Mediastinal mass in a dog

Figure 1. CT image showing a large isodense to the muscle, poorly contrast enhancing mass on the left side of the thorax.
Figure 2. FNA cytology of a cranial mediastinal mass (Wright-Giemsa stain, 50x objective).
Figure 2. FNA cytology of a cranial mediastinal mass (Wright-Giemsa stain, 50x objective).
Figure 3. Clonality testing (PCR for antigen receptor rearrangements, PARR) of a cranial mediastinal mass showing polyclonal T and B-cell populations.

Maciej Guzera DVM PhD DipECVCP MRCVS


A 11 year old, male neutered, Labrador presented to the veterinarian with a recent history of excessive panting, polyuria/polydipsia and exercise intolerance. Ultrasound performed by the referring veterinarian identified a large hypoechoic cranial mediastinal mass.


On clinical examination he was found to be panting continuously with increased respiratory noise and effort. On cardiac auscultation the heart was audible only in the right caudal thorax. His abdomen was mildly distended.


Haematology and biochemistry

Blood work was performed as part of routine diagnostic workup. Haematology revealed mild leukocytosis (17.83x10^9/L, reference interval, RI: 6-12x10^9/L) due moderate lymphocytosis (11.1x10^9/L, RI: 1-3.6x10^9/L) and mild basophilia (0.89x10^9/L, RI: <0.04x10^9/L). Peripheral blood smear evaluation was unremarkable. Biochemistry showed mild to moderate total hypercalcaemia (3.24 mmol/L, RI: 2.3-3 mmol/L).


A computed tomography (CT) scan was performed in order to better assess the location and the extent of the thoracic mass (Figure 1). CT showed a large mediastinal mass which spanned the entire height of the thorax to the level of the 10th pair of ribs. The lungs and heart were displaced caudally.


Next, a fine-needle aspiration (FNA) cytology of the mediastinal mass was performed (Figure 2).

Molecular diagnostics

Clonality testing (PCR for antigen receptor rearrangements, PARR) performed on the aspirate showed a polyclonal pattern (Figure 3).


Further testing which was performed in order to better characterize hypercalcaemia showed an increased concentration of parathyroid hormone-related protein (PTHrP) (1.9 pmol/L, RI: <1.0 pmol/L) and a decreased parathyroid hormone (PTH) level (0.4 pmol/L, RI: 0.5-5.8 pmol/L).


What is your interpretation of the results and diagnosis?

Interpretation of results

Figure 4. Histopathology of thymoma from the dog showing aggregates of lymphocytes admixed with clusters of neoplastic cells arranged in sheets (haematoxylin and eosin stain, 10x objective).

The most likely cause of leukogram changes and hypercalcaemia was neoplasia.

The cytology of the cranial mediastinal mass showed a high number of lymphocytes (mostly small and few intermediate) and a low number of well-differentiated mast cells. A low number of medium-sized epithelial cells was also present. These cells were individualized or present in small clusters. They were oval or polygonal and had abundant light blue cytoplasm with medium-sized, round to oval and centrally-located nuclei. The chromatin was finely reticular. The nucleoli were mostly single, medium-sized and round. The background was clear and haemodiluted. The cytological findings were indicative of neoplasia and were highly suggestive of a thymoma (lymphocyte-predominant thymoma), although a thymic lymphoma could not be totally excluded.

A polyclonal PARR result excluded lymphoma without a need to perform a core biopsy.

The PTHrP and PTH results confirmed the paraneoplastic origin of hypercalcaemia.



The dog underwent median sternotomy to remove the thymoma. Histologic examination of the excised mass confirmed a thymoma (stage III type B2) (Figure 4). Two months later the dog was doing well with no respiratory signs and good exercise tolerance.


Thymoma is a tumour originating in the thymus. Its development is a consequence of a neoplastic transformation of supporting thymic epithelial cells. Neoplastic cells are frequently accompanied by a non-neoplastic proliferation of lymphoid cells. Thymoma is the second most common (after lymphoma) cranial mediastinal tumour in the dog but its general prevalence is low. They usually occur in older dogs with the incidence being the highest at the age of 9.

Definitive cytological diagnosis of thymoma can be hampered by a low number of epithelial cells in the sample and both lymphomas and thymomas may be composed of small lymphocytes. This emphasizes the necessity of adjunct diagnostic tests (e.g. clonality testing, flow cytometry) and histopathology.

Paraneoplastic syndromes (e.g. hypecalcaemia) are a common finding in patients with thymoma – reported in up to 67% of cases.