Lingual rhabdomyoma in a dog

A neutered male mixed breed dog, with a history of progressive dyspnoea, was presented for surgical removal of an enlarging lingual neoplasm (60 x 40 x 70 mm), previously diagnosed as a granular cell tumour. Microscopically the excised neoplasm comprised sheets of pleomorphic cells; large granular eosinophilic cells, multinucleated giant cells and smaller angular cells. A tentative diagnosis of rhabdomyoma was based on the light microscopic changes. This diagnosis was confirmed by the ultrastructural identification of bundles of intracytoplasmic filaments (interpreted to be myosin) within the neoplastic cells and the strong positive reaction of these cells to markers for "muscle-specific" actin (HHF35).