Page 163 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

142 PART II Diagnostic Procedures for the Cancer Patient

lymphocytes, and histiocytes. Transmissible venereal tumors
(TVTs) are also included in this category, and there are a variety
of epithelial and mesenchymal tumors that sometimes appear as
VetBooks.ir round cell tumors. These tumors share certain cytomorphologic
features. Cells exfoliate easily leading to highly cellular specimens
in which the cells are individualized in noncohesive monolayers.
As the moniker indicates, the cells are round and have distinct
cytoplasmic margins and round nuclei, although nuclear shape
may vary in pleomorphic forms of these tumors.

Mast Cell Tumors
MCTs consist of cells with numerous purple cytoplasmic granules
that fill the cytoplasm and often obscure the nucleus (see Figs. 7.2
and 7.5). In cats, granules are finer than they are in canine mast
cells. Even in poorly granulated MCTs, there often are enough
granules in some cells to suggest that they are mast cells. One
notable exception is when the granules fail to take up stain when • Fig. 7.32 Fine-needle aspirate of a plasmacytoma. Many cells have the
one of the aqueous quick stains is used, so clinicians using these characteristic appearance of plasma cells. Multinuclearity is a common
stains should be alert to this artifact (see Fig. 7.2). The nucleus feature of this tumor.
is generally centrally located but may be eccentric. Criteria of
malignancy are observed infrequently and may include anisocyto- chromatin. Multinuclearity is common in plasmacytomas, and in
sis, anisokaryosis, binuclearity, multinuclearity, nuclear pleomor- more pleomorphic forms of this tumor the nuclei may be multi-
phism, and the presence of mitotic figures. Marked pleomorphism lobulated (Fig. 7.32). Neoplastic plasma cells may appear imma-
is uncommon and when present suggests a higher-grade tumor; ture and resemble large lymphocytes with higher N:C ratios and
however, at present grading is based on histologic findings. Mark- finer chromatin. Sometimes the cells contain Russell bodies, col-
edly pleomorphic MCT cells may have sparse or absent granu- lections of immunoglobulin within the endoplasmic reticulum,
lation, marked variation in cell and nuclear size, and lobulated and are termed Mott cells. Plasma cell tumors may occur in the
or ameboid nuclei. In dogs, aspirates of MCTs often contain skin, oral mucosa, bone marrow, liver, and spleen, and the spe-
numerous eosinophils along with a small proportion of reactive cific diagnostic criteria for plasma cell myeloma, extramedullary
fibroblasts and thick bands of collagen (see Fig. 7.5). Proposed plasma cell myeloma, and plasmacytoma are presented in Chapter
cytologic grading systems for mast tumors in dogs have not been 33, Section D. Reactive plasma cell proliferations consist of a mix-
validated yet; these systems suggest that marked pleomorphism, ture of inflammatory cells and are rarely mistaken for a neoplastic
poor granulation, and the presence of mitotic figures may pre- process in a cutaneous mass; however, when plasmacytosis is iden-
dict a more aggressive behavior. A novel cytologic grading scheme tified in bone marrow, reactive and neoplastic conditions must be
adapted from two-tier histologic grading criteria was predictive of distinguished.
a poorer outcome and had high sensitivity and specificity com-
pared with histologic grading. 14 Lymphoma
Determining the presence of metastasis in tissues, including
LNs, liver, and spleen, that have resident mast cells, may be dif- Lymphoma comprises many variants, and entire chapters are
ficult. Features that support metastatic disease include the pres- written on their cytologic features. Definitive diagnosis of lym-
ence of large numbers of mast cells suggestive of tissue effacement, phoma based on examination of cytologic specimens is often
mast cells with pleomorphic features, and mast cells arranged in possible; however, some types of lymphoma or lymphoma in
groups instead of singly. If cytologic evaluation cannot distin- certain tissues may be difficult to diagnose cytologically. As with
guish between resident mast cells and metastasis, a surgical biopsy many discrete round cell neoplasms, it is the homogeneity of the
should be evaluated. population, rather than the morphologic features, that suggest a
Other neoplasms in which the cells contain cytoplasmic gran- neoplastic process. In lymphoid organs or other tissues in which
ules may be mistaken for mast cell tumors and include granulated there is a reactive or polyclonal infiltrate of lymphocytes, small
T-cell lymphoma, natural killer (NK) cell lymphoma, and granu- lymphocytes should predominate and comprise more than 50%
lar cell tumors. When cells from MCTs are agranular or when of the lymphoid cells, even as the proportion of large and inter-
the granules fail to stain with aqueous stains, the tumor may be mediate lymphocytes increases. Plasma cells and other inflam-
mistaken for plasmacytoma, histiocytoma, atypical lymphoma, or matory cells also may be found in these reactive lesions. As the
squamous cell carcinoma. Reactive nonneoplastic mast cells may proportion of intermediate and large lymphocytes approaches or
be found in increased numbers at sites of fibrosis because of the exceeds 50%, it becomes more difficult to differentiate between
role mast cells play in wound healing. a reactive and neoplastic process; this is especially true for the
spleen and certain LNs, such as mandibular and mesenteric
Plasma Cell Tumors nodes, that are continuously exposed to antigen. Because of this,
sampling of other LNs or tissues is preferred. In addition, cats
Plasma cell tumors composed of well-differentiated plasma can mount strong lymphocytic responses that can cytologically
cells are easily recognized owing to the characteristic features of resemble lymphoma. In contrast, there are certain types of lym-
plasma cells—abundant royal blue cytoplasm, paranuclear clear phoma, such as T-cell-rich B-cell lymphoma and Hodgkin’s-like
zone (Golgi apparatus), eccentric round nucleus, and clumped lymphoma, that contain a mixture of clonal (neoplastic) and