Thymoma in dogs: 23 cases (1980–1991)

Abstract

Summary 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 the cranial mediastinum. Eleven dogs had mega-esophagus, and myasthenia gravis was confirmed in 7 of these 11. One dog developed clinical signs of myasthenia gravis after removal of the thymoma. Concurrent, nonthymic neoplasms were found in 5 dogs, and 2 had hypercalcemia. Three dogs developed third-degree atrioventricular heart block, 1 of which had generalized myositis involving the cardiac muscle. None of the dogs had evidence of distant metastasis. Histologically, the predominant tumor types were differentiated epithelial type (9/23) and lymphocyte-rich type (6/23). Clear cells (large cells with nonstaining cytoplasm) comprised ≥ 50% of the cell population in tumors from 5 dogs. Mast cells were detected histologically in 85% of the thymomas evaluated. Sixteen dogs were treated, and in 15 of these, surgery was the primary means of treatment. Six of the 9 dogs with megaesophagus that underwent surgery died or were euthanized within 1 week of diagnosis; whereas only 1 of the 4 dogs without megaesophagus that underwent surgery died within 1 week of diagnosis. Two dogs underwent surgery and received adjuvant chemotherapy. One dog died of complications associated with chemotherapy. One dog was treated with chemotherapy alone and survived 14 months. Seven dogs did not undergo treatment; 4 of these were euthanatized immediately after the mass was first discovered. By means of univariate analysis, age (≤ 8 years old vs > 8 years old), megaesophagus (present vs not present), and histologic type were found to be significantly (P ≤ 0.05) associated with survival time. Only megaesophagus was found to be 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 megaesophagus could not be calculated, because most dogs died of causes unrelated to the thymoma and were censored. Kaplan-Meier 1-year survival rate was 83% for dogs without megaesophagus.