Answer: Carcinomatous transformation of an inverted Schneiderian papilloma

Histology: The epithelium lining is markedly thickened. In areas the thickened epithelium is thrown into papillary projections, while in other areas it is inverted downward into the subepithelial tissues. The epithelium exhibits severe atypia in the form of dyspolarization, increased mitotic activity with atypical mitotic forms, and pleomorphism. Entrapped mucus cells are present within the broad bands of neoplastic epithelium. Although the tumor demonstrates downward extension, definite tumor invasion is difficult to identify. Specifically, the inverted nests maintain rounded contours and they do not elicit a desmoplastic stromal reaction.

Discussion: Inverted Schneiderian papillomas are prone to local tumor recurrence and to carcinomatous transformation. About 3 – 10% of inverted papillomas undergo malignant degeneration. The majority of these carcinomas are squamous cell carcinomas, often non-keratinizing squamous cell carcinomas. These squamous cell carcinomas may occur as a synchronous tumor together with the inverted papillomas, or as metachronous carcinomas that occur after treatment of an inverted papilloma. There are no reliable histologic features that are predictive of carcinomatous transformation of an existing inverted papilloma. The distinction between inverted and invasive tumor growth is not always straightforward. Even benign inverted papillomas can deeply invert and even erode bone.