Low-grade tracheal adenocarcinoma with ETV6::NTRK3 fusion: unique morphology akin to subsets of sinonasal low-grade non-intestinal-type adenocarcinoma.

The prevalence of NTRK fusions in non-small cell carcinoma (NSCLC) is only approximately 0.2%, most of which harbor NTRK1 fusions. NSCLCs with NTRK3 fusions are extremely rare. Herein, we report a case of low-grade tracheal adenocarcinoma in a 64-year-old woman. Histologically, areas of complicated tubule-papillary or cribriform patterns constituted a major component of the tumor and comprised cuboidal to columnar epithelial tumor cells with pale eosinophilic cytoplasm and cytoplasmic mucin, similar to subsets of sinonasal low-grade non-intestinal-type adenocarcinomas. Immunohistochemically, the tumor was positive for MUC5AC and MUC4 and showed nuclear expression of the pan-Trk antibody. ETV6::NTRK3 was identified by reverse transcription-polymerase chain reaction using formalin-fixed paraffin-embedded tissues. To the best of our knowledge, this is the first case of low-grade tracheal adenocarcinoma with ETV6::NTRK3 fusion. Our case illustrates that low-grade adenocarcinomas with ETV6::NTRK3 fusion may exist throughout the respiratory tract.