Thymoma in the dog: two case reports and review of 20 additional cases.

A 7-year-old German Shepherd Dog had a large cranial mediastinal mass that had resulted in chronic weight loss. A diagnosis of benign thymoma was made by transthoracic needle biopsy. The thymoma was removed surgically and the dog recovered without complications. At 4 months after surgery, the dog's activity was improved and it had gained weight. A 12-year-old mixed-breed dog was admitted for evaluation of dependent edema of the neck. Thoracic radiography revealed pleural effusion and lesions in the right lung and left scapula. Neoplastic epithelial cells were found in a needle aspiration specimen from the scapular lesion. The dog was euthanatized and necropsied. The primary tumor, a malignant thymoma, filled the cranial mediastinum and had invaded the cranial vena cava. Metastasis was confirmed in the right lung, liver, and left scapula. Eight cases of thymoma in the dog were retrieved from the Veterinary Medical Data Program. In 5 of the cases, the tumor was benign. The 3 dogs with malignant thymoma developed cranial vena caval syndrome, and they were euthanatized or died. Four of the dogs with benign thymoma were euthanatized for reasons unrelated to the tumor. One dog with benign thymoma was free of tumor when examined 5 years after surgical removal of the tumor.