Liquid biopsy is emerging as an essential tool in tumor monitoring and a potential alternative and supplement to tissue biopsy for tumor genotyping, especially in relapsed or metastatic diseases. Liquid biopsy methods for detecting T790M in ctDNA can be qualitative or quantitative, including amplification refractory mutation system PCR (ARMS-PCR), digital droplet polymerase chain reaction (ddPCR), and next generation sequencing (NGS)-based methods. Comparison of multiple detecting platform for EGFR mutations in plasma samples has been undertaken in studies to determine the most feasible assay in clinical practice. In this study, we will investigate the usefulness of ddPCR for quantitative detection of EGFR T790M mutation in peripheral blood, and compared the utility of ddPCR and NGS for guiding decisions regarding osimertinib therapy in NSCLC patients who had develop resistance to first- or second generation EGFR-TKIs.

Inclusion Criteria:

• histologically/cytologically confirmed stage IV NSCLC patients harboring sensitizing EGFR mutations;
• progression from first or second-generation EGFR-TKI confirmed by radiographic imaging;
• received both NGS test and ddPCR from peripheral blood simultaneously.

Exclusion Criteria:

• progression from first- or second-generation EGFR-TKIs as adjuvant therapy;
• having received third-generation EGFR-TKIs prior to the gene tests;
• having received NGS test only or ddPCR test only.