Driver mutation detected in cerebrospinal fluid despite negative liquid biopsy results.

We report a case where a KRAS G12V driver mutation was identified in cerebrospinal fluid (CSF) but not in peripheral blood cell-free DNA (cfDNA) in a patient with advanced lung adenocarcinoma and significant central nervous system involvement. A 67-year-old man presented with hemoptysis and was diagnosed with stage IVB TTF-1-positive lung adenocarcinoma with brain and bone metastases. Standard chemotherapy was ineffective. While cfDNA analysis detected only a RAD21 mutation, CSF analysis revealed the KRAS G12V mutation. Despite the identification, no effective targeted therapy was available. This case highlights that CSF may be more suitable than peripheral blood cfDNA for detecting driver mutations in patients with predominant CNS lesions.