Third-line Treatment of Gefitinib in NSCLC Patients

* Primary Study Objective: To evaluate the disease control rate (DCR) of gefitinib as third-line retreatment in stage IIIB/IV NSCLC with EGFR 19del/L858R positive mutation patients who had benefited from first-line gefitinib treatment with EGFR 19del/L858R positive mutation and tumor progression after the second-line chemotherapy * Secondary Study Objectives: To evaluate objective response rate (ORR), progressive-free survival (PFS), overall survival(OS) and quality of life (QoL) of gefitinib as third-line retreatment in NSCLC patients To evaluate the safety of gefitinib as third-line treatment in NSCLC patients * Exploratory analyses: To dynamically monitor EGFR mutation status and explore the relationship with clinical outcome