Cytologic description:
This is a hypercellular spindle cell lesion with large cohesive fragments composed of spindled cells. Most of the nuclei are oval and mostly uniform in size. Few nuclei are larger but no mitosis or necrosis is appreciated. These features suggest that the lesion is a benign neoplasm of mesenchymal origin.

Differential diagnosis:
The top differential diagnosis given the morphology and location include leiomyoma, schwannoma, and gastrointestinal stromal tumor (GIST). A malignant counterpart of these lesions is less likely, as malignant mesenchymal tumors tend to be more discohesive, pleomorphic, mitotically active and necrotic. Leiomyoma tends to have cigar-shaped nuclei with indentation. Schwannoma tends to form large cohesive fragments and have wavy nuclei with palisading. Although characteristic cytomorphologic features may suggest a specific entity or line of differentiation, a definitive distinction depends on immunohistochemical studies. A reasonable panel in this case includes desmin for smooth muscle origin, c-kit and DOG-1 for GIST, and S100 for schwannoma. In this case, S100 was diffusely positive while the other markers were negative, supporting a diagnosis of schwannoma.