 Patients without enough tissue for EGFR mutation detection can also have opportunity to use EGFR-TKI as first line therapy. Meanwhile, this study was also proposed to explore the best intervention time of anti-resistant drugs such as AZD 9291, a T790M inhibitor through the quantitative and dynamic analysis of EGFR sensitive and resistant mutation in plasma cf-DNA during EGFR-TKI treatment process. 