Malignant mediastinal extra-adrenal paraganglioma with spinal cord invasion in a dog.

A 1.5-year-old castrated, male, mixed-breed dog was evaluated because of a 2-week history of lethargy, stiffness, and progressive paraparesis. Spinal radiographs, myelography, and computed tomography of the region showed a locally invasive mass involving the thoracic wall. Upon necropsy, an encapsulated, fluctuant mass was noted attached to the right dorsal body wall in the region of the fifth to seventh thoracic vertebra. Churukian-Schenk staining revealed positive granules within the neoplastic cell cytoplasm and immunohistochemistry was positive for expression for cytoplasmic neuron-specific enolase and synaptophysin. Chromogranin A and S100 expression were found to be negative. Immunohistochemistry and silver staining did not allow further differentiation of the tumor, and the diagnosis remains consistent with either a chromaffin paraganglioma or a nonchromaffin paraganglioma (chemodectoma) with some production of catecholamines. Extra-adrenal paragangliomas are rare neoplasms in dogs but should be considered as a possible differential diagnosis for a primary paraspinal thoracic mass.