After discontinuing osimertinib and switching to selpercatinib, rapid onset of cancerous meningitis was observed, and the lack of amplification of the CCDC6 [1]RET [11] fusion boundary by qRT-PCR (quantitative reverse-transcription polymerase chain reaction) using spinal fluid-derived cell-free DNA suggested that the brain metastases were likely predominant EGFR exon 19 p. E746_A750del mutated clones. The gene discussed is RET; the disease is malignant tumor of meninges.