Diagnosis of a cystic lymphocyte-rich thymoma in a young cat

Thymoma is one of the thymic epithelial tumors arising from the thymic epithelial cells of a variety of animal species including cat, cattle, dog, goat, horse, and pig. The cancer cells of thymoma are always localized within thymus. Feline thymoma was usually identified in the cranial mediastinum of elder cats. In this report, we present the full diagnostic characteristics of a young cat with a cranial mediastinal mass diagnosed as a cystic type B1 thymoma. A 3-year-old male neutered domestic short-haired cat was referred for further diagnosis due to the shortness of breath and pleural accumulation. One single cystic extra-pericardial mass was seen and the boundary between the mass and the surrounding tissue was clearly demarcated in three-view thoracic radiographs. Ultrasound showed pleural effusion and intra-thoracic mass. Pleural fluid biochemistry showed high level of triglycerides and complete blood count revealed the elevated number of basophils and high level of feline serum amyloid A (fSAA). Pleural effusion sediment cytology showed a large number of round cells, mainly small lymphocytes, as well as a moderate number of neutrophils and a small number of large lymphocytes. The high level of triglycerides in pleural fluid instead of serum and high level of fSAA seem to provide informative clues to the diagnosis of thymoma and are worthy of further investigation.