Answer: Schneiderian papilloma, inverted type

Histology: The specimen consists of a polypoid fragment of respiratory epithelium with submucosal edema and chronic inflammation. There is thickening of the surface epithelium with direct downward extension of the surface epithelium deeply into the submucosa. The inverted lobules of epithelium seem to erode into craniofacial bone. The inverted lobules of epithelium are lined by a mixture of respiratory and epidermoid cells. They do not demonstrate any significant pleomorphism. The epithelium is permeated by neutrophils, and collections of these neutrophils form discrete intraepithelial microabscesses.

Discussion: Uncertainties regarding the nature and classification of papillomas of the sinonasal tract are reflected in the plethora of synonyms historically used to designate this lesion. At one extreme, the term “papillary hypertrophic sinusitis” conveys the notion that these lesions are a manifestation of chronic inflammation of the sinonasal tract. At the other extreme, the terms “papillary respiratory epithelial carcinoma” and “cylindrical cell carcinoma” suggest that these papillomas are, at the very least, pre-malignant. Currently, papillomas arising from the Schneiderian membrane are designanted as Schneiderian papillomas and include the fungiform papilloma, the cylindrical cell papilloma, and the inverted Schneiderian papilloma (IP). IP is distinctive in its propensity for expansive downward (i.e. inverted) growth of epithelial cells into the underlying submucosa. Its neoplastic nature is manifest by: 1) its ability to grow along the mucosal surfaces with contiguous involvement of multiple sinonasal sites, 2) its high recurrence rate following surgical resection, 3) its ability to compress and erode into adjacent anatomic structures if left untreated, and 4) its significant association with malignant transformation.

Clinically, IP typically presents as a polypoid mass unilaterally involving contiguous regions of the sinonasal tract. Its light microscopic appearance is dominated by expansive lobules of epidermoid and/or respiratory epithelium that extend into the submucosa. These lobules are typically permeated by neutrophils that form intraepithelial microabscesses. IP has a propensity to recur and is associated with carcinomatous transformation. Approximately one-half of IPs recur following surgical excision, and about 5% to15% arise in association with a synchronous or metachronous squamous cell carcinoma of the sinonasal tract. Conventional light microscopic examination is not very useful in predicting recurrence or malignancy. Of note, extension into craniofacial bones can occur as a consequence of pressure erosion, and this finding by itself is not to be taken as evidence of malignant transformation.