Inverted papilloma is the most common type of sch-neiderian papilloma. This lesion occurs almost exclusively in the lateral wall of the nasal cavity and in the paranasal sinuses, although on rare occasions it may also arise on the nasal septum . Grossly, they frequently have a polypoid appearance, but they differ from nasal polyps of the common type by their histological features. Inverted papillomas are composed of invaginat-ing crypts, cords and nests covered by non-keratinising squamous epithelium, which alternates with columnar ciliated respiratory epithelium and with intermediate or transitional epithelium (Fig. 2.4a). This newly formed duct system is similar to the embryonic development of the nasal mucosa . The multilayered epithelium typically contains mucous cells and mucin-filled micro-
cysts. The invagination of the mucosa may result in the presence of apparently discontinuous cell masses lying deep to the epithelial surface, but the basement membrane is intact and may be shown in continuity with that of the surface epithelium . An inverted growth is the hallmark of inverted papilloma, but varying degrees of papillary growth may be seen at the surface . The surface is characteristically lined by a respiratory type of epithelium; nevertheless, foci of surface keratinisa-tion are occasionally present . A few regular mitoses may be found in the basal and parabasal layers. Although the nuclei may show mild nuclear irregularities and hyperchromatism, no disturbances of the cellular polarity are found. An abundant and oedematous connective tissue stroma is a common feature of inverted papillomas. It usually contains macrophages and neu-trophils, but eosinophils may also be present. This inflammatory infiltrate may also be present between the epithelial cells, within the dilated lumens of invaginated crypts, and within the numerous microcysts that usually occur in the respiratory epithelium. Seromucinous glands are absent, but branching gland ducts are often present. The tumour grows by extension to involve the contiguous sinonasal epithelium.
If treated only by local surgical excision, recurrence occurs in up 75% of cases. Therefore, lateral rhinotomy and medial maxillectomy are advisable for tumours of the lateral nasal wall . Carcinoma develops in about 10-15% of inverted papillomas [122, 211, 236]. Carcinoma may coexist with inverted papilloma at the initial presentation or originate subsequently [122, 273]. According to the experience of Michaels and Hellquist , carcinoma does not usually develop in the course of recurrences of inverted papilloma. In the presence of severe atypia or marked keratinisation in an inverted papilloma malignant transformation is always suspected (Fig. 2.4b). In these instances the entire specimen should be thoroughly examined to exclude an associated carcinoma. Most associated carcinomas are squa-mous , although other types may also occur such as verrucous carcinoma .
Oncocytic papilloma, also known as "columnar" or "cy-lindric" cell papilloma , is the least common type of schneiderian papilloma. It constitutes less than 5% of all sinonasal papillomas [18, 122, 173, 262]. Both sexes are equally affected. Bilaterality has not been documented. Tumours are in general small, although occasionally may reach various centimetre measurements in their greatest dimension. They are composed of exophytic fronds and endophytic invaginations lined by pseudostratified or multilayered columnar cells with prominent onco-
cytic features. The cells have uniform hyperchromatic nuclei and abundant eosinophilic, occasionally granular, cytoplasm that contains abundant mitochondria and stains for the mitochondrial enzyme cytochrome C oxidase . Goblet cells are not found. Cilia may be occasionally encountered on the superficial epithelial layer. Intraepithelial microcysts containing mucin and neutrophils are usually present. These microcysts are larger than the similar structures also seen in inverted papilloma. The tumour resembles inverted papilloma in its sites of occurrence, the lateral wall of the nasal cavity and the maxillary antrum. The rate of recurrence is considered to be 36%, which is lower than in inverted papilloma. The low frequency of this tumour makes it difficult to evaluate its true malignant potential, which seems to be similar to that of inverted papilloma . Atypical hyperplasia and carcinoma in situ changes can be occasionally found (Fig. 2.5). Surgical excision with wide margins is the treatment of choice. Invasive squamous cell carcinoma, high-grade mucoepidermoid
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