A Study to Predict Gefitinib' s Efficacy for Lung Cancer by Plasma Free Nucleic Acids EGFR Gene Mutation Test

Inclusion Criteria: * Participants in this research must meet the following criteria: histologic diagnosed of lung adenocarcinoma. * Late stage (stage IIIB or IV) lung adenocarcinoma according to the International Union against cancer (UICC) the latest version of standard cancer TNM staging. * tumor tissue EGFR mutation negative, and ctDNA EGFR mutation positive correlated with EGFR TKI sensitivity. * Participants should have signed and dated on the official informed consent. * Not less than 18 years old. * At least 1 lesion can be exactly measured. * Patients whose plasma free nucleic acids EGFR mutation test were positive, and willing to use gefitinib for therapy by their own expense. Exclusion criteria: * Patients already received EGFR-TKI treatment previously. * Patients have pressure on the spinal cord or brain metastases, no symptom, conditions stabled, do not need to use steroid medications for 4 weeks before the start of the study. * Patients with clinical evidence of any symptoms of severe or uncontrolled hypertension, for example, researchers believe that patients with uncontrolled hypertension and active bleeding will affect the study reliability. * Patients with uncontrolled nausea or vomiting, chronic gastrointestinal disease, unable to swallow drugs or underwent major bowel resection may affect the full absorption of gefitinib. * Patients meet any one of the following heart standard: in static State, 3 times ECG check shows average correction QT period (QTc) \>470 msec; clinical abnormal symptom of rhythm, conduction, and static ECG, for example completely left beam conduction block, III degrees conduction block, II degrees conduction block, PR period \>250 msec, various factors with risks of extended QTc or rhythm disorders, for example, congestive heart failure, low potassium blood syndrome, congenital long QT Syndrome, direct relatives has long QT syndrome or sudden unexplained death in less than 40 years old, drug combination that may prolong the QT interval. * Patients Have the following history: ILD, drug-induced ILD, radiation Pneumonitis require steroid treatment, clinically active interstitial lung disease. * Abnormal marrow reservation or organ function. * Women who are breastfeeding. * Fertile male or female, who did not taken effective contraceptive measures, women have been pregnant or breast feeding, or positive pregnancy test prior to study (urine or serum). * Patients don't agree with the study processes, constraints and requirements, and were evaluated by researchers that can't participate in the study.