Inverted papilloma of the nasal cavity: pathological aspects in a follow-up study.

The initially removed tumors from 42 patients with inverted papillomas of the nasal cavity and paranasal sinuses were subjected to an extensive histopathological examination in order to define the characteristics associated with multiple recurrences and development of malignancy. Two of the patients subsequently developed squamous cell carcinoma and eight patients had more than one recurrence. No specific histological characteristic was determined to be a predictor of multiple recurrences or development of malignancy. However, three patterns of characteristics emerged from the data which were found to be related to the development of malignancy, multiple recurrences, and benign behaviour. Malignancy was found to be associated with bilateral inverted papilloma, a predominance of mature squamous epithelium, the presence of all three epithelial types (metaplastic squamous, mature squamous, and cylindrical), severe hyperkeratosis, a mitotic index greater than or equal to 2 per high-power field (HPF), absence of inflammatory polyps among the papillomas, an abundance of plasma cells, and an absence of neutrophils. Multiple recurrences (without malignancy) were associated with a mitotic index greater than or equal to 2 per HPF and an absence of inflammatory polyps. Benign behaviour was associated with predominantly mucinous tumors, a mitotic index less than 1 per HPF, a ratio of neoplastic epithelium/connective tissue stroma greater than or equal to 6, and the presence of inflammatory polyps among the papillomas.