Cytomorphologic features that distinguish schwannoma from other low-grade spindle cell lesions

Fine-needle aspiration biopsy of spindled mesenchymal lesions, including schwannomas, are challenging because of overlapping cytomorphological features. The objective of the current study was to identify key diagnostic cytological criteria for the diagnosis of schwannoma and to distinguish it from its common mimics.

A total of 58 schwannomas were evaluated and compared with 98 benign and low-grade spindle cell lesions, including 17 gastrointestinal stromal tumors and 20 fibromatosis cases. Biopsy confirmation was available for all cases. The authors semiquantitatively evaluated cellularity, quality of stroma (fibrous and fibrillar), the presence of single cells with bipolar cytoplasmic processes, and marked nuclear pleomorphism (in the absence of other features of malignancy). Nuclear features evaluated included fishhook-type nuclei, intranuclear inclusions, chromatin pattern, and nucleoli.

Schwannomas demonstrated cohesive tissue fragments with fibrillary and occasionally fibrous stroma. The presence of intranuclear inclusions and marked nuclear pleomorphism and the absence of single cells with bipolar cytoplasmic processes emerged as statistically significant differences between the schwannoma and the nonschwannoma groups. When 5 criteria were present (high numbers of clusters, few to no single cells, fibrillary stroma, nuclei with pointed tips, and anisonucleosis) the sensitivity, specificity, positive predictive value, and negative predictive value for the diagnosis of schwannoma were 22%, 97%, 81%, and 68%, respectively.

Cohesive tissue fragments with fibrillary/fibrous stroma, intranuclear inclusions, marked nuclear pleomorphism, and the absence of spindled cells with bipolar cytoplasmic processes are strongly suggestive of schwannoma and assist in excluding potential mimics.