Metastatic thymoma in the liver of a dog

Abstract

A 12-y-old neutered male Portuguese Water dog was presented because of a 1-y history of persistent hyporexia, diarrhea, and recurrent pyelonephritis. Abdominal ultrasound revealed hepatic nodules and diffuse splenomegaly, and radiographs revealed a mediastinal mass. Fine-needle aspirates of the liver, spleen, and mediastinal mass were suspicious for lymphoma. Flow cytometry identified small T cells that co-expressed CD4 and CD8 at all sites, most suspicious for thymoma, but lymphoma could not be ruled out. PCR for antigen receptor rearrangements analysis identified polyclonal amplification of the T-cell receptor genes, more consistent with thymoma than lymphoma. Histopathology of the liver and thymic mass confirmed thymoma with hepatic metastasis.

Keywords: Dogs; flow cytometry; hepatic metastasis; metastatic thymoma.

Figure 1.

Flow cytometry of A, a mediastinal mass, and B, combined spleen–liver sample from a dog with metastatic thymoma, compared to C, the thymus of a normal dog. A, B. Flow cytometry revealed an expansion of T cells that express CD5 (purple) with co-expression of CD4+ and CD8+ and do not express CD3. There were phenotypically identical small lymphocytes in both the mediastinal mass and spleen–liver aspirate. C. Normal thymocytes are identified as small T cells that express CD5 (purple) with co-expression of CD4+ and CD8+ and no expression of CD3. The CD5+ thymocytes are negative for MHC class II, a feature of normal thymocytes. There is an increased percentage of CD3+ and CD5+ T cells in the normal thymus compared to the mediastinal mass in panel A (13.6% and 3.1%, respectively).

Figure 2.

PCR for antigen receptor rearrangements of the spleen of a dog with metastatic thymoma. Molecular clonality assessment of genomic DNA was conducted using capillary electrophoresis and BioCalculator (Qiagen, Hilden, Germany) and GeneMarker (Soft Genetics, State College, PA) analysis. There is a broad polyclonal result when assessed with T-cell receptor gamma primers. The higher molecular weight PCR products (blue) in the 470 nucleotide range were amplified with V gamma 2 primer. The PCR products in the 420 range (black) were amplified with V gamma 3 primer.

Figure 3.

Immunohistochemistry of A, C, D, mediastinal mass, and B, liver of a dog with metastatic thymoma. A. Within the mediastinal mass, there are heterogeneous small lymphocytes intermixed with individualized and aggregated polygonal cells with abundant eosinophilic, flocculent cytoplasm and round nuclei with dispersed chromatin and single-to-multiple small prominent nucleoli. H&E. 400×. B. Hepatic architecture is effaced by a predominance of heterogeneous small lymphocytes with fewer intermixed aggregates of polygonal cells similar to those described in the mediastinal mass and highlighted in the inset. There are few remaining hepatocytes (bottom of figure). H&E. 200×. Inset: higher magnification of epithelial cells intermixed with lymphocytes. 400×. C. The polygonal cells within the mediastinal mass show diffuse cytoplasmic immunoreactivity for cytokeratin AE1/AE3. 400×. D. The majority of lymphocytes within the mediastinal mass demonstrate cytoplasmic immunoreactivity for CD3. 400×.