EQUINE VETERINARY EDUCATION / AE / AUGUST 2019
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After considering the diagnosis of lymphoma with
concern for multicentric disease, the potentially poor prognosis, the treatment options and the well-being of the horse, the owners elected for humane euthanasia.
Post-mortem findings
Necropsy revealed a multilobular, soft, pale, tan, fleshy, irregular and infiltrative mass within deep connective tissue that measured approximately 23 9 18 9 18 cm and enveloped significant vasculature, including the common carotid artery and its branches (Fig 4). Also present were multifocal zones of necrosis deeper in the mass. The mass was expansile within soft tissue, obliterating normal architecture. The mass compressed the pharyngeal and epiglottic region, but did not involve the pharyngeal mucosa. Histological examination of the pharyngeal wall near the tumour revealed prominent submucosal lymphoid nodules with occasional mitoses, but with a mixture of medium and small sized lymphocytes and plasma cells. Overall, the submucosal lymphocytes were smaller than cells within the mass tissue, and, based on cellular differentiation, were considered to be reactive rather than neoplastic. Adjacent salivary tissue and thyroid lobes were normal in appearance. No enlarged lymph nodes, mediastinal masses or other internal masses were found, save a 3–4 cm sessile lipoma on the surface of the jejunum. The remainder of the necropsy was unremarkable. Sections of the primary mass underwent immunohistochemical staining for CD20 and CD3. The larger
a)
neoplastic cells had marked expression of CD20 (Fig 3), consistent with a B cell phenotype. Smaller cells scattered throughout the mass were CD3 positive, consistent with surrounding T cells. Histological and immunophenotypical analyses were consistent with a T cell rich, large B cell lymphoma (TCRLBCL). Stained and unstained aspirates were submitted3 for clonal assessment by polymerase chain reaction for analysis of antigen receptor rearrangement (PARR testing). Molecular clonality assessment of genomic DNA was performed using capillary zone electrophoresis4,5. Clonality analysis of IgH2, IgH3 and KDEi (B cell) genes yielded clonal rearrangements, and analysis of TCRG (T cell)
yielded polyclonal gene rearrangements (Fig 5). These findings supported the presence of a B cell neoplasm with T cell proliferation consistent with TCRLBCL.
Discussion
The current case represents one of only few reported cases of a solitary lymphoid tumour in a well fleshed horse with no evidence of multicentric disease. With a previously undescribed presentation and a lack of additional signs, this case demonstrates that lymphoma should be on the differential diagnoses list of practitioners investigating a solitary soft tissue mass in the horse. Lymphoma in animals has been divided into four main
categories based upon location: multicentric, intestinal, mediastinal and cutaneous (Meyer et al. 2006; Munoz et al. 2009). Additionally, in horses, solitary tumours of extranodal
b)
c)
d)
Fig 3: Incisional biopsy of retropharyngeal mass. a) The mass contains a dense population of round cells with large areas of necrosis. H&E stain; 23 objective; bar = 1 mm. b) The mass is composed of a mixed population of round cells. There are large round cells (approximately 2–3 times an erythrocyte) with euchromatic chromatin and irregularly round nuclear membranes and euchromatic chromatin. These are interspersed by many small mature lymphocytes. There are frequent mitotic figures with individual cell death and tingible body macrophages. H&E stain; 603 objective; bar = 20 lm. c) The population of large round cells has intense membranous CD20 immunoreactivity (B cells). CD20 immunohistochemical staining; bar = 20 lm. d) Small round lymphocytes within the mass have intense membranous immunoreactivity (T cells). CD3 immunohistochemical staining; bar = 20 lm.
© 2017 EVJ Ltd
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