Answer: Hemangiopericytoma

Histology: This highly cellular tumor is comprised of oval-to-spindled uniform cells that are tightly packed around endothelial-lined vascular spaces. The vascular channels are eye-catching. They are open, dilated and branching, giving rise to a so-called “staghorn” appearance. Despite the cellularity of this tumor, there is no appreciable mitotic activity or tumor necrosis. Immunohistochemical stains for actin, S-100 protein, and other markers were non-contributory.

Discussion: Hemangiopericytomas usually arise in the deep soft tissues, but they can also involve the sinonasal tract. In these instances, they typically arise in the paranasal sinuses, secondarily extend into the nasal cavity, and present with non-specific nasal obstruction and epistaxis. On clinical grounds they are usually mistaken for an inflammatory/allergic sinonasal polyp. On histologic grounds, they may be confused with several nasal lesions that are characterized by a prominent vascular component including juvenile angiofibroma and nasal glomangioma. Juvenile angiofibromas, however, arise in the nasopharynx and are generally restricted to males in their second decade of life; and glomangiomas have distinct epithelioid cells that are actin positive. Other mesenchymal tumors may also display a prominent HPC-like vascular pattern and should always be considered in the differential of an hemangiopericytoma. These include synovial sarcoma and mesenchymal chondrosarcoma. There is no specific marker for the pericyte from which hemangiopericytomas are believed to arise, and the major role of immunohistochemistry in diagnosing hemangiopericytomas is to exclude other entities.
There is much confusion regarding the appropriate nomenclature for hemangiopericytomas involving the sinonasal cavity. Despite their similarity to soft tissue hemanigiopericytomas, it has been noted that hemangiopericytomas of the sinonasal tract much more consistently behave in a benign fashion – they may recur if inadequately excised, but they do not metastasize. Consequently, it has been suggested that sinonasal hemangiopericytomas may be biologically distinct from their soft tissue counterpart, and hence the term hemangiopericytoma-like tumor of the sinonasal cavity. This distinction is probably erroneous. Many now believe that this more innocent behavior may simply reflect earlier presentation and removal of low-stage tumors, and that given the opportunity for tumor progression they could behave in a more aggressive fashion. Indeed, examples of sinonasal hemangiopericytomas that have metastasized are now well-documented. Malignant behavior is predicted by tumor necrosis and a mitotic rate of greater than 1 mitosis per 10 high-power fields.