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


A 7-year-old intact male boxer was evaluated because of a preputial dermal mass that had been present for about one year.

A 7-year-old intact male boxer was evaluated because of a preputial dermal mass that had been present for about one year. The mass was on the left cranial aspect of the prepuce; was nonulcerated, round, and raised; and measured about 1 x 1.5 x 1 cm. No other abnormalities were noted on physical examination. The mass was aspirated, and two unstained direct smears were prepared for cytologic evaluation.

The slides were stained with a Wright's-Giemsa stain, and examination of the smears revealed a highly cellular sample with a predominant population of discrete round cells (Figure 1A). These cells had small to moderate amounts of pale basophilic cytoplasm with varying numbers of purple intracytoplasmic granules. When visible, nuclei were round to oval and usually centrally located, with a dispersed chromatin pattern (Figure 1B). Some nuclei contained a single prominent nucleolus. Mild to moderate anisocytosis and anisokaryosis were present. These round cells were morphologically consistent with mast cells. Low numbers of nondegenerate neutrophils, eosinophils, and occasional macrophages were also observed. The cytologic diagnosis was mast cell tumor.

Figure 1A & 1B.

The cytologic pleomorphism and variable granularity were suggestive of a less well differentiated and potentially more aggressive mast cell tumor. The tumor was excised with wide surgical margins, and a histologic examination confirmed a completely excised, poorly differentiated (grade III) mast cell tumor. The results of postoperative staging tests were unremarkable, and multiagent chemotherapy was initiated. Six months later, the patient was still receiving chemotherapy and was reportedly tumor-free.


Cutaneous mast cell tumors are a common finding in dogs, comprising about 16% to 21% of all dermal and subcutaneous tumors.1 Breeds reportedly predisposed to mast cell tumors include boxers, Boston terriers, beagles, and Labrador retrievers. Most mast cell tumors are found in middle-aged to older dogs, but younger dogs are sometimes affected, including dogs as young as 3 weeks old.1 No gender predilection has been documented.1-3 There is wide disparity in the gross appearance of these tumors, but they classically occur as solitary dermal or subcutaneous masses. A small percentage of affected dogs may have multiple masses.1-3

Cytologic characteristics

In general, cytologic examination is useful in diagnosing round cell tumors. These tumors tend to exfoliate well when aspirated, so samples are often highly cellular. Mast cell tumors tend to be easily identified by their characteristic dark-purple-staining (metachromatic) cytoplasmic granules; however, the number of cytoplasmic granules seen may vary depending on several factors, including the type of stain used. Unfortunately, some mast cell tumors' granules may stain poorly or not at all with water-based Wright's stains (e.g. Diff-Quik—Dade Behring, Hema III—Biochemical Sciences), so if these stains are used, the degree of granularity may be difficult to interpret.

Figure 2

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

Figure 3

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.

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).

Figure 4A & 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

Historically, certain anatomical locations (e.g. inguinal, perineal, scrotal, preputial, and muzzle areas) have been associated with more malignant-acting (biologically aggressive) tumors,1,3,5 but recent reports have challenged this dogma.6 Clinical staging (evaluation of regional lymph nodes, lymphoid organs, and bone marrow) is used to detect possible metastatic mast cell disease.1 Information gathered from clinical staging and histologic grading is used to determine the optimal approach to treatment.

Treatment options include excision with wide surgical margins, chemotherapy, and radiation therapy, or some combination of modalities, depending on the size, location, and malignant potential of the tumor.1

This case report was provided by Maria Vandis, DVM, and Joyce S. Knoll, VMD, PhD, DACVP, Department of Biomedical Sciences, Cummings School of Veterinary Medicine,Tufts University, North Grafton, MA 01536.


1. Thamm DH, Vail DM. Mast cell tumors. In: Small animal clinical oncology. 3rd ed. Philadelphia, Pa: WB Saunders Co, 2001;261-266.

2. Meinkoth JH, Cowell RL. Recognition of basic cell types and criteria of malignancy. Vet Clin North Am Small Anim Pract 2002;32:1209-1235.

3. Gross TL, Ihrke PJ, Walder EJ, et al. Mast cell tumors. In: Skin diseases of the dog and cat. 2nd ed. Ames, Iowa: Blackwell Publishing, 2005:853-858.

4. Raskin RE. Skin and subcutaneous tissues. In: Atlas of canine and feline cytology. Philadelphia, Pa: WB Saunders Co, 2001;77-87.

5. Gieger TL, Theon AP, Werner JA, et al. Biologic behavior and prognostic factors for mast cell tumors of the canine muzzle: 24 cases (1990-2001). J Vet Intern Med 2003;17:687-692.

6. Sfiligoi G, Rassnick KM, Scarlett JM, et al. Outcome of dogs with mast cell tumors in the inguinal or perineal region versus other cutaneous locations: 124 cases (1990-2001). J Am Vet Med Assoc 2005;226:1368-1374.

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