Aberrant Desmin Expression in a Middle Ear Neuroendocrine Tumor: Expanding the Immunophenotypic Spectrum.

Middle ear neuroendocrine tumors (NETs) are uncommon neoplasms that exhibit both epithelial and neuroendocrine differentiation. Aberrant desmin expression in middle ear NET has not been documented previously and may create a diagnostic pitfall due to morphologic overlap with myogenic and rhabdoid tumors. A 52-year-old man presented with right ear pain, fullness, and facial deviation. Imaging revealed a soft-tissue mass involving the middle ear cavity and the mastoid air cells, with ossicular erosion. Histopathology showed an infiltrative neoplasm arranged in nests, trabeculae, cords, and pseudoacini composed of monomorphic cells with plasmacytoid/rhabdoid morphology. Immunohistochemically, the tumor cells showed diffuse positivity for KRT, synaptophysin, chromogranin, and diffuse strong desmin expression, while myogenin, MYOD1, p63, calponin, TTF1, CDX2, and GATA3 were negative. SMARCB1/INI1 expression was retained, and the Ki67 index was <2%. A diagnosis of middle ear NET with aberrant desmin positivity was rendered. The patient remained disease-free 18 months following surgical excision. The index patient expands the immunophenotypic spectrum of middle ear NET by documenting diffuse aberrant desmin expression. Awareness of this unusual finding is important to avoid misdiagnosis as rhabdomyosarcoma or other rhabdoid neoplasms. Accurate diagnosis requires careful integration of morphologic and immunohistochemical features, and long-term follow-up remains essential despite the generally indolent behavior of middle ear NETs.