Oncocytic SPs have a very pink appearance. The lining epithelium is composed of columnar cells with eosinophilic granular cytoplasm filled with mitochondria.
Higher power shows tufting into the lumen; collections of neutrophils and mucous cysts are characteristic features.
Both exophytic and endophytic growth patterns can be seen in oncocytic SPs; note the rounded cysts containing mucin, which can occasionally be mistaken for rhinosporidiosis.
The oncocytic type of Schneiderian papilloma (a.k.a cylindrical cell papilloma, columnar cell papilloma) is the least common of the three subtypes, comprising 3-8% of Schneiderian papillomas (Fletcher).
Similar to the inverted type, it arises in the lateral nasal wall or in the maxillary or ethmoid sinuses, and there a subset (4-17%) may undergo malignant transformation. Unlike the inverted and exophytic subtypes, there is no association with HPV (Fletcher, Barnes).
Microscopically, oncocytic SPs exhibit both endo and exophytic patterns of growth. The epithelium is composed of columnar cells with finely granular cytoplasms that resemble oncocytes. Small mucous cysts and microabscesses are usually scattered within the epithelium.
Most of oncocytic SPs transform into squamous cell carcinomas, but cases of malignant transformation into mucoepidermoid carcinomas and sinonasal undifferentiated carcinomas have also been reported (Barnes).
Because of the prominent rounded mucin-filled cysts typical of oncocytic SPs, it may (on occasion) be mistaken for rhinosporidiosis (a chronic granulomatous disease caused by Rhinosporidium seeberi). However, the rounded structures containing the pathogen in rhinosporidiosis is present not only in the epithelial, but also the stroma. Furthermore, in rhinosporidiosis, the epithelium is not oncocytic (Barnes).
Barnes L. Schneiderian Papillomas and Nonsalivary Glandular Neoplasms of the Head and Neck. Mod Pathol 2002;15(3):279-297.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 83-7.