Answer: More than one of the above

Histology: The light microscopic appearance of this polypoid mass is dominated by the expansive downward (i.e. inverted) growth of epithelial cells into the underlying submucosa (Image 1). In some regions, the epithelium is thrown into papillary folds (Image 2). At higher power, the epithelial layer is heavily infiltrated by neutrophils that form numerous intraepithelial microabscesses. In some regions, the epithelial cells have an oncocytic features including the presence of abundant pink granular cytoplasm and uniform round nuclei with prominent nucleoli. Within these areas are intraepithelial mucinous cysts that are reminiscent of certain fungal organisms (i.e. Rhinosporidium; Image 3). In other regions, there is evidence of malignant epithelial transformation including marked increase in cellularity, absence of cellular maturation, presence of cellular pleomorphism, and presence of abundant mitotic figures (Image 4). Despite the deep submucosal extension of this process, the neoplastic cells form nests with smooth and rounded contours.

Discussion: This Schneiderian papilloma represents a cylindrical cell variant with carcinomatous transformation. Schneiderian papillomas are neoplastic proliferations of the ectodermally-derived Schneiderian mucosa lining the sinonasal tract. Three variants are recognized: fungiform, inverted, and cylindrical (oncocytic). The cylindrical cell variant is the least common. It is architecturally characterized by multilayered papillary epithelial proliferations, and cytologically by oncocytic appearing cells with associated intraepithelial mucin cysts containing pink acellular material. These cysts should not be mistaken for sporangia of rhinosporidiosis.

In addition to the propensity for disease recurrence following incomplete excision, the inverted and cylindrical cell types share a significant potential for malignant transformation. Although the reported incidence of malignant transformation is widely variable, in our experience about 3-5% of Schneiderian papillomas (of the non-fungiform types) are associated with a carcinoma. The vast majority of these malignancies are squamous cell carcinomas. For this reason, Schneiderian papillomas should be entirely submitted for histologic evaluation of a carcinomatous component. There are no reliable histologic indicators to indicate those papillomas that are most likely to harbor a carcinoma.

For those Schneiderian papillomas harboring a squamous cell carcinoma, the distinction between in-situ and invasive disease can be problematic. Deep tumor extension is not by itself indicative of invasion given the inverted nature of the benign epithelial proliferations. The presence of irregular tumor nests surrounded by a desmoplastic stroma is a much more reliable feature of invasiveness.