Results of the study will clarify that if the circulating tumor DNA (ctDNA) EGFR mutation could be used as a screening method for EGFR-TKI targeted therapy for late stage lung cancer patients, as well as a standard for the therapy efficacy. This study can be used as a standard to assess gefitinib' s efficacy after 8 weeks of treatment in IIIB or stage IV lung cancer through tumor tissue negative, ctDNA positive EGFR mutation.