Canine peripheral nerve sheath tumor with eosinophilic cytoplasmic globules.

A 13-year-old male Shetland Sheepdog had a subcutaneous tumor in the left brachium. The tumor was removed and recurred several times at 5, 13, 16, 22, and 31 months after the initial presentation. Histologically, the removed nodules from the fourth resection were composed of neoplastic proliferation of round to fusiform cells, which possessed eosinophilic globules in their cytoplasm. The globules were periodic acid-Schiff positive and diastase resistant. Positive reactions for acid phosphatase were observed in the cytoplasm of the tumor cells. Ultrastructurally, these globules consisted of membrane-bound, dense structures containing dense granules, lucent vacuoles, and homogeneous materials. The recurrent tumors removed at the fifth resection consisted of spindle cell proliferation arranged in interlacing fascicles with wavy nuclei and containing a small number of cells with cytoplasmic globules. The tumor cells were immunoreactive to vimentin, S-100 protein, myelin basic protein, and neuron-specific enolase. The tumor was diagnosed as a peripheral nerve sheath tumor with eosinophilic cytoplasmic globules. These findings are unique for the histogenesis of granular cell tumors.