Malignant melanotic nerve sheath tumor in pleural effusion: Deceitful cytology with significant repercussions.

Malignant melanotic nerve sheath tumor (MMNST) is an exceedingly rare and aggressive neoplasm of Schwann cell origin that has seldom been described in the cytopathology literature. Herein we present a case of a 60-year-old female with a 3.8 cm presacral mass that was diagnosed as a MMNST. A molecular workup demonstrated TERT promoter -124C > T and TET2 Q891* gene mutations. Within 2 years of her initial diagnosis, she had developed widespread metastasis and pleural effusions. A cytologic workup of the pleural fluid revealed clusters of vacuolated epithelioid cells with enlarged nuclei, prominent nucleoli, and occasional multinucleation. The lesional cells were positive for SOX10, S100-protein, Melan-A, and HMB45, while negative for Calretinin, MOC31, and monoclonal CEA. In this clinicopathologic context, a diagnosis of metastatic MMNST was rendered. Awareness of this entity and its clinical presentation, along with a critical understanding of its molecular findings and that of imitators, is crucial in achieving an accurate diagnosis.