Gingival rhabdomyosarcoma accompanied by an immature myogenic population immunoreactive for α-smooth muscle actin in a dog.

A 3-year-old female shih tzu was presented with a white to dark red mass arising from the gingiva. Because of the rapid and invasive growth of the mass, the dog was humanely destroyed. Microscopically, round to polygonal anaplastic cells with strongly eosinophilic cytoplasm grew in an alveolar pattern separated by fibrous stroma. Mitotic figures were numerous. Multinucleated cells and 'strap cells' were observed, but cross striation and glycogen accumulation were absent. Immunohistochemically, the tumour cells were positive for vimentin, desmin, muscle-specific actin and MyoD1, and a small number of tumour cells were positive for α-smooth muscle actin (α-SMA). Based on the morphological and immunohistochemical features, the gingival mass was diagnosed as alveolar rhabdomyosarcoma accompanied by α-SMA-positive immature myogenic cells.