Aggressive, Solid Variant of Alveolar Rhabdomyosarcoma with Cutaneous Involvement in a Juvenile Labrador Retriever

An 8.5-month-old male Labrador retriever presented with a cutaneous mass in the right maxillofacial region and swelling of the gingiva. The dog received antibiotic and anti-inflammatory treatment. After 3 weeks the dog returned, presenting with disseminated cutaneous tumours on the neck, trunk and groin. One of the nodules was resected and a cutaneous round cell tumour was diagnosed on microscopical examination. The dog was humanely destroyed. Necropsy examination revealed disseminated tumours in the skin, internal organs and skeletal muscles. Microscopically, all of the tumours were composed of small round cells, arranged in nests. Immunohistochemically, the neoplastic cells expressed vimentin, desmin, MyoD1, myogenin and smooth muscle actin, but were negative for CD3, CD18, CD79αcy, cytokeratin AE1/AE3, chromogranin A, class II molecules of the major histocompatibility complex, neuron-specific enolase and S100. The average Ki67 index was 89.5%. The final diagnosis was a solid variant of alveolar rhabdomyosarcoma (ARMS). This is the first report of the cutaneous multifocal form of ARMS in veterinary oncology.