BACKGROUND: Cerebrospinal fluid (CSF) stands as an easily accessible reservoir for circulating tumor DNA (ctDNA) analysis in patients with CNS tumors, although evidence is still limited. Our aim was to prospectively evaluate the feasibility of detecting ctDNA for mutational analysis in CSF and plasma in patients with glioma. METHODS: Prospective study of patients with gliomas diagnosed at four third-level hospitals in Spain. A customized next-generation sequencing (NGS) 8-gene panel (IDH1, IDH2, ATRX, TP53, PTEN, PIK3CA, EGFR, BRAF) was used in paired CSF, plasma and tumor samples. Mutation concordance occurred when the same pathogenic gene variant was detected in tumor and ctDNA. The prognostic value of ctDNA and that of its median variant allele frequency (mVAF) were analyzed. RESULTS: Between February 2017 and March 2020, 37 patients with glioma were enrolled. Among 32 patients with analyzable CSF samples: new diagnosis (n=23)
relapse (n=9). WHO 5th Ed: IDH-mut astrocytoma (n=10), IDH-mutant oligodendroglioma (n=6), IDH-wildtype glioblastoma (n=16). CSF-ctDNA-positive (ctDNA+): 19/32 (59%). CSF-ctDNA-negative (ctDNA-): 13/32 (41%). No. of mutations in CSF: 1 (10/19), 2 (7/19), 3 (2/19). Frequency of CSF-ctDNA mutated genes: EGFR (8/19: 42%), PTEN (7/19: 37%), TP53 (6/19: 32%), IDH1 (5/19: 26%), PIK3CA (4/19: 21%). Tumor-CSF mutation concordance: 16/19 (84%). Progression-free and overall survivals were significantly shorter in patients with ctDNA+ ≥ mVAF compared to ctDNA+ <
mVAF. No association was found between ctDNA in CSF and distance to closest CSF reservoir, tumor size or IDH status. ctDNA was detected in 2 out 14 (14%) individual plasma samples, in both cases concordant with the primary tumor. CONCLUSION: CSF is a reliable reservoir for ctDNA analyses in patients with gliomas. ctDNA is detectable in plasma although at a lower rate. Larger, prospective studies should be conducted to refine the potential role of liquid biopsy in this disease.