Keratoameloblastoma with unique histological architecture: an undescribed variation of ameloblastoma

Keratoameloblastoma is an extremely rare variant of ameloblastoma, and a review of the English language literature yields only several documented cases of ketatoameloblastoma. This paper reports a keratoameloblastoma showing unique histological architecture. The patient was a 76-year-old Japanese man with a multilocular radiolucent lesion of the mandible extending from the left canine to the second molar area. Microscopically, the lesion was characterized by multicystic spaces lined by papillary projections of proliferating odontogenic epithelium with extensive surface parakeratinization in a lamellar accumulation of keratin. In addition, "hair-like" extensions of keratin were frequently found. There was no ghost cell type keratinization. Histological features of the odontogenic epithelium were similar to those of conventinal ameloblastoma. An additional prominent feature of the present ameloblastoma was formation of hard tissue in continuation, in part, of the accumulated keratin in the fibrous tissue. These hard tissues showed a woven bone- or cellular cementum-like appearance and were not in contact with odontogenic epithelium. The present case was finally diagnosed as "keratoa meloblastoma," although such a type of keratoameloblastoma has not been documented previously in the spectrum of ameloblastoma.