Pan-Trk immunohistochemistry reliably identifies ETV6-NTRK3 fusion in secretory carcinoma of the salivary gland

Secretory carcinoma of the salivary gland is a newly recognized entity that morphologically resembles breast secretory carcinoma and has a characteristic t(12;15)(p13;q25) ETV6-NTRK3 translocation. Fluorescence in situ hybridization (FISH) or reverse transcription polymerase chain reaction (RT-PCR) analyses can detect the ETV6-NTRK3 fusion; however, both tests are expensive and not widely available. In this study, we aimed to determine whether pan-Trk immunohistochemistry (IHC) could detect ETV6-NTRK3 fusions as reliably as RT-PCR and FISH. We performed pan-Trk IHC in 70 salivary gland cancer samples, including secretory carcinomas, acinic cell carcinomas, and hybrid carcinomas. Nineteen tumors exhibited positive pan-Trk staining, including 16 secretory carcinomas, 2 hybrid carcinomas with a secretory carcinoma component, and 1 acinic cell carcinoma. Pan-Trk IHC staining was localized in the nucleus in 16 (84.2%) cases and in the cytoplasm and/or membrane in 3 (15.8%) cases. RT-PCR analysis for the ETV6-NTRK3 transcript was conducted in 45 samples; the fusion transcript was present in 11 of 12 secretory carcinomas and absent in 32 acinic cell carcinomas and 1 mucoepidermoid carcinoma. Pan-Trk IHC was positive in 10 of 11 salivary tumors that were positive for ETV6-NTRK3 by RT-PCR and negative in all 34 tumors that were negative for the fusion by RT-PCR. Therefore, in comparison with RT-PCR, pan-Trk IHC had a sensitivity of 90.9% and specificity of 100%. In conclusion, our data showed that pan-Trk IHC is a reasonable screening test for diagnosing secretory carcinoma of the salivary gland.