Thoracoscopic Cranial Mediastinal Mass Resection

Cranial mediastinal masses in dogs are most frequently diagnosed as either thymoma or lymphoma with ectopic thyroid carcinoma, branchial cysts and chemodectomas being much less common. If a cytological or histopathological confirmation of a thymoma is made, surgical resection is usually recommended. Thymoma is a tumor arising from the thymus gland and is a rare disease in dogs. In humans it is often detected incidentally during a chest radiograph or as part of a work-up for myasthenia gravis, but in dogs it is often clinically silent for long periods and therefore presents at a more advanced stage.1-3 Canine thymoma has been described as benign or malignant, but this is based more upon phenotypic behavior than histologic appearance.4 Typically benign or “non-invasive” tumors remain within the thymic capsule and do not invade vascular or other structures in the region. Malignant or “invasive” thymomas are locally aggressive and may also metastasize.1,5 In both humans and dogs, surgery is the treatment of choice for non-invasive thymomas and complete excision, without violation of the capsule, is the critical technical element that correlates with a good prognosis.5,6

Dogs with thymoma usually present with symptoms attributable to compression or invasion of surrounding structures or paraneoplastic syndromes. Signs include coughing, dyspnea or symptoms of venous congestion secondary to occlusion of the cranial vena cava. The most common paraneoplastic syndrome associated with thymoma, in both dogs and humans, is myasthenia gravis. Thymomas can be asymptomatic and diagnosed as an incidental finding on thoracic radiographs performed as part of the diagnostic evaluation for another condition.  [...]