Oral malignant melanoma with osteoid formation in a dog

Melanomas are the most frequent malignant neoplasms of the oral cavity of d ogs. Osteogenic melanoma is a rare variant of melanoma that has been descri bed in humans but not in animals. We describe a case of malignant melanoma with bone production affecting the gingiva of a 12-year-old Miniature Dachs hund dog. The tumor was a round, lightly pigmented mass that measured 1 cm in diameter and was hard to cut. Histologically, the tumor was composed of nests and lobules of anaplastic, round, melanin-containing cells supported by a stroma displaying abundant osteoid matrix. Many of the neoplastic mela nocytic cells were in close contact with the osteoid trabeculae. Immunohist ochemical tests revealed that the neoplastic cells were immunoreactive for S-100 and vimentin and showed no staining with cytokeratin. The animal had a local recurrence of the tumor 3 months after the initial excision. This i s the first case of malignant melanoma with osteoid formation reported in a nimals.