The primary purpose of this study is to compare the positive rate between the cell-free DNA and cytological examination of cerebrospinal fluid in Advanced lung cancer with leptomeningeal metastases.

Inclusion Criteria:

1. Patient who was confirmed primary lung cancer by pathologic histology or cytology
2. Adult patients (≥18 years and ≤75 years). ECOG Performance Status 0 or 1.Life expectancy of at least 12 weeks.
3. Clinical symptoms and imaging tests are highly suspicious of advanced lung cancer with leptomeningeal metastases
4. Without contraindications to lumbar puncture

Exclusion Criteria:

1. Patients who have primary benign or malignant brain tumor and metastatic malignant brain tumor
2. Patients who have contraindications to lumbar puncture
3. Serious uncontrolled systemic disease including active infection,uncontrolled hypertension,diabetes,unstable angina,congestive heart failure,myocardial infarction,severe arrhythmia which needs drugs,hepatic、renal and metabolic disease

Clinical symptoms and imaging tests are highly suspicious of advanced lung cancer with leptomeningeal metastases