Alveolar rhabdomyosarcoma in a dog confirmed using myogenin immunohistochemistry: A case report

A large mass from the right forelimb of a five-year-old, male, Maltese dog, was resected surgically and examined histopathologically. Grossly, the 4 × 6 cm mass was well-demarcated and firm. Microscopically, the mass was characterised by neoplastic mononuclear cells with large round hyperchromatic nuclei and scant cytoplasm. The neoplastic tumour cells were separated by thick or thin collagen septa and were arranged in an alveolar pattern forming cell nests. Immunohistochemical detections of the tumour revealed positive reactions for vimentin, desmin and myogenin, but the tumour was negative for alpha smooth muscle actin (α-SMA), S-100, CD3, CD79a, CD68, cytokeratin 8 (CK8) and cytokeratin 18 (CK18). These results showed that the tumour cells originated from skeletal muscle; therefore, the tumour was diagnosed as an alveolar rhabdomyosarcoma.