Malignant peripheral nerve sheath tumor (MPNST) is an uncommon soft tissue sarcoma with a predilection for the extremities and trunk of adults, rarely occurring in the oral cavity. To date, 10 cases of MPNSTs affecting the lower labial mucosa have been reported in the English language literature. We describe an additional case of MPNST in the lower labial mucosa of a 67-year-old male patient, who exhibited a painful swelling with a history of 4 months of evolution. Microscopic examination showed dense plexiform fascicles of spindle cells with wavy atypical nuclei. Mitotic figures and nuclear pleomorphism were evident. Immunohistochemical analysis of tumor cells revealed positivity for S-100 protein, CD56, CD34, and neuron-specific enolase but was negative for neurofilament protein, glut-1, claudin-1, desmin, and smooth muscle actin. Ki-67 labeling was 20%. The final diagnosis was MPNST. The lesion was surgically removed with wide margins, with no signs of recurrence after 4 years of follow-up.
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