Magnetic resonance imaging and ultrasonography in the diagnosis of a malignant peripheral nerve sheath tumor in a dog

A 9-year-old male neutered mixed breed dog had a two-month history of progr essive left thoracic limb lameness. There was electromyographic evidence of denervation potentials in all muscles of this limb. In magnetic resonance images a multilobulated, hyperintense mass was visible caudal to the mid-di aphysis of the left humerus on T-2 weighted images. The mass, which was iso intense with surrounding tissue on T1 weighted images, extended proximally towards the brachial plexus. The mass was also visible as a fusiform struct ure of mixed Echogenicity sonographically, although fine-needle aspiration performed at this time was nondiagnostic. A malignant peripheral nerve shea th tumor was diagnosed histopathologically.