Patients with extensive-stage small cell lung cancer receive six cycles of chemotherapy(etoposide plus platinum) as first-line treatment,who achieve Complete Response(CR)/Partial Response(PR) will accept small doses of etoposide as maintenance treatment. The objective of this study is to evaluate the progression free survival,overall survival,objective response rate,disease control rate and safety of etoposide as maintenance therapy. Based on previous studies on maintenance therapy in small cell lung cancer,the hypothesis of this study is maintenance therapy using etoposide may improve progression free survival,overall survival for selected patients. The investigators will use the peripheral blood to assess circulating tumor cell and cell-free DNA,which may help us to screen a subgroup of patients with better response to etoposide maintenance therapy.

Inclusion Criteria:

• Pathological diagnosis of small cell lung cancer
• Extensive stage according to American Veteran Staging System
• At least one measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST )1.1
• Eastern Cooperative Oncology Group (ECOG) scores for performance status of 0 to 2
• Age≥18 years
• Adequate organ function
• Consent form according with Guideline for Good Clinical Practice of the International Conference on Harmonisation (ICH-GCP)guidelines
• The expected survival≥2 months

Exclusion Criteria:

• Pathological diagnosis of mixed small cell lung cancer
• Limited stage according to American Veteran Staging System
• Other preexisting or existing malignant tumors,not including non-melanoma skin cancer with effective treatment,cervical cancer in situ or tumor response has been 3 years which is considered cured by the investigator
• Known pre-existing interstitial lung disease
• Pre-existing or uncontrolled gastrointestinal disease which may affect drug absorption or worsen existing disease believed by the investigator
• Any other medical history or coexisting disease that may affect compliance of patients or the assessment of safety and effectiveness of the drug believed by the investigator
• Pregnant or lactating women
• Active phase of hepatitis B virus infection,Active phase of hepatitis C virus infection or known HIV carriers