Study of the efficacy and safety of maintenance Irinotecan in patients with extended small cell lung cancer after 4 courses of Cisplatin plus Irinotecan chemotherapy

INTERVENTION: Four cycles of Cicplatin(60mg/m2 day1)and Irinotecan(60mg/m2 day1, 8,15) followed by maintenance Irinotecan(CPT‐11)(60mg/m2 day1, 8,15) until disease progression. CONDITION: extended small cell lung cancer PRIMARY OUTCOME: feasibility at 6 months SECONDARY OUTCOME: overall survival; progression‐free survival; response rate; rate of adverse events INCLUSION CRITERIA: 1)Pathologically diagnosed as small cell lung cancer 2)Diagnosed as extended small cell lung cancer based on chest CT, abdominal CT, bone scintigraphy or brain MRI at the onset of disease. 3)Age 20=<,70=> 4)ECOG PS0or1 5)Measurable or nonmeasrurable lesions 6)CR or PR is achieved after the prior chemotherapy of Cisplatin (60mg/m2) and Irinotecan(60mg/m2 day1,8,15) . 7)Administered at least 9 times Irinotecan in prior IP chemotherapy 8)Registered after irradiated PCI, in the case of brain metastasis was not found before the prior chemotherapy. PCI is not always required. 9)Registered after irradiated WBRT(3Gy/Fr total 30Gy) after the prior IP chemotherapy , in the case of brain metastasis was found before the prior chemotherapy. WBRT is required. 10)No history of chemotherapy for other than lung cancer, and radiotherapy or surgery for primary lung cancer. 11)Adequate organ function, evaluated within 14 days before enrollment a