Case report of three EGFR TKI naïve lung adenocarcinoma containing double EGFR mutations (L858R/T790M or Exon 19 Deletion/T790M); Comparing genetic information and histology.

EGFR T790M mutation is a crucial gene alteration causing EGFR TKI resistance. However, the implication of T790M mutation is still unknown for the stepwise progression of EGFR TKI naïve lung adenocarcinoma. In this study, we studied site-related EGFR T790M mutation analysis in EGFR TKI naïve lung adenocarcinomas harboring double EGFR mutation (L858R and T790M or Exon 19 deletion (Del.19) and T790M) by droplet digital (dd) PCR method. We examined three resected lung adenocarcinoma cases harboring EGFR double mutation including T790M. These cases didn't receive EGFR TKI treatment. We divided formalin-fixed and paraffin embedded (FFPE) unstained slide tissues into 11-18 areas in each tumor and extracted DNAs from each area separately. The DNAs were analyzed by ddPCR. T790M mutation ratio (T790M/L858R or T790M/Del.19) were calculated. For three cases, we also performed EGFR FISH for analyzing EGFR copy number.In Case 2 and 3, T790M mutation ratio were 100% and 30% homogeneously and showed increased EGFR copy number also homogeneously. However, in case 1, it was different between invasive and non-invasive areas. EGFR copy number was also heterogeneous and showed increasing only in invasive area. We indicated a peculiar case harboring T790M heterogeneity and only invasive area had T790M mutation even though the case was not treated by EGFR TKI. It suggests that T790M is possibly significant not only for EGFR TKI resistance but also the progression in lung adenocarcinoma.