Pathological and histological examination and outcome
The results of a necropsy revealed, a large volume of free fluid in the thoracic cavity, with blood adhered to the diaphragm, and atelectasis of the edges of the lung lobes (Fig. 13). A large mass was found in ​​the cranial mediastinum, approximately 15x15 cm in size, which surrounded the trachea and esophagus (Fig. 14), and continued up to the aorta. Enlarged lymph nodes were found in the stomach area. The stomach showed evidence of mucosal hemorrhages in the pyloric area. The intestines were without visual pathological changes except for the ileum, where small hemorrhages on the mucosa and enlarged mesenteric nodes were observed. The liver was intact, and the spleen was normal on first sight, but on palpation and later on disection there were obvious changes in the area of the gastrosplenic vein.The vein was firm, with a thickened wall and the corresponding lymph nodes were markedly enlarged. While the kidney parenchyma remained intact, there were also changes in the lymphatic nodes around the kidneys.
Samples of the fluid on smear testing showed moderate cellularity. Cell preservation was fair. Cytologically, a small population of lymphocytes with round nuclei and reduced cytoplasm were observed (cross sections of the nuclei <1.5x the size of an erythrocyte). These were the predominant cells on the smears from the fluid and were the second most common nucleated cells in the pre-made smears. The cells showed mild anisocytosis and anisokaryosis. Mitotic figures were not found. Low numbers of neutrophils were found in the smears from the fluid samples and moderate numbers were found in the pre-made smears. Very low numbers of eosinophils and macrophages were found.
Microscopic examination of the spleen showed no lesions. The lymph node adjacent to the spleen had an intact capsule with some retention of normal cellularity and architecture. However, most of the medullary sinuses, some vascular spaces, and occasional foci of the lymph node parenchyma had a population of neoplastic cells. The normal cell population were instead replaced by monomorphic populations of large round cells. These cells had slightly ill-defined borders, moderate amounts of eosinophilic cytoplasm, and varying numbers of oval to convoluted nuclei with fine chromatin and some with particularly large nucleoli. Mitotic figures were moderately frequent and occasionally atypical. There was an increase in cortical small lymphocytes (perhaps reactive). Additionally the perinodal adipose tissue was infiltrated with tumourous round cells. The stomach and kidney lymph nodes showed the same findings as described in the lymph node above.
Findings in the mediastimum lymph node, thymus, and the surrounding adipose tissue were similar to those described in and around the splenic lymph node, above. In addition, rare large cells in the thymic sinuses showed erythrophagocytosis. These cells also resembled the aforementioned tumourous cells.
The ileum showed a moderate infiltration of plasma cells with eosinophils present in the mucosa. There were also a moderate number of eosinophils present in the submucosa, and surrounding glands. No neoplastic cells were found. Thses findings were indicative of eosinophilic to plasmacytic duodenal enteritis, ranging from a mild to moderate level of disease progression
A histopathological examination was repeated to dermine the type of the tumour, but the exact identity of the tumour cells remained unclear. The tissue showed large to huge blasts (Fig. 15) with a bizzare appearance, as they were surrounded by a rich mixture of small (probably T-cell) lymphocytes,which would support a diagnosis of an agressive form of equine lymphoma.
Immunostaining was carried out for antibodies against CD3, CD79a. The residual lymphocytes within the follicles and paracortex show immunopositive staining for CD79a and CD3 respectively. The atypical neoplastic cells percolating throughout the sinuses failed to express CD79a and CD3.
Histopathological examination and immunostaining were not sufficient to determine the exact type of the tumour, although lymphoma or histiocytic sarcoma was suspected. Upon repeating the histological examination, the samples were found to have a greater population of poorly differenciated cells, and thus the results of which were more compelling towards supporting the diagnosis of an aggressive lymphoma.
Serological oncomarker testing for thymidine kinase was performed with the results obtained two weeks after eutanasia however with negative results 0.696 U/l (< 3).