Thymofibrolipoma in a labrador retriever.

While thymomas are uncommon but well-known mediastinal masses, collagen-rich variants are exceedingly rare. Thymofibrolipoma and sclerosing thymoma tumor variants have been recently recognized in medical pathology, and thymofibrolipoma has been only rarely reported in dogs. A cranial thoracic mass was identified in a 6-year-old Labrador Retriever that was characterized by robust collagenous stroma dissected by thin cords of cytokeratin-positive neoplastic epithelial cells and bordered by mildly pleomorphic epithelial cells with occasional lymphocytic aggregates and rare Hassall corpuscles. To the authors' knowledge, this is only the second report of thymofibrolipoma in veterinary medicine and the first to describe a variant with a mitotically active and relatively pleomorphic, adjacent thymic epithelial population.