A study of patients with CNS malignancies showed that 62% of patients had detectable genomic alterations in cfDNA collected from the cerebral spinal fluid.53 A second study showed that pretreatment ctDNA is detectable in 78% of plasma samples and 100% of CSF samples54; moreover, cases of isolated CNS relapse of DLBCL captured by plasma ctDNA have been reported.45 The MYD88 L265P mutation is a characteristic of PCNSL and ddPCR assays probing the hotspot MYD88 L265P mutation have a sensitivity rate of 60% in plasma samples39 and could potentially be used on CSF samples. This evidence concerns the gene MYD88 and primary central nervous system lymphoma.