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A Firm Mass at the Base of Ear

Kimberly J. Caruso, DVM

Clinical Pathology

|August 2003|Peer Reviewed

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A 6-month-old, neutered male Dalmatian was presented to the veterinarian for evaluation of a firm mass at the base of the ear.

The lesion was approximately 1 cm in diameter, raised, hairless, pink, shiny, nonulcerated, and nonpainful. The owner had noticed the growth in the past few weeks. The veterinarian sampled the growth using the following nonaspiration sampling technique: A 1-inch, 22-gauge needle is placed into the mass, redirected two or three times, and withdrawn. A syringe filled with air is attached to the needle, the luminal contents of the needle are expelled onto one or more glass slides, and cytology smears are prepared. One smear obtained by this method was stained to ensure adequate cellularity, and the others were sent to a nearby diagnostic laboratory for evaluation (Figures 1 and 2, stained with Wright's-Giemsa).

Diagnosis: Histiocytoma.

Cytologic evaluation. Several smears with moderate cellularity were examined. The predominant cells consist of moderate numbers of individual, discrete-appearing cells. These cells have round, purple nuclei with occasionally flattened nuclear borders and a moderate amount of pale, bluish-gray cytoplasm with well-defined cytoplasmic borders. The nuclear chromatin pattern is coarse with nucleoli that are absent to single and prominent. Minimal anisocytosis and anisokaryosis are present. The background is made up of a thick, moderately grayish-blue proteinaceous fluid. The cytologic diagnosis is histiocytoma.

Foundation. There are five types of discrete (round) cell tumors: Lymphoma, mast cell tumor, plasma cell tumor, transmissible venereal tumor, and histiocytoma.1 Cytologically, discrete cell tumors exfoliate in high numbers individually, lack cell to cell adhesion, and appear round morphologically. The term discrete cell may be preferred by some clinical pathologists over the term round cell since the cells are not always perfectly round.1 Because these cells often exfoliate in high numbers, they can mimic epithelial neoplasia by appearing cohesive when in reality this feature is attributed to the cellular crowding present on the smear. Discrete cell tumors are often easily diagnosed by cytologic evaluation; however, poorly differentiated neoplasms make it more difficult to distinguish among the types of discrete tumors. To the inexperienced cytologist, histiocytomas can be among the more challenging discrete cell tumors to recognize because they can appear similar to lymphoma and poorly differentiated mast cell tumors. Histiocytomas have a moderate amount of light, bluish-gray cytoplasm and angular nuclei, whereas lymphoblasts have a small amount of deeper blue cytoplasm with round nuclei and prominent single to multiple nucleoli. In addition, histiocytomas lack the eosinophilic infiltration that often accompanies mast cell neoplasia. The signalment and tumor location are helpful in making a definitive diagnosis.2

Histiocytomas are benign cutaneous tumors originating from Langerhans macrophages in the dermis and subcutis. They are a common skin tumor, and approximately 50% are found in dogs younger than 2 years of age. Great Danes, boxers, dachshunds, cocker spaniels, Shetland sheepdogs, and bull terriers seem predisposed. The head, and especially the pinnae, is the most common site. Other frequently involved sites include the distal limbs and feet; less frequently involved sites are the neck, trunk, and tail. Grossly, the tumors have a dome-shaped, button-like appearance. They are often hairless and pink and may be ulcerated, resembling the eraser on the end of a pencil. These tumors grow rapidly but typically do not exceed 2 cm in diameter. They usually regress spontaneously within 3 to 4 months. Cytologic evidence of regression is characterized by lymphocytes admixed with histiocytes and may be confusing to an inexperienced viewer.

• Does the cytologic specimen have adequate cellularity?
• Are the microscopic findings indicative of an inflammatory lesion or neoplasia (round cell, mesenchymal, epithelial)?
• What are the differential diagnoses?
• What is the definitive diagnosis, and how would you treat this patient?

Did you answer ...
Yes, the specimen has adequate cellularity.
The monotonous cell population is compatible with neoplasia, most likely a discrete (round) cell tumor-cells are individual, appear round, and are numerous and noncohesive.
Histiocytoma, lymphoma, mast cell tumor, plasma cell tumor, and transmissible venereal tumor are differential diagnoses for a discrete cell tumor.
Based on the microscopic findings, histiocytoma is the definitive diagnosis. This diagnosis is supported by the age of the patient as well as the physical characteristics and location of the mass.
The tumor will not metastasize and will probably regress spontaneously within 3 to 4 months.


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