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Malignant peripheral nerve sheath tumors (MPNSTs) are rare sarcomas of the nerve sheath that infrequently involve the head and neck region. The few reports of MPNSTs arising from benign schwannoma precursors typically exhibit epithelioid morphology. We describe an exceptional case of a conventional MPNST arising in a benign trigeminal nerve schwannoma of a 21-year-old man without neurofibromatosis or familial schwannomatosis. Histologically, the tumor consisted of spindle-shaped cells in a fascicular and focal whorling growth pattern, with extensive geographic necrosis, cytologic atypia, rare atypical mitotic figures and focal histologic features of cellular schwannoma. Immunohistochemically, the tumor cells showed strong and diffuse S100 positivity and loss of SMARCB1. Subsequent genomic sequencing revealed a single nucleotide deletion (c.1148delC) in the SMARCB1/INI1 gene Diffuse S100 positivity and/or loss of SMARCB1 expression is extremely rare in conventional MPNST. We discuss diagnostic considerations for MPNSTs and review the literature concerning treatment of these rare neoplasms.

Malignant Peripheral Nerve Sheath Tumor with Loss of SMARCB1 and Diffuse S1000 Expression Arising in a Trigeminal Nerve Schwannoma