A neoplastic nodular lesion, 2 × 3 cm in diameter, was found in the larynx of a
6-year-old spayed female dog. The tumor was ill-circumscribed, consisting histologically
of large round cells with abundant cytoplasm interspersed with small round cells with less
cytoplasm and occasional multinucleated cells (myotubes). Immunohistochemically, tumor
cells were positive for myoglobin, desmin and vimentin in varying degrees, but negative
for S-100 protein, GFAP or cytokeratin. Cytoplasmic myofilaments/myofibrils with a dense
Z-line-like structure were seen, the fine structures of which were complemented by PTAH
stain. Based on these findings, the tumor was diagnosed as a rhabdomyosarcoma, a very rare
tumor in the larynx of dogs.