The purpose of this study is to evaluate the relevance of intratumoral washing for detection of EGFR mutation (including T790M positivity).

This is a prospective, single-arm, open-label study to assess evaluate the relevance of intratumoral washing by ultrathin bronchoscopy (outer diameter; 3mm) for detection of EGFR mutation (including T790M positivity) using cobas real-time PCR and droplet digital PCR (DDPCR) in patients with NSCLC.

Inclusion Criteria:

• Age ≥ 20 years
• Obtained written informed consent
• Patients diagnosed with NSCLC by histology or cytology and inoperable stage IV at the time of study enrollment
• Patients with the following EGFR gene mutations: E19Del, L858R alone or concurrent rare EGFR gene mutations (T790M, G719X, exon 20 insertion, S768I)
• Patients previously treated with EGFR-TKIs such as gefitinib, erlotinib, afatinib, dacomitinib as first line therapy
• Patients who had shown clinical benefits (CR, PR, SD) from EGFR-TKIs and had been confirmed PD on those therapy according to RECIST v 1.1.
• Patients who underwent liquid biopsy (plasma) for EGFR mutation at the time of PD on EGFR-TKIs
• Patients who plan to undergo tissue biopsy for EGFR mutation at the time of PD on EGFR-TKIs

Exclusion Criteria:

• Patients who withdraw informed consent
• Patients who are unable to undergo liquid biopsy (plasma) and tissue biopsy for EGFR mutation based on the investigator's judgement