Use of circulating tumor DNA to guide treatment of primary central nervous system lymphoma: a case report.

Primary central nervous system lymphoma (PCNSL) is a rare tumor that can be difficult to diagnose as it can mimic other CNS diseases. Tissue biopsy is the gold standard for diagnosis, but this is not always feasible if the lesions are in deep structures of the brain. Tissue samples may also be non-diagnostic if steroids have been administered prior to the procedure. Genomic testing of circulating tumor DNA (ctDNA) in the cerebrospinal fluid is a technique that may provide an alternative method for diagnosis. Prior case reports and studies have shown a correlation between PCNSL and MYD88 mutation, but patients were treated only in the setting of a diagnostic biopsy. Here we present a case of a patient with inconclusive biopsy results whose CSF ctDNA revealed a MYD88 mutation suggestive of PCNSL and guided her treatment with high-dose methotrexate.