Veterinary Medicine - June 2008 - (Page 303)

Clinical Exposures PEER-REVIEWED 4A. A ﬁne-needle aspirate from a cutaneous mass in a dog illustrating lymphoma. Note the extremely high nuclear to cytoplasmic ratio. The nuclei are round and eccentric with coarsely stippled chromatin, marked by irregularly shaped areas of dark heterochromatin. Numerous irregularly round, dark-blue lymphoglandular bodies (cytoplasmic fragments) are scattered throughout the background (Wright’s-Giemsa stain; 100X). 4B. A ﬁne-needle aspirate from a cutaneous mass in a dog illustrating plasmacytoma. Note the moderate amount of basophilic cytoplasm and eccentric nuclei, with dark, tightly clumped chromatin. These cells often display moderate anisocytosis and mild anisokaryosis. Binucleate and multinucleated cells are relatively common in these tumors (Wright’s-Giemsa stain; 100X). 4C. A ﬁne-needle aspirate from a cutaneous mass in a dog illustrating histiocytoma. Note that both the cells and their nuclei are a bit irregular in shape. Nuclear chromatin is loose and lacy compared with the other tumor types (Wright’s-Giemsa stain; 100X). lymph nodes, skin, eyes, liver, musculature, abdominal viscera, lungs, and brain.3-5 Etiology The origin of this tumor has been extensively studied. Although a viral cause has been postulated but not veriﬁed, recent research conﬁrms that the tumor is clonal in origin, and the development of this tumor requires transmitting the neoplastic cells (not cell-free ﬁltrates or killed cells) from one dog to another.1,2 In addition, unique chromosomal gains and losses in tumors from distinct geographic regions suggest local lineages.1,2 Lastly, a diagnostic long interspersed nuclear element (LINE-1) inserted near the c-myc gene occurs in all tumors, even samples collected two decades apart.1,2 LINEs are long DNA sequences that represent reverse-transcribed RNA molecules (also called retrotransposons). These can be used to generate genetic ﬁngerprints. The LINE-1 insertion appears to be a speciﬁc marker of canine transmissible venereal tumor resulting from either an insertion during the somatic evolution of the tumor or its presence in the germ line of the original host.2 The histogenesis of canine transmissible venereal tumor cells is inconclusive. Although the tumor is often regarded to be of histiocytic origin, more recent studies suggest an immature leukocyte (likely myeloid) origin.6 Cytologic characteristics As with other round cell tumors, canine transmissible venereal tumor cells tend to readily exfoliate, and ﬁne-needle aspiration usually yields high numbers of individualized round cells.5,7 These cells have abundant, lightly basophilic to sometimes deeply basophilic cytoplasm, often with small punctate vacuoles.5,7 The nuclei are round with coarse to clumped chromatin and one or two prominent nucleoli,5,7 and mitotic activity is often high.5,7 Several of these features can be used to distinguish canine transmissible venereal tumors from other round cell tumors such as mast cell tumors, histiocytomas, plasmacytomas, lymphoma, and some melanomas. Canine transmissible venereal tumor cells typically have a lower nuclear to cytoplasmic ratio than lymphoma cells (Figure 4A). The characteristic sharply deﬁned cytoplasmic vacuoles can be helpful in distinguishing this tumor from lymphoma (Figure 4A), plasmacytoma (Figure 4B), or histiocytoma (Figure 4C). Cells in a plasmacytoma or histiocytoma are also less likely to have the prominent nucleoli seen in canine transmissible venereal tumor cells (Figures 4B & 4C). Generally, cells from mast cell tumors and melanomas contain distinct cytoplasmic granules (Figures 5A & 5B). An exception may be observed with poorly granulated mast cells, which are round cells with a moderate amount of pale cytoplasm and central to slightly eccentric nuclei resulting in a typical fried egg appearance (Figure 6). Although these cells may lack the dense granulation of typical mast cells, close inspection will usually reveal some ﬁne metachromatic granules, an important distinguishing characteristic. Cytologic examination can also help you determine the tumor’s status. Canine transmissible venereal tumors are highly antigenic, and a dog’s immunologic response plays an important VETERINARY MEDICINE June 2008 303

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Veterinary Medicine - June 2008 Contents Leading Off Clinical Exposures Idea Exchange A Challenging Case: Conjunctival Lymphoma in a Cat Updates on Canine Hypoadrenocorticism Product Preview CE Form Advertiser Index Marketplace/Classifieds Mind Over Miller Clinical Edge - June 2008 Table of Contents Managing Diabetes Mellitus in Cats: What Makes it Work? Porcine Insulin Zinc Suspension in the Management of Feline Diabetes Mellitus Monitoring Diabetes Mellitus in Diabetic Cats Feline Diabetes Mellitus: Important Client Communication Points

Veterinary Medicine - June 2008

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