Cytologic features of an acanthomatous ameloblastoma in a dog.

An 8-year-old mixed breed male dog was presented with a mass on the rostral mandibular gingiva that quickly emerged 2-3 weeks prior to presentation. The mass was firm, smooth, well-circumscribed, and approximately 2 × 1 × 0.5 cm in size rostral to the left mandibular canine tooth (304). Clinical examination and radiographs were unremarkable. Cytology revealed two distinct cell populations, consisting of numerous uniform-appearing epithelial cell clusters and low numbers of individual spindle cells. Epithelial cells had mild anisocytosis and anisokaryosis, round nuclei with finely stippled chromatin, no prominent nucleoli, high N:C ratios, and low amounts of pale basophilic cytoplasm. Slender spindle cells observed had oval nuclei with no prominent nucleoli and wispy cytoplasm. On histopathologic examination, the lamina propria of the gingiva was dissected by numerous irregular and anastomosing trabeculae and islands of neoplastic epithelial cells. Neoplastic cells were focally in connection with the hyperplastic overlying epithelium. The trabeculae were surrounded and embedded by cell-rich fibrous stroma. Peripheral to the islands and trabeculae, cells were arranged in palisades, and the nuclei had an antibasilar location. The epithelial cells had prominent intercellular bridges, low amounts of cytoplasm, and one round to oval nucleus. Anisocytosis and anisokaryosis were mild to moderate, and six mitoses/10 HPF were present. Tumor cells reached the deep sample margins. Histopathologic evaluation was consistent with acanthomatous ameloblastoma. This locally aggressive neoplasm causes alveolar bone lysis and often extends beyond alveolar bone margins. Acanthomatous ameloblastoma is an important differential for rostral mandibular gingival masses containing numerous uniform epithelial cell clusters with rare slender spindle cells.