Thymic carcinosarcoma with melanocytic differentiation in a dog.

Carcinosarcomas are very rare tumors in dogs. Although carcinosarcomas with melanocytic differentiation arising from organs other than the thymus have been described in humans, this type of tumor has not been reported in dogs in any part of the body. We observed such a tumor in the cranial mediastinum of an 11-year-old spayed female dachshund. The dog was admitted to the clinic because of coughing, sporadic regurgitation, and dyspnea. Thoracic ultrasonography and computed tomography revealed a large mediastinal mass that was surgically removed via sternotomy. The tumor was of thymic origin and demonstrated 3 distinct components: an epithelial component positive for pancytokeratin (AE1/AE3) and high molecular weight cytokeratin (CK5/CK6) with some cystic spaces; a mesenchymal component positive for vimentin; and in association with the epithelial part, a minor melanocytic component positive for Melan A. Histologic metastasis of the epithelial and melanocytic components was present within a tracheobronchial lymph node. The dog died 105 d after surgery, after an episode of acute dyspnea. Key clinical message: To the authors' knowledge, this is the first report of thymic carcinosarcoma with melanocytic differentiation.