Case of the month #6
by Ashley Parsley (DipACVP)
A 12-year-old Quarter Horse gelding presented for multiple cutaneous to subcutaneous nodules on the neck, ranging in size from 1.5 – 4.0 cm in diameter. The largest nodule had been present for an unknown period of time, and new smaller nodules had recently begun forming. Fine needle aspirates were taken from multiple nodules, which show the same cytologic findings, as seen in the images.
What’s your diagnosis?
Fig 1-3 Lymph node in a horse, Wright Giemsa, 20-50x.
Evaluation of the cytology shows two populations of lymphocytes. Around 90% are small with round nuclei, clumped chromatin, no visible nucleoli, and scant blue cytoplasm. The other 10% are intermediate to large with round to cleaved nuclei, stippled chromatin, no visible nucleoli, and mildly expanded blue cytoplasm with a small paranuclear clearing zone. Rare macrophages are also present.
Lymphoma, likely T-cell rich B cell lymphoma (TCRBCL).
In equine cutaneous lesions, lymphoma is the most likely diagnosis for these cytologic findings, specifically, T cell-rich large B cell lymphoma (TCRLBCL). However, since this is a difficult diagnosis to make on cytology, histology for confirmation was recommended, performed, and agreed with the diagnosis of TCRLBCL. The diagnostic features of TCRLBCL include a lymphoid population that consists primarily of small, mature, non-neoplastic T cells (see CD3 image), with a lower number of large, neoplastic B cells (see Pax5 image).1,2 Variable numbers of macrophages, epithelioid macrophages, and occasionally multinucleated giant cells can also be present.1,2 While other conditions such as lymphocytic inflammation and immune-mediated diseases can cause cutaneous to subcutaneous infiltrates of lymphocytes, in this case, there was a distinct lack of other inflammatory cells (such as plasma cells and eosinophils) to support these other differentials.
While TCRLBCL is rare in other species, it is potentially the most common lymphoma subtype in horses.1,2 Studies have shown that TCRLBCL comprises 71-84% of equine cutaneous lymphomas,2,3 and 14-43% of lymphoma overall in horses.2,4 TCRLBCL in horses can present as multicentric, cutaneous, or gastrointestinal lymphoma. The cutaneous form most often presents as multiple dermal to subcutaneous haired nodules that can present anywhere on the body (head, neck, trunk, and/or limbs); fewer cases of solitary masses and uncommon ulceration are described.1-3
- Kelley LC, Mahaffey EA. Equine malignant lymphomas: morphologic and immunohistochemical classification. Vet Pathol. 1998; 35:241–252.
- Durham AC, Pillitteri CA, Myint MS, Valli VE. Two hundred three casesof equine lymphoma classified according to the World Health Organization (WHO) classification criteria. Vet Pathol. 2012; 50(1): 86-93.