Canine thymoma.

Thymoma is an uncommon canine neoplasm of thymic epithelial cells. It is seen in various breeds but may occur more frequently in German Shepherd Dogs. Middle-aged or older dogs can be affected and no sex predilection exists. A paraneoplastic syndrome of myasthenia gravis, nonthymic malignant tumors, and/or polymyositis occurs in a significant number of dogs with thymoma. Clinical signs are variable and are related to a space-occupying cranial mediastinal mass and/or manifestations of the paraneo-plastic syndrome. Dyspnea is the most common presenting clinical sign. Thoracic radiographs usually show a cranial mediastinal mass. Lymphoma is the main differential diagnosis. A definitive diagnosis may be made by closed biopsy but is more likely to be confirmed by thoracotomy. Thymomas may be completely contained within the thymic capsule or may spread by local invasion or metastasis. A staging system allows for an accurate prognosis and a therapeutic plan. Surgical removal of encapsulated thymomas may result in long-term survival or cure. Invasive or metastatic thymomas carry a guarded prognosis. Manifestations of the paraneoplastic syndrome complicate treatment. Adjuvant radiation and chemotherapy may be of value for advanced cases; however, adequate clinical trials have not been done in the dog.