Primary NTRK -rearranged Spindle Cell Neoplasm of the Gastrointestinal Tract: A Clinicopathological and Molecular Analysis of 8 Cases.

NTRK-rearranged spindle cell neoplasm occurs predominantly in the superficial or deep soft tissues of extremities or trunk. Occurrence in the visceral organs is extremely rare. Herein, we describe 8 cases of NTRK-rearranged spindle cell neoplasm that arose primarily in the gastrointestinal tract. Patients included 5 males and 3 females with age at presentation ranging from 6 to 63 years (median: 29.5 years). Tumors occurred in the colon (n=3), small intestine (n=2), rectum (n=2), and stomach (n=1). Tumor size ranged from 3.5 to 9 cm (median: 5 cm). Morphologically, 4 tumors were low-grade, composed of haphazard or intertwining fascicles of spindle cells, with prominent interstitial collagen fibers and ring-like perivascular hyalinization being present in 2 tumors. The other 4 tumors were histologically high-grade sarcomas, consisting of sweeping fascicles of atypical spindle cells showing increased cellularity and brisk mitotic activity. Immunohistochemically, 6/6 cases (100%) showed diffuse and strong cytoplasmic staining of pan-TRK. Variable expression of TrkA, CD34, and S100 was noted in 5/5 (100%), 5/8 (62.5%), and 4/7 (57.1%) cases, respectively. Fluorescence in situ hybridization analysis showed NTRK1 rearrangement (n=7) and NTRK2 rearrangement (n=1). In cases with available materials, RNA sequencing identified LMNA::NTRK1 (n=3), TPM3::NTRK1 (n=2), and STRN::NTRK2 (n=1) fusions. At follow-up (range: 4 to 30 months; median: 12.5 months), 6 of 7 patients who underwent surgery had no evidence of disease at last follow-up. One patient was succumbed to the disease at 12 months despite adjunctive treatment with TRK inhibitor larotrectinib after surgery. One patient was treated with larotrectinib alone. He showed significant response at 7 months after treatment. NTRK-rearranged spindle cell neoplasm represents an exceptionally rare entity in the gastrointestinal tract. The presence of interstitial collagen fibers and ring-like perivascular hyalinization and co-expression of CD34 and S100 are diagnostic clues to low-grade neoplasms. However, high-grade sarcomas pose a considerable diagnostic challenge to pathologists owing to the lack of specific features. The final diagnosis relies on molecular assays. Patients with advanced disease may benefit from TRK inhibitor treatment.