Thymoma is an uncommon canine and feline neoplasm of thymic epithelial cells. It is seen in various breeds but may occur more frequently in Labrador Retriever and German Shepherd Dogs. Middle-aged or older dogs (average age of 11 years) can be affected and no sex predilection exists. Affected cats are usually older than 9 years of age. 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 paraneoplastic 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 fine needle aspiration of the mass under ultrasound guidance or 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. 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 or cat. Most dogs and cats with a cranial mediastinal mass will present with signs of dyspnea, coughing, and/or exercise intolerance. Other signs may include regurgitation, vomiting, or gagging secondary to esophageal compression or paraneoplastic myasthenia gravis. Generalized myasthenia gravis may also occur with a primary complaint by the owner of recurrent weakness or collapse. Precaval syndrome (swelling of the head, neck, and/or thoracic limbs) is possible if the mediastinal mass causes compression of or invades the cranial vena cava. On physical examination, if the cranial mediastinal mass is extremely large, muffled lung sounds will be noted. While most cranial mediastinal masses are usually thymoma or lymphosarcoma, other causes may include ectopic thyroid tissue, branchial cyst, chemodectoma, or thoracic wall tumor. Fluid within the cranial mediastinum (transudate, exudate, hemorrhage) can occasionally mimic a mediastinal mass.
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Randhurst Animal Hospital
212 East Rand Rd., Mt. Prospect, IL 60056
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