Answer: Glomangiopericytoma

Histology: The subepithelial connective tissue is infiltrated by a highly cellular proliferation of oval-to-spindled cells. The cells stream between small thin-walled blood vessels. Despite the high cellularity, the tumor cells are very bland and uniform. The cells are not immunoreactive for CD34, actin, S-100, cytokeratin or EMA.

Discussion: Glomangiopericytoma is now the consensus terminology for a low grade spindle cell tumor of the sinonasal tract. The term reflects its HPC-like microscopic appearance along with its presumed origin from a modified perivascular glomus-like myoid cell. On clinical grounds they are usually mistaken for an inflammatory/allergic sinonasal polyp. On histologic grounds they may be confused with various spindle cell mesenchymal neoplasms (e.g. cellular schwannoma, solitary fibrous tumor, 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 or S100.

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).