Clinical Exposures: Cytologic examination of a cutaneous mast cell tumor in a boxer - Veterinary Medicine
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Clinical Exposures: Cytologic examination of a cutaneous mast cell tumor in a boxer


Figure 3
In well-differentiated mast cell tumors, numerous cytoplasmic granules may obscure the nucleus of the cell, while in less well differentiated tumors, few granules may be seen. Because components of mast cell granules are chemotactic for eosinophils, eosinophils are often observed in association with mast cell tumors (Figure 2) and may even be more numerous than the mast cells. Collagenolysis occurs in some mast cell tumors, and strands of collagen may be seen in a cytologic examination of a tumor aspirate as ribbons of eosinophilic material (Figure 3).3 Spindle-shaped or stellate cells may also be observed in the background (Figures 2 & 3), and these mesenchymal cells may represent a reactive fibroblastic response to components in mast cell granules or other changes within the mast cell tumors (e.g. necrosis, edema, collagenolysis). If numerous fibroblasts are found, it may give a false impression of a mesenchymal tumor (soft tissue sarcoma) with secondary mast cell infiltration.2,4

Differentiating round cell tumors

Differentiating mast cells from other neoplastic round cells cytologically may be challenging if the mast cell granules stain poorly or if cells contain few cytoplasmic granules. In general, mast cell tumors must be differentiated from other discrete round cell tumors such as lymphoma, plasma cell tumors, transmissible venereal tumors, histiocytomas, and melanomas. Identifying the characteristic metachromatic granules, even in small numbers, is the key to diagnosing a mast cell tumor.

Figure 4A & 4B.
Lymphoma involving large granular lymphocytes can be easily mistaken for a mast cell tumor, and in some cases immunologic markers are needed to distinguish between these two cell types. Like mast cells, large granular lymphocytes have prominent granules (Figure 4A), but in large granular lymphocytes, the granules are azurophilic (magenta), are often larger than those in mast cells, and may have a clear halo surrounding the azurophilic core. In addition, large granular lymphocyte granules tend to be clustered to one side of the nucleus (Figure 4A), while mast cell granules are often distributed evenly around the nucleus.2-4 Occasionally, both lymphoma and mast cell tumors are accompanied by an eosinophilic infiltrate, but neoplastic lymphocytes characteristically have scant amounts of basophilic cytoplasm and, with the exception of large granular lymphocytes, lack granules. While neoplastic melanocytes also contain cytoplasmic granules, melanin tends to look dark-brown to green (to almost black in heavily granulated cells) depending on the stain used (Figure 4B).

Histologic grading and clinical staging

Mast cell tumors are graded histologically in an attempt to predict biologic behavior and prognosis. Histologic grading is based on the location within the dermis, degree of pleomorphism, cytoplasmic granularity, and mitotic activity.1,3 Other important criteria include tissue invasiveness and the presence of hemorrhage and necrosis. Tumors are commonly classified histologically as well-differentiated (Grade I—usually less malignant behavior), intermediate (Grade II), or poorly differentiated (Grade III—more malignant behavior). Cytologic grading of mast cell tumors is considered unreliable because it does not allow evaluation of tumor invasion into surrounding tissues or other architectural features and, as mentioned before, evaluation of the degree of granularity of some mast cells may be problematic if the cells are treated with water-based stains. However, in general, sparse or variable granularity and prominent cellular pleomorphism (anisocytosis, binucleate cells, prominent nucleoli) on cytologic preparations are considered suggestive of a more malignant-acting tumor.2,3


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