Answer: Nasopharyngeal angiofibroma

Histology: The histopathologic picture is dominated by two components – a vascular component and a fibrous component. The blood vessels have thin walls with absent or incomplete smooth muscle. They are lined by a single layer of endothelial cells. The lumens are compressed or open forming staghorn configurations. The stroma consists of spindled to stellate cells in a collagenized background. The stellate cells are uniformly dispersed throughout the collagenized stroma, and they lack significant atypia and mitotic activity. An immunohistochemical stain for beta-catenin shows strong nuclear staining in the stromal cells.

Discussion: Nasopharyngeal angiofibromas are uncommon tumors that typically arise in the nasopharynx/posterior nasal cavity of adolescent males. Their growth and development appear to be hormonally (testosterone) driven. Females rarely if ever develop these tumors. Most cases occur in the second decade of life, and they are rarely encountered after the age of 30.
The histologic hallmark feature of nasopharyngeal angiofibroma is the intimate admixture of thin-walled vessels and collagenous stroma. For those pathologists familiar with the characteristic histopathologic picture of nasopharyngeal angiofibroma, they are rarely confused with other spindle cells lesions of the sinonasal tract. For example, they lack the dense cellularity and mitotic activity of fibrosarcoma. Likewise, hemangiopericytomas tend to be much more cellular tumors with less intervening collagen. Indeed, the presence of individual stellate cells scattered within a collagenized stroma would be unusual for a hemangiopericytoma.
Diagnostic problems sometimes arise when the clinical picture is atypical for angiofibroma. In this present case, the age of the patient and location of the tumor was unusual for angiofibroma (As it turns out, the tumor was in fact arising from the nasopharynx and was secondarily involving the sphenoid sinus by direct local extension.) In those cases of diagnostic uncertainty, an immunohistochemical stain for beta-catenin can be useful in establishing the diagnosis. It has recently been shown that activating beta-catenin gene mutations are important in the pathogenesis of nasopharyngeal angiofibromas, both in those tumors that occur in patients with adenomatous polyposis coli and in those that arise sporadically. Accordingly, immunohistochemical localization of beta-catenin to the nuclei of stromal cells is a consistent finding in nasopharyngeal angiofibromas that does not occur in other stromal lesions such as inflammatory sinonasal polyps.