Answer: Schneiderian papilloma, fungiform type

Histology: The surface epithelium is thickened and thrown into exophytic papillary fronds. The fronds are lined by a metaplastic squamous epithelium, and there are scattered intraepithelial mucinous microcysts representing entrapped residual mucinous cells. There is increased mitotic activity within the basal and parabasal layers, but the epithelium matures in an orderly fashion. In-situ hybridization studies are positive for human papillomavirus (HPV)-6/11.

Discussion: For intranasal masses, it is often important to determine precise anatomic location of the lesion. The fungiform type of Schneiderian papilloma is a neoplastic but benign papillomatous proliferation arising from the Schneiderian membrane (i.e. the ectodermally-derived mucosa lining the nasal cavity and paranasal sinuses). In contrast, the simple squamous papilloma is a papillomatous proliferation arising from the skin of the nasal vestibule. Although there is a tendency to group all Schneiderian papillomas into a single group, its members are clinically and pathologically distinct and should be subtyped accordingly. The fungiform papilloma arises almost exclusively from the nasal septum. Conversely, the inverted papilloma almost always arises exclusively from the lateral wall of the nasal cavity and from the paranasal sinuses (especially the maxillary sinus). Both the fungiform papilloma and the inverted papilloma (and for that matter, the cylindrical cell papilloma) are associated with significant rates of local tumor recurrence, but the fungiform papilloma (unlike the inverted papilloma) almost never undergoes carcinomatous transformation.

Histologic distinction between fungiform and inverted papillomas is primarily based on architectural growth patterns: fungiform papillomas demonstrate an exophytic growth pattern, while inverted papillomas exhibit an endophytic growth pattern. In those cases where the growth pattern is mixed, subclassification is based on the predominant pattern. In effect, the finding of focal areas of invaginated epithelium does not exclude the diagnosis of fungiform papilloma in a septal lesion with a predominant exophytic component.

Like squamous papillomas, the fungiform papilloma is often positive for low-risk HPV. In effect, HPV studies are not useful in distinguishing squamous papillomas and Schneiderian.