Thymic carcinoma and branchial cyst in a cat

A 10‐year‐old female spayed Maine Coon presented with lethargy, weight loss and severe dyspnoea. Radiographs showed mediastinal enlargement and marked pleural effusion. Thoracocentesis yielded a highly protein‐rich transudate with macrophages predominating. Computed tomography identified a large cystic mediastinal structure with a typical peripheral contrast‐enhancing rim, but fluid contents with a high attenuation of 35HU and internal contrast‐enhancing projections raising concern for neoplasia. However, mediastinal cytology failed to demonstrate neoplastic cells, but instead ciliated columnar epithelium most consistent with a thymic branchial cyst. The cat underwent median thoracotomy and mediastinal mass resection and is well 6 months postoperatively, with no evidence of recurrence. Histopathology with immunohistochemistry diagnosed thymic carcinoma, with branchial duct remnants. This report highlights the diagnostic challenge of mediastinal cystic lesions and the possible concurrent presence of a non‐neoplastic and a malignant neoplastic process.