By Kylie Gamber, c/o 2022
Signalment & History
7-year-old male neutered Labrador retriever who presented with a history of anemia, thrombocytopenia, progressive weakness and periods of tachypnea.
Findings
- Within the left ventral portion of the cranial mediastinum, there is a well-defined, smoothly marginated and rounded soft tissue opaque mass, which causes mild focal rightward deviation of the trachea and partial border effacement of the cranial aspect of the cardiac silhouette.
- The cardiac silhouette and pulmonary parenchyma are normal.
- Between the right cranial and right middle lung lobe, there is a thin pleural fissure line.
- Along the ventral thoracic body wall at the level of the 5th through 7th sternebral segments, there are multifocal well defined, smoothly marginated, broad-based and round fat opacities, the largest of which measures 3.9 cm in diameter, and is superimposed with the ventral aspect of the cardiac silhouette on the right lateral projection.
Diagnosis
- Cranial mediastinal mass. Differential diagnoses primarily include thymoma or thymic lymphoma.
- Ventral thoracic body wall lipomatous masses, likely representing lipomas.
Conclusions
Fine needle aspirates of the liver, spleen and mesenteric lymph nodes were consistent with lymphoma. Thymic lymphoma was considered a most differential for the cranial mediastinal mass.
An alternative differential included thymoma, which are neoplastic growths arising from epithelial remnants of the thymus. Thymomas will rarely metastasize and if caught early can be resected with good success rates.1
Flow cytometry analysis showing a mix of CD4 and CD8 cells would indicate a mass of the thymus origin, as these cells are the predominant t-cell precursors present in the thymus throughout development. If the mass was thymic lymphoma, flow cytometry analysis would show a single mature CD4 or CD8 cell, or a cell expressing neither CD4 nor CD8.1
Although it is rare, it is also advised to look for signs of metastasis. In the cases of metastatic thymoma, there is often both local and disseminated disease present. Reported cases have shown urinary tract infections, immune mediated disease, muscle myositis, keratoconjunctivitis sicca, polyarthritis, and diabetes mellitus. 1 Additionally, patients thymomas may have concurrent megaesophagus as a paraneoplastic response.2
References:
- Wiles, V., Haddad, J., Leibman, N., Avery, A. C., & Hughes, K. L. (2018). Metastatic thymoma in the liver of a dog. Journal of Veterinary Diagnostic Investigation, 30(5), 774–778. https://doi.org/10.1177/1040638718791222
- Atwater, S. W., et al. “Thymoma in dogs: 23 cases (1980-1991).” Journal of the American Veterinary Medical Association 205.7 (1994): 1007-1013.
