IntroductionApproximately one-half of patients with epidermal growth factor receptor (EGFR) mutation-positive advanced/metastatic non-small-cell lung cancer (NSCLC) develop resistance to first- or second-generation EGFR tyrosine kinase inhibitors (TKIs) due to a secondary T790M mutation. This study investigated the pattern of T790M testing after EGFR TKI treatment in a real-world setting in Japan.MethodThis prospective observational study enrolled patients with EGFR mutation-positive advanced/metastatic NSCLC who reported disease progression during treatment with first- or second-generation EGFR TKIs. Data regarding sampling methods for T790M mutation testing (plasma sample, cytology or tissue biopsy) and the treatment strategies after disease progression were recorded prospectively.ResultsA total of 236 patients were included in the study (female, 67.4%; median age, 73.0 years), and 205 patients (86.9%) underwent rebiopsy by any of the three possible methods: plasma sampling in 137 patients (58.1%) and tissue/cytology sampling in 68 patients (28.8%) during the first rebiopsy. Overall, 80.6% of the tissue/cytology samples contained tumor cells, and 40% of these samples were positive for the T790M mutation. T790M mutations were detected in only 19.7% of plasma samples. Of the 199 patients who underwent T790M testing, 61 (30%) tested positive, and 56 (91.8%) subsequently received osimertinib.ConclusionAmong the 87% of Japanese patients who underwent rebiopsy after progressing on treatment with a first- or second-generation EGFR TKI, approximately 30% tested positive for the T790M mutation and were eligible to receive osimertinib. Although plasma sampling is non-invasive, this rebiopsy method is less sensitive for T790M detection compared with tissue or cytology sampling (UMIN identifier: UMIN000024928).FundingAstraZeneca Japan.