Oral malignant melanoma with osteoid formation in a dog.

Melanomas are the most frequent malignant neoplasms of the oral cavity of dogs. Osteogenic melanoma is a rare variant of melanoma that has been described 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 Dachshund 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 melanocytic cells were in close contact with the osteoid trabeculae. Immunohistochemical 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 is the first case of malignant melanoma with osteoid formation reported in animals.