Osimertinib With or Without Bevacizumab for EGFR- Mutant Non-small Cell Lung Cancer With Leptomeningeal Metastasis

Inclusion Criteria: * Age in 18-80 years * Pathologically proven NSCLC * EGFR mutation , the EGFR status was identified from primary lung tumors using the amplification refractory mutation system (ARMS) or next-generation sequencing (NGS) analysis. * LM diagnosis was based on the detection of malignant cells in the CSF, the focal or diffuse enhancement of leptomeninges, and nerve roots or the ependymal surface on gadolinium-enhanced MRI . * No severe abnormal liver and kidney function; * No other severe chronic diseases; * Signed informed consent form Exclusion Criteria: * Patients with the clinical manifestation of nervous system failure including severe encephalopathy, grade III-IV white matter lesions confirmed by imaging examination, moderate or severe coma, and glasgow coma score less than 9 points; * Allergic to osimertinib or bevacizumab * Any of the following: Pregnant women ;Nursing women ;Men or women of childbearing potential who are unwilling to employ adequate contraception * History of myocardial infarction or other evidence of arterial thrombotic disease (angina), symptomatic congestive heart failure (New York Heart Association ≥ grade 2), unstable angina pectoris, or cardiac arrhythmia; Note: allowed only if patient has no evidence of active disease for at least 6 months prior to randomization; * History of cerebral vascular accident (CVA) or transient ischemic attack (TIA)≤ 6 months prior to randomization * History of bleeding diathesis or coagulopathy * History of hemoptysis da≥ grade 2 (defined as bright red blood of at least 2.5 mL) ≤3 months prior to randomization * Leukocytes below 2\*10\^9/L, neutrophils below 1\*10\^9/L; platelets below 50\*10\^9/L; * Had major surgery within 60 days; * History of arteriovenous thrombosis * Gastrointestinal perforator in the past 6 months * Inadequately controlled hypertension (systolic blood pressure of \> 150 mmHg or diastolic pressure \> 100 mmHg on anti-hypertensive medications); Note: history of hypertensive crisis or hypertensive encephalopathy not allowed * Grade 4 proteinuria