Multimodal Liquid Biopsy Improves Longitudinal Circulating Tumor DNA Detection in Children with Central Nervous System Tumors

Background: Circulating tumor DNA (ctDNA) is a promising biomarker for pediatric central nervous system (CNS) tumors, but limitations in sensitivity and universally accepted detection criteria remain. We evaluated multimodal ctDNA detection using a novel composite scoring framework integrating mutation, copy number, and fragmentomic analysis in longitudinal plasma and CSF samples from children with CNS tumors. Methods: Tumor, blood and/or CSF samples were collected from pediatric CNS tumor patients. DNA was extracted from plasma, CSF, tumor, and whole blood. All samples underwent targeted and whole genome sequencing. Plasma and CSF samples were evaluated for ctDNA positivity--defined by a composite score [≥]2--utilizing a tri-modal approach of whole genome sequencing for copy number alterations and fragmentomics and targeted sequencing. Results: The cohort included 28 patients: medulloblastoma (39%), diffuse midline glioma (25%), high-grade glioma (18%), pilocytic astrocytoma (11%), and ependymoma (7%). A total of 140 plasma and 27 CSF samples were collected longitudinally. Combining all three modalities identified ctDNA in 70% of CSF and 17% of plasma samples, exceeding rates achieved by any single modality alone. Post-diagnosis plasma ctDNA positivity was significantly associated with worse progression-free and overall survival. CSF demonstrated significantly higher ctDNA signal intensity than plasma across all assay modalities. Conclusions: A multimodal composite scoring approach improves longitudinal ctDNA detection in pediatric CNS tumors. Although CSF remains the most sensitive biofluid, plasma ctDNA reflects disease progression in select patients. These findings support integrated, multimodal liquid biopsy strategies for disease monitoring and highlight the need for prospective validation of standardized detection criteria.