We present a case in which a CCDC6 (coiled-coil domain containing 6)-RET fusion (CCDC6 [1]RET [11] fusion) was detected as an osimertinib-resistant mutation, and a patient was switched from osimertinib to selpercatinib monotherapy, resulting in a rapid onset of carcinomatous meningitis. The gene discussed is RET; the disease is infectious meningitis.