Answer: Inverted papilloma

Histology: The lesion consists of a polyp composed of proliferating columnar and transitional epithelium and mucin containing cells. There is an inverted growth of the epithelium into the stroma. The growth of this epithelium consists entirely of rounded nests. The cells lack cytologic atypia.

Discussion: Inverted papillomas fall under the category of sinonasal papillomas, which are benign but may be locally aggressive neoplasms that occur in the nose and paranasal sinuses. Inverted papillomas are most commonly seen arising in the lateral sinus walls and paranasal sinus. They may be lined by squamous, ciliated columnar, or intermediate (transitional) epithelium. These lesions are associated with human papillomavirus (HPV) infection. About 5% of these lesions occur in association with an invasive carcinoma.

Invasive squamous carcinoma would not grow in rounded nests, as seen in the inverted papilloma. The presence of desmoplasia is also a helpful feature in distinguishing invasive squamous cancer from inverted papilloma. Inverted papillomas differ from inflammatory polyps by showing a proliferation of epithelial elements. Inflammatory polyps consist mainly of edema, inflammatory cells including eosinophils, and glandular hyperplasia, but not proliferating epithelial cells.

The other two types of sinonasal papillomas include exophytic (or fungiform) papilloma and cylindrical cell papilloma. The fungiform papilloma tends to arise from the septum, usually occurs in a younger age group, and is also associated with HPV infection. The cylindrical cell papilloma or oncocytic schneiderian type of sinonasal papilloma does not appear to be associated with HPV. The lesion consists of cells with oncocytic cytoplasm with a ciliated surface, mucin droplets and intraepithelial spaces containing inflammatory cells.