Case Name: Thymoma Signalment: 4-year old castrated male Domestic Shorthair cat  Presenting Complaint and Patient History: The cat presented with a history of mild lethargy.   The owners also described an intermittent cough that had become progressively more frequent over the past 2-3 weeks.  The cat was current on vaccinations, and had tested negative for FeLV and FIV. Physical Examination: The respiratory rate was slightly increased, but no cough was elicited during examination.  Heart and lung sounds were dull in the cranial thorax, and compressibility of the cranial thorax was decreased. Additional Information: On thoracic radiographs (not shown here) a large soft tissue density mass filled the cranial mediastinum, causing dorsal elevation of the trachea.  The heart and caudal lung fields appeared normal.  Fine needle aspirates of the mass were obtained for cytology.  A median sternotomy was subsequently performed for surgical removal of the mass. Cytology Images: 50x magnification 100x magnification   Cytology Description Smears made of needle aspirates from the mediastinal mass are quite cellular, and most of the nucleated cells are small mature lymphocytes.  A few intermediate-sized and large immature lymphocytes are also present.  Scattered well-granulated mast cells are seen.  A few large mononuclear cells are found singly in the smears.  These cells have an abundant amount of cytoplasm, with no evidence of phagocytic activity.   Opinion:  The cytologic findings are not consistent with lymphoma, as the lymphocyte population is heterogeneous and consists mostly of small mature cells.  Thymoma is a consideration, but a definitive diagnosis cannot be reached from the cytology, as the epithelial cell population characteristic of thymoma was not evident in these samples. Histopathology Images: low magnification of tumor architecture low magnification with capsule Hassel's corpuscle CD3 staining of T lymphocytes cytokeratin staining of epithelial cells   Histopathology Description The mass consists of densely packed sheets of lymphocytes, admixed with a smaller number of epithelial cells that are pleomorphic in appearance.  Scattered mast cells and macrophages are present, and rare Hassel’s corpuscles are seen.   Many of the lymphocytes stained positive for CD3 (T-lymphocytes) while the epithelial cells were positive for cytokeratin.   Diagnosis:  Thymoma, lymphocyte predominant