Clinical Exposures: Canine transmissible venereal tumor: The cytologic clues - Veterinary Medicine
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Clinical Exposures: Canine transmissible venereal tumor: The cytologic clues



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

Figure 4A,B,C
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

Figure 5A&B
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.

Figure 6
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 role in inhibiting the growth and spread of the neoplasm.3 Similar to observations in histiocytomas, increased lymphocytes and plasma cells may be observed cytologically on impression smears and aspirates in regressing tumors, compared with progressing tumors.3


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