Today's Veterinary Practice

MAY-JUN 2014

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May/June 2014 Today's Veterinary Practice 21 LymPh Node CyToLogy | tvpjournal.com nodes are enlarged and the dog is asymptomatic, because infectious causes of lymphadenopathy more often lead to clinical signs of illness. Histopathology can identify marginal and T-zone lym- phomas that are indolent; biopsy specimens should ideally include the entire node and should be sent to a patholo- gist with expertise in this area. These lymphomas may not require treatment; if treated, they have a lower response rate to treatment but longer survival times. Flow cytometry can determine the immunophenotype of a labeled cell. Immunophenotype of lymphoma affects prognosis; median survival for T-cell lymphoma is typically one-half the median survival for B-cell lymphoma for a given chemotherapy protocol. Flow cytometry is performed on a needle aspirate sample suspended in a special medium. Immunohistochemistry (biop- sy samples) or immunocytochem- istry (fine-needle aspirate samples) can also be used to determine immunophenotype. While knowledge of immunophe- notype may provide an improved understanding of expected out- come, and while some clinicians favor certain protocols based on immunophenotype (some clinicians prefer alkylator-heavy protocols for T-cell lymphoma), it has never been shown that modifying the chemother- apy protocol based on knowledge of immunophenotype improves out- come, and multidrug (CHOP-based) protocols are still effective for all high-grade lymphomas. PCR for Antigen Receptor Rear- rangement (PARR) can be performed on aspirate slides when a diagnosis of lymphoma is elusive. With this tech- nique, DNA of the variable regions encoding the immunoglobulin and T-cell receptors is amplified. • In a normal immune system, there is great variety to protect the body from as many antigens as possible. • When a lymphocyte or lympho- blast undergoes a malignant trans- formation, then clonally expands, there is great redundancy in the lymphoid population of a sample. • In other words, all malignant lymphocytes are pro- grammed to make the same receptor because they all came from the same progenitor; PARR detects this monotony. Thus, detection of monoclonal population of cells by PARR confirms the presence of neoplasia. False positive tests are rare, but may be seen with ehrlichiosis. Solid Tumor Metastasis When lymph nodes are completely effaced with tumor cells, the diagnosis of metastatic neoplasia is often straight- forward. However, in the absence of background lympho- When using flow cytom- etry, the laboratory that will be processing the sample should be contacted before sample collection, as spe- cial handling (suspending the aspirate in a specific medium, and overnight ship- ping on ice) is required. Figure 2. Monomorphic population of lymphoblasts consistent with diagnosis of lymphoma; note the relatively nor- mal mitotic figure in the upper left and neutrophil for size comparison in the center. A few small, mature lympho- cytes remain near the top of the slide, demonstrating how deeply they stain compared with neoplastic lympho- blasts. Courtesy Dr. Tamara Hancock Figure 3. Reactive lymph nodes exhibit increased numbers of lymphoblasts and plasma cells, with variable inflam- matory cells, such as neutrophils, eosinophils, and occasional mast cells. In this figure, increased numbers of lymphoblasts and intermediate lym- phocytes give the appearance of many different types of lymphocytes, rather than the monomorphic appearance of lymphoma. Courtesy Dr. Natalie Hoepp TaBLe 4. appearance of metastatic Cells Found in Lymph Nodes METASTATIC CELLS APPEARANCE CELLS ORIgINATE FROM: Mesenchymal Cells • Round to ovoid nuclei • Indistinct cell borders • Trailing/wispy cytoplasm Sarcoma Epithelial Cells • Round nuclei • abundant, angular cytoplasm • Tend to occur in clusters or sheets* • Background population of mostly small lymphocytes Carcinoma Round Cells • Round nuclei • Cytoplasm of varying amounts histiocytoma Lymphosarcoma mast cell tumor melanoma** Plasmacytoma Transmissable venereal tumor * These may be infrequent, requiring that the entire slide be carefully examined. ** Melanoma may occasionally be categorized differently, depending on the characteristics of individual tumors. TVP_2014-0506_Lymph Node Cytology.indd 21 5/24/2014 10:39:00 AM

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