LAUSR

Mesenchymal tumors driven by NTRK fusions are clinically and morphologically heterogeneous. With an increasing number of clinicopathological entities being associated with NTRK fusions, the diagnostic and predictive value of the identification of NTRK fusions is uncertain. Recently, mesenchymal tumors in the gastrointestinal tract with NTRK fusions were described as gastrointestinal stromal tumors (GIST), but the nosology of such neoplasms remains controversial. We report eight mesenchymal tumors involving the gastrointestinal tract with NTRK1 or NTRK3 rearrangements. The tumors occurred in six children and two adults, five males and three females (age range 2 months–55 years; median 3.5 years), and involved the small intestine ( n  = 4), stomach ( n  = 2), rectum ( n  = 1), and mesentery ( n  = 1). Clinical outcomes were variable, ranging from relatively indolent ( n  = 2) to aggressive diseases ( n  = 2). Morphologically, the tumors were heterogeneous and could be classified in the following three groups: (1) infantile fibrosarcoma involving the gastrointestinal tract ( n  = 4), enriched for NTRK3 fusions; (2) low-grade CD34-positive, S100 protein-positive spindle-cell tumors, associated with NTRK1 fusions ( n  = 2); and (3) unclassified high-grade spindle-cell sarcomas, with NTRK1 fusions ( n  = 2). By immunohistochemistry, the tumors demonstrated diffuse pan-TRK expression, of variable intensity, and lacked a specific line of differentiation. Four cases expressed CD34, which was coexpressed with S100 protein in three cases. Expression of SOX10, KIT, and DOG1 was consistently absent. Molecular genetic testing identified TPM3–NTRK1 ( n  = 3), TPR–NTRK1 , LMNA–NTRK1 , and ETV6–NTRK3 ( n  = 2), and SPECC1L–NTRK3 in-frame gene fusions. We conclude that the evaluation of mesenchymal spindle-cell neoplasms of the gastrointestinal tract without a definitive line of differentiation should include interrogation of NTRK alterations, particularly in pediatric patients. Mesenchymal tumors of the gastrointestinal tract with NTRK rearrangements are clinically and morphologically heterogeneous, and few, if any, seem related to GIST.