Answer: Glomangiopericytoma

Histology: The surface epithelium is intact. The subepithelium is expanded by sheets of loosely packed cells with round to oval nuclei and lightly eosinophilic cytoplasm. Despite the high cellularity, the tumor cells are very bland and uniform. Mitotic figures are not observed. Small intervening blood vessels exhibit perivascular hyaline. The tumor cells are immunoreactive for actin and factor XIIIa; and they are not immunoreactive for CD34 or EMA.

Discussion: Glomangiopericytoma is now the consensus terminology for a low grade spindle cell tumor of the sinonasal tract. The term reflects its hemangiopericytoma (HPC)-like microscopic appearance along with its presumed origin from a modified perivascular glomus-like myoid cell. On histologic grounds they may be confused with various mesenchymal neoplasms including meningioma, cellular schwannoma, solitary fibrous tumor, and soft tissue HPC. Histologically, the tumor is characterized by a proliferation of uniform round to spindled cells with interspersed vascular channels. Immunohistochemistry is often very useful in establishing the diagnosis. Glomangiopericytomas are uniformly immunoreactive for Factor XIIIA and actin; and they are not immunoreactive for CD34, S100 or EMA.

Despite its morphologic resemblance to soft tissue HPC, glomangiopericytoma of the sinonasal tract more consistently behave in a benign fashion – they may recur if inadequately excised, but they do not metastasize. Consequently, it is now accepted that glomangiopericytoma and soft tissue HPC are biologically and clinically distinct. Although very rare, truly malignant glomangiopericytomas have been reported. Parameters used to identify malignancy include large tumor size (>5 cm), bone invasion, tumor necrosis, and increased mitotic activity (>4 mitoses per 10 hpf).