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Case Reports

Abstract

We present the case of a 61-year-old woman with a progressively enlarging subcutaneous nodule on her right lower extremity. MRI of the affected leg revealed a solid, enhancing mass within the subcutaneous soft tissues, raising suspicion for a malignant neoplasm. The excised tissue was submitted for histopathologic evaluation. The neoplasm exhibited a well-circumscribed architecture with spindled cells arranged haphazardly, lacking significant cytologic atypia. A prominent inflammatory infiltrate, composed mainly of plasma cells, lymphocytes, and histiocytes was noted. The tumor cells were predominantly spindled in shape and had elongated plump nuclei and minute but distinct nucleoli, as well as a moderate amount of pale eosinophilic cytoplasm. Immunohistochemical analysis revealed tumor cell positivity for CD34, while markers for melanocytic, epithelial, smooth muscle, and ALK proteins were negative. Next-generation sequencing identified an ETV6-NTRK3 fusion, placing this neoplasm within the differential of ALK-negative inflammatory myofibroblastic tumor or the emerging spectrum of NTRK-rearranged spindle cell neoplasms. NTRK-rearranged spindle cell neoplasms are a recently characterized group of tumors defined by activating fusions involving receptor tyrosine kinases—most commonly NTRK1. These rearrangements lead to constitutive MAP kinase pathway activation via chimeric proteins such as LMNA::NTRK1, TPR::NTRK1, or TPM3::NTRK1. The ETV6-NTRK3 fusion is uncommonly reported in spindle cell neoplasms of the skin and soft tissue. These tumors typically affect children and young adults, with a median reported age of 19 years, making this adult case notably unusual. Accurate diagnosis requires molecular confirmation and identification of actionable kinase fusions for appropriate classification and potential targeted therapy with TRK inhibitors.