Answer: Schneiderian papilloma, fungiform type

Histology: The lesion has a papillary growth pattern with fibrovascular cores supporting a thickened epithelium. The lining is comprised of a multilayered squamous epithelium with entrapped mucinous cells giving rise to intraepithelial mucinous microcysts. The squamous epithelium exhibits orderly maturation, and it lacks any significant cellular atypia.

Discussion: The fungiform type of Schneiderian papilloma is a neoplastic but benign papillomatous proliferation arising from the Schneiderian membrane (an ectodermally-derived mucosa lining the nasal cavity and paranasal sinuses). The squamous papilloma, on the other hand, 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 and is sometimes referred to as a septal papilloma. The inverted papilloma arises almost 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 local 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.

Again, one of the more important reasons in recognizing the fungiform variant of Schneiderian papilloma is its very low threat of carcinomatous transformation. Unlike papillary squamous cell carcinoma, fungiform papilloma does not exhibit nuclear pleomorphism or high mitotic activity.