The puppy was presented with a 1 cm diameter subcutaneous nodule at the base of one ear. The lesion was removed at 6 weeks of age and submitted for microscopic evaluation. Section of skin contains a densely cellular, multilobular, pleomorphic spindle-cell neoplasm. Groups of neoplastic cells are packeted by a thin encircling rim of fibrous connective tissue. Neoplastic cells are elongate with ample granular cytoplasm and indistinct cytoplasmic borders, large round to oval open-faced nuclei, and a prominent single nucleolus.
Mitotic figures are frequent.
Some cells have cytoplasmic cross-striations, which were better illustrated with PTAH staining. Neoplastic cells stained positively for desmin and actin. Rhabdomyosarcoma, Golden Retriever, canine. These tumors are highly invasive and tend to metastasize early and disseminate widely. Common sites of metastasis are lymph nodes, lung, heart, spleen, adrenal glands, kidneys, and skeletal muscle.