CNS tumors is challenged by the blood-brain barrier, low analyte abundance, and heterogeneous assay performance. Recent advances have expanded the spectrum of detectable tumor-derived components, including circulating tumor DNA (ctDNA), cell-free DNA, extracellular vesicles, RNA species, nucleosomes, metabolites, and lipids, across multiple biofluids such as cerebrospinal fluid (CSF), plasma, serum, urine, saliva, and tears. The aim of this study was to review the biological foundations, analytes, biofluids, clinical applications, and technical limitations of liquid biopsy in primary CNS tumors, with emphasis on diagnostic, prognostic, and surveillance value. We synthesized current evidence on tumor-derived analytes detectable through liquid biopsy, their molecular correlates, and their performance across biofluids. CSF is consistently the most informative biofluid for CNS tumors, with higher analyte concentration and superior concordance with tumor tissue compared with plasma. CtDNA in CSF reliably identifies hallmark alterations, including IDH1/2, H3K27M, TERT, BRAF, ATRX, TP53, 1p/19q codeletion, and MYCN amplification, thereby enabling better diagnosis, molecular classification, and therapeutic stratification. Plasma-based assays are less sensitive but remain valuable for longitudinal monitoring, especially when combined with ultrasensitive sequencing or fragmentomic approaches. Emerging biomarkers, including nucleosome footprints, exosomes, proteins, microRNA (miRNA)/long noncoding RNA (lncRNA)/circular RNA (circRNA) signatures, lipidomic panels, and metabolites such as D-2-hydroxyglutarate, show potential for integration into multimodal diagnostics. Liquid biopsy provides a powerful and rapidly evolving tool for the minimally invasive molecular assessment of CNS tumors. While CSF remains the optimal matrix for diagnosis and characterization, advances in ultrasensitive detection methods increasingly support the feasibility of plasma and urine for longitudinal follow-up. Integrating liquid biopsy with advanced imaging and tissue-based data will likely transform diagnostic accuracy, therapeutic decision-making, and real-time monitoring of CNS tumors.