THYMOMA IN DOGS - 23 CASES (1980-1991)

Medical records of 23 dogs in which thymoma was diagnosed between Jan 1, 1980 and Dec 31, 1991 were reviewed. All thymomas were located in t he cranial mediastinum. Eleven dogs had megaesophagus, and myasthenia gravis was confirmed in 7 of these 11. One dog developed clinical sign s of myasthenia gravis after removal of the thymoma. Concurrent, nonth ymic neoplasms were found in 5 dogs, and 2 had hypercalcemia Three dog s developed third-degree atrioventricular heart bloch, 1 of which had generalized myositis involving the cardiac muscle. None of the dogs ha d evidence of distant metastasis. Histologically, the predominant tumo r types were differentiated epithelial type (9/23) and lymphocyte-rich type (6/23). Clear cells (large cells with nonstaining cytoplasm) com prised greater than or equal to 50% of the cell population in tumors f rom 5 dogs. Mart cells were detected histologically in 85% of the thym omas evaluated. Sixteen dogs were treated, and in 15 of these, surgery was the primary means of treatment. Six of the 9 dogs with megaesopha gus that underwent surgery died or were euthanized within 1 week of di agnosis; whereas only 1 of the 4 dogs without megaesophagus that under went surgery died within 1 week of diagnosis. Two dogs underwent surge ry and received adjuvant chemotherapy. One dog died of complications a ssociated with chemotherapy. One dog was treated with chemotherapy alo ne and survived 14 months. Seven dogs did not undergo treatment; 4 of these were euthanatized immediately after the mass was first discovere d. By means of univariate analysis, age (less than or equal to 8 years old vs > 8 years old), megaesophagus (present vs not present), and hi stologic type were found to be significantly (P less than or equal to 0.05) associated with survival time. Only megaesophagus was found to b e significantly associated with survival time by multivariate analysis . Dogs with megaesophagus had a Kaplan-Meier median survival time of 4 days. Kaplan-Meier median survival time for dogs without megaesophagu s could not be calculated, because most dogs died of causes unrelated to the thymoma and were censored. Kaplan-Meier 1-year survival rate wa s 83% for dogs without megaesophagus.