Undifferentiated Embryonal Rhabdomyosarcoma in a German Shepherd Dog: Macroscopic, Histopathologic and Immunohistochemical Features

This case report describes an 8-year-old female German Shepherd dog with undifferentiated embryonal rhabdomyosarcoma. The mass measured 13 x 12 x 9 cm, weighed 900 grams, and had an elastic consistency. Histopathologic examination revealed a large necrosis area in the center of the mass. We determined cells with spindle-oval and rounded morphology, hyperchromatic nuclei, unclear cytoplasmic borders, tightly arranged atypia, and mitosis around the necrosis. We noted long cells with nuclei arranged in a row and wreath-like multinucleated giant cells among these cells. In the immunohistochemical examination, neoplastic cells were stained with vimentin, desmin, skeletal muscle myosin, sarcomeric actin, and SMA positively, while Iba1, HLA-DR, pancytokeratin, S100B, SOX10, and GFAP were stained negatively. Myogenin was intranuclearly positive in approximately half of the cells. The case was diagnosed as RMS and was classified as undifferanted varyant of embryonal type on the basis of histopathologic and immunohistochemical findings. The original morphological and immunophenotyping structure of the tumor, along with the intriguing structure of the giant cells, led us to believe that sharing the case would be beneficial. This case will contribute to the pathomorphological knowledge of canine striated muscle tumors for studies in the field of veterinary oncology.