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This case series describes the clinical appearance, radiographic appearance and histopathologic features of 4 patients (3 cats and 1 dog) with unique cystic oral lesions that are consistent with a diagnosis of calcifying odontogenic cyst (COC). COC is a rare odontogenic lesion in humans that has not been previously reported in companion animals. Historically, COC in humans has been classified as a benign cystic neoplasm and as a non-neoplastic odontogenic cyst. Current classification favors the latter. The cases in this series also have ambiguous features regarding classification of the lesion as an odontogenic cyst versus benign neoplasia.

This paper describes the microscopical features of a rare odontogenic tumour that occurs in dogs and cats and which has been referred to as the calcifying epithelial odontogenic tumour (CEOT), although it is not the counterpart of the human tumour of that name. We have proposed amyloid-producing odontogenic tumour (APOT) as an appropriate alternative term. The tumour is composed of irregularly shaped strands of squamous epithelium, which in some areas exhibit palisading of the basal cells, similar to that seen in ameloblastomas. Stellate reticulum occurs focally in some examples. The second prominent feature is the presence of amyloid which tends to calcify. Finally, in some examples, a collagenous matrix, which is apparently a form of dentine, is present focally, adjacent to the epithelium. These histological features are compared with those of the canine keratinizing ameloblastoma and the human CEOT. To date, to few examples have been reported to determine accurately the biological behaviour of APOTs, but some have recurred after excision; none has metastasized.