A novel EML4-NTRK3 fusion in lung adenocarcinoma with dramatic response to entrectinib.

In-frame fusions in NTRK genes, with intact kinase domain, have been reported to occur at higher frequencies in rare tumors like infantile fibrosarcoma, congenital mesoblastic nephroma, and secretory carcinoma, whereas they occur at very low frequencies in common malignancies like NSCLC and colon cancers (0.1%-1%). Despite the rare occurrence, these alterations have gained importance owing to approval of drugs like entrectinib and larotrectinib targeting the kinase domain of the gene. More than 50 fusion partners have been described, and only in-frame fusions result in constitutive ligand-independent kinase activity leading to oncogenesis. The commonly reported NTRK fusions in the lung include SQSTM1-NTRK1, ETV6-NTRK3, and SQSTM1-NTRK3. Detection of these rests on the use of conventional modalities like Immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH); however, accurate characterization requires direct sequencing methods. We report an interesting case of an NTRK fusion-positive NSCLC, exhibiting good response to entrectinib.