Lymphoepithelial thymoma characterized by proliferation of spindle cells in a Samoyed dog.

Lymphoepithelial thymoma was diagnosed in a 14-year-old Samoyed dog with clinical symptoms of myasthenia gravis at 6 months of age. At necropsy, dark red-colored mass with many nodular protuberances was found in the anterior mediastinal area. Histologically, the mass consisted of solid proliferation of neoplastic cells with spindle nuclei and cytoplasm and a few lymphocytes, which is separated by an abundant fibrous and adipose tissue. Immunohistochemically, spindle cells were positive for cytokeratin, and infiltrating lymphocytes were positive for CD3. On the basis of these findings, this tumor was diagnosed as lymphoepithelial thymoma, which is morphologically similar to type A thymoma in humans.