Case 180706 -3

Fourteen-year-old male, castrated Golden retriever dog

One month prior to presentation, the patient was seen at the referring veterinarian for an episode of collapse. Chest radiographs showed a mediastinal mass. The patient presented one month later in respiratory distress. Thoracic ultrasound was suggestive of a left cranial and caudal lung lobe torsion and the patient went to surgery for a left lung lobectomy and mediastinal mass debulking. Sternal lymph nodes were noted to be enlarged and were also submitted with the mass.

Three samples of the mediastinal mass were submitted for histopathology. The largest was 7 cm x 5 cm x 3 cm, and consisted of a hollow pouch lined by a margin of smooth, 0.3-cm-thick tissue that was soft and multifocally expanded by pale-tan nodules up to 2.5 cm in diameter. The two smaller pieces of the mass were 3.5 x 2.5 x 1.5 cm and 3 x 1.5 x 2 cm.

Four sections of tissue are examined which are composed entirely of a densely cellular, unencapsulated, mass. The mass is composed of sheets of lymphocytes and smaller numbers of mast cells, punctuated by neoplastic epithelial cells. The neoplastic cells are round to polygonal, have indistinct cell margins and contain a moderate amount of homogenous eosinophilic cytoplasm. Nuclei are round, with finely stippled chromatin and one to two distinct magenta nucleoli. Large cells with abundant clear cytoplasm are occasionally scattered throughout the mass. Anisocytosis is moderate, and anisokaryosis is marked. No mitotic figures are seen in ten 400X fields. Dense fibrous connective tissue dissect through the mass and extend from it. This connective tissue has a few islands of the previously described neoplastic tissue, and multifocal regions of hemorrhage, characterized by extravasated erythrocytes admixed with yellow pigment (hematoidin). In one region, abundant acicular clefts surrounded by pigment-laden macrophages.

Neoplastic cells throughout the mass have cytoplasmic immunoreactivity for an immunohistochemical stain using a pan-cytokeratin antibody.

Mediastinal mass: Thymic carcinoma

The mass in the mediastinum is consistent with a thymic neoplasia, based on cytoplasmic immunoreactivity to pancytokeratin (an epithelial cell marker) and the infiltration of mixed leukocytes including lymphocytes and mast cells which is typical of thymic neoplasia. Two mediastinal lymph nodes were also examined; one was markedly expanded by a similar population of cells and the architecture was partially obliterated by the infiltrating population. These cells were are also immunoreactive to pancytokeratin. The architecture of the lymph node was markedly distorted, complicating a definitive identification as nodal tissue. However, based upon the surgeon's identification of the tissue as a node, the presence of subcapsular sinuses and prominent lymphoid follicles , we are confident that this is in fact a lymph node with metastatic neoplastic thymic tissue (thymic carcinoma).

Conference attendees discussed what distinguished thymoma from a thymic carcinoma, and whether the distinction has prognostic value. Some would call this carcinoma based solely on capsular invasion while others felt that metastasis would have to be established in order to make the distinction. Regardless, it was agreed upon that if the behavior of the tumor is to metastasize and invade, then the prognosis will be worse than for a benign thymoma. We thank Dr. Peter Moore for contributing this case.

Meuten, DJ. Tumors in Domestic Animals. Ames, Iowa: John Wiley & Sons Inc., 2017.