A retrospective study of 15 thymomas in domestic animals showed four animals had antemortem signs of dyspnea and grossly visible lumps in the thoracic inlet. The neoplasms were single, multilobulated, encapsulated masses in the anterior mediastinum or thoracic inlet. Cells with clear cytoplasm comprised the bulk of two canine thymomas, whereas the remaining neoplasms had a mixed population of ovoid and spindle-shaped cells. There were lymphocyte populations of varying density in all thymomas. Three thymomas contained structures similar to hyalinized cells and Hassall's corpuscles of normal thymus. Metastatic lesions were not seen but there were implantations on the pericardium in one cat. There was capsular invasion in the ovine thymomas.
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