Immunophenotype of Pediatric NTRK fusion mesenchymal tumors

Background: While ETV6-NTRK3 fusion is common in infantile fibrosarcoma, NTRK1/3 fusion in pediatric tumors is scarce and, consequently, not well known. Herein, we evaluated NTRK1/3 fusion pediatric mesenchymal tumors clinicopathologically and immunophenotypically. Methods: We reviewed nine NTRK -fusion pediatric sarcomas confirmed by fluorescence in situ hybridization and/or next-generation sequencing from Seoul National University Hospital between 2002 and 2020. Results: One case of TPR - NTRK1 fusion-positive intracranial extra-axial high-grade undifferentiated sarcoma (12-year-old boy), one case of LMNA-NTRK1 fusion-positive low-grade infantile fibrosarcoma of the forehead (3-year-old boy), one case of ETV6-NTRK3 fusion-positive inflammatory myofibroblastic tumor (IMT) (3-months-old girl), and six cases of ETV6-NTRK3 fusion-positive infantile fibrosarcoma (median age: 2.6 months, range: 1.6–5.6 months, M: F = 5:1) were reviewed. The Trk immunopositive pattern was distinctive according to the fusion genes. We notified nuclear positivity in TPR-NTRK1 fusion sarcoma, nuclear membrane positivity in LMNA-NTRK1 fusion sarcoma, and both cytoplasmic and nuclear positivity in ETV6-NTRK3 fusion-positive IMT and infantile fibrosarcomas. Also, the TPR-NTRK1 fusion sarcoma showed robust positivity for CD34/nestin and high mitoses. The LMNA-NTRK1 fusion sarcoma revealed CD34/S100 protein/nestin/CD10 coexpression, and a low mitotic rate. The IMT with ETV6-NTRK3 fusion expressed SMA. Six infantile fibrosarcomas with ETV6-NTRK3 fusion showed variable coexpression of nestin (6/6)/CD10 (4/5)/ S100 protein (3/6). Conclusions: All cases of NTRK1 and NTRK3 fusion-positive pediatric tumors robustly expressed the Trk protein. A Trk immunopositive pattern and CD34/S100/nestin/CD10/SMA immunohistochemical phenotype may suggest NTRK fusion partner genes and diagnoses. LMNA-NTRK1 fusion sarcoma might be a low-grade subtype of infantile fibrosarcoma. Interestingly, more than half of the infantile fibrosarcoma cases were positive for S100 protein and CD10. The follow-up period of TPR-NTRK1 and LMNA-NTRK1 fusion tumors are not enough to predict prognosis. However, ETV6-NTRK3 fusion infantile fibrosarcomas showed an excellent prognosis with no evidence of disease for an average of 11.7 years after gross total resection of the tumor.