Using pan-TRK and RET Immunohistochemistry for the Detection of Fusion Types of Salivary Gland Secretory Carcinoma

Secretory carcinoma (SC) is a low-grade salivary gland carcinoma characterized by recurrent ETV6 rearrangements. Most cases have ETV6-NTRK3 fusions, while the minority of cases have non-NTRK3 fusions, including ETV6-RET and ETV6-MET. Detection of the fusion partner has become important, as there are TRK or RET inhibitors that may benefit patients with advanced SC. Currently, there are different methods to detect gene rearrangement in SCs, such as next-generation sequencing, reverse transcription-polymerase chain reaction, or fluorescence in situ hybridization. Immunohistochemistry (IHC) has greater accessibility, quick turnaround time, and can serve as a screening tool for confirmatory molecular tests. Pan-TRK and RET antibodies have been used to detect gene fusions in different tumors. Here, pan-TRK and RET IHC assays were performed on 28 salivary gland SCs, including 27 cases with ETV6-NTRK3 and one with ETV6-RET fusion confirmed by fluorescence in situ hybridization. Pan-TRK staining was positive in 26/27 (96.3%) of NTRK3 fusion-positive SCs with a nuclear staining pattern in more than 50% of tumor cells, and negative in the RET-rearranged case. RET IHC showed positive staining in most cases (26/28), but only three cases (including the RET-rearranged case) had diffuse and strong staining. RET IHC can be considered an effective screening test when diffuse/strong reactivity is present in pan-TRK IHC-negative cases. This study showed that pan-TRK staining has high sensitivity and specificity for SC with NTRK3 fusion. Whereas pan-TRK IHC is a useful screening method, further studies are needed to assess the value of RET IHC as a second sequential step.