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
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