Page 721 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 33 Hematopoietic Tumors 699

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• Fig. 33.8 Diagnostic algorithm for peripheral lymphadenopathy in dogs. PARR, polymerase chain reac-
tion for receptor gene rearrangement.

is identified by determining the expression of molecules specific Clonality Assays
for B cells (e.g., CD79a, CD20) and T cells (e.g., CD3), and have Information about the clinical presentation and morpho-
96
been recently reviewed. Although tumor cells sometimes have logic and immunophenotypic characteristics of the lympho-
morphologic characteristics that typify a particular immunophe- cytes, obtained by IHC or flow cytometric analysis, must be
notype, exceptions occur and morphologic appearance cannot be integrated to select appropriate targets for clonality testing
used as the sole determinant of immunophenotype. For example, and to interpret results accurately. 193 Occasionally, diagnosis
in a series of nine high-grade T-cell lymphomas and leukemias in of lymphoma and differentiation of malignant versus benign
dogs, the cells had a plasmacytoid appearance typically associated proliferation of lymphocytes are not possible based on stan-
with B-cell lymphoma. 123,192 Similarly, anatomic location does dard histologic and cytologic criteria. In these cases, advanced
not always predict the immunophenotype. molecular analyses may be helpful to confirm a diagnosis.
For accurate determination of immunophenotype, antibodies Clonality is the hallmark of malignancy; that is, the malignant
against lymphocyte markers are applied to tissue sections (IHC), cell population theoretically should be derived from expansion
cytologic specimens (immunocytochemistry), or individual cells in a of a single malignant clone characterized by a particular DNA
fluid medium (flow cytometry). Flow cytometric evaluation of cells region unique to that tumor. For example, in a dog with T-cell
obtained by needle aspiration is also feasible. For T cells, markers lymphoma, all the malignant cells theoretically should have
include CD3 (pan T), CD4 (helper T), and CD8 (cytotoxic T); for the same DNA sequence for the variable region of the T-cell
B cells, the markers are CD79a (see Fig. 33.7B), CD20, and CD21, receptor gene. Likewise, in a dog with B-cell lymphoma, the
although dogs with indolent TZL can express CD21. 178 Increas- tumor cells should have identical DNA sequences in the vari-
ingly, aberrant expression of CD molecules has been reported in able region of the immunoglobulin (Ig) receptor gene. Con-
canine lymphoma. In a study of 59 dogs with lymphoma, tumor versely, in reactive lymphocytosis, the cells are polyclonal for
cells from six dogs were positive for both T- and B-cell markers; their antigen receptors. Using this knowledge, investigators
however, a clonality assay revealed clonality either of the T-cell or the have used PCR technology to amplify the variable regions of
immunoglobulin receptor, but not both. This indicates that, in some the T-cell and immunoglobulin receptor genes to detect the
cases, the malignant cells may coexpress B- and T-cell markers. 118 presence of clonal lymphocyte populations in dogs (see Fig. 8.3
Antibodies against these molecules are used to determine the immu- of Chapter 8). These techniques are reviewed in Chapter 8
nophenotype; however, they also have potential utility as a therapeu- and elsewhere. 158,180,193–195 In physician-based medicine, such
tic modality if tumor cells could be targeted using these antibodies. assays of clonality are approximately 70% to 90% sensitive and
Table 33.3 presents the histologic and immunophenotypic charac- have a false-positive rate of approximately 5%, and recent stud-
teristics of the more common lymphoma subtypes in dogs. ies report similar rates in dogs. False-negative and false-positive