Subcutaneous embryonal rhabdomyosarcoma in a dog

Introduction: Canine rhabdomyosarcomas (RBDs) are classifiedinto embryonal, botryoid, alveolar and pleomorphic forms. Althoughrare, the embryonal type is the most common and is reported to havea locally invasive behaviour and metastatic potential.Materials and Methods: An 11-year-old pointer had a subcutaneousmass in the left forearm and an enlarged prescapular lymph node.Cytology, immunocytochemistry (ICC), histology and electron microscopy(EM) were performed.Results: Cytology of the mass and lymph node showed neoplasticcells with morphological features consistent with an undifferentiatedsarcoma. Cytological atypia was severe. Histology revealed a subcutaneous,unencapsulated neoplasm composed of sheets of cells with thesame morphology observed in aspirates. The lymph node was positivefor metastasis. Expression of vimentin, desmin and myoglobin wasdemonstrated using ICC and indicated a diagnosis of RBD. EM revealednumerous mitochondria, large nuclei and nucleoli and raredense masses of tangled myofilaments, suggestive of poorly differentiatedrhabdomyoblasts. The findings were consistent with an embryonalRBD. Six months after treatment the dog remains free of disease.Conclusions: Cytology and ICC are pivotal for the diagnosis ofRBD. Despite the guarded prognosis, in this case of metastatic embryonalRBD the dog responded well to therapy.[...]