The Dynamic Monitoring of Cerebrospinal Fluid ctDNA

Inclusion Criteria: * Advanced NSCLC patient with EGFR mutation, associated with leptomeningeal metastasis * Patients who meet the requirements of OMMAYA cystlateral ventricle chemotherapy * Age \> 18 years * Complete serologic tumor markers (CEA, SCC) and imaging data (enhanced CT and/or MRI, PET-CT) * Liver, kidney, and hematologic measures were normal (as measured by laboratory testing within 1 week before enrollment in the absence of ongoing supportive care), with a neutrophil count of more than 1.5×109/L, a platelet count of more than 100×109/L, and a hemoglobin level of more than 9.0g/dl ，Bilirubin normal or \<1.5×ULN; AST(SGOT), ALT(SGPT) \<2.5×ULN; Serum creatinine \<1.5×ULN * The patients were fully aware of the study, provided voluntary written informed consent, and were able to adhere to the protocol-defined visit and follow-up procedures Exclusion Criteria: * Patients who do not meet the requirements of lateral ventricular chemotherapy * History of allergy to vormetinib and pemetrexed * Severe complications occurred during the treatment * Known history of allogeneic organ transplantation and allogeneic hematopoietic stem cell transplantation * Severe infection in active stage or with poor clinical control * Mentally ill, substance abusers and pregnant or lactating women * No informed consent was signed * Eligibility as judged by the other investigators