Signalment

A 3-year-old male neutered crossbreed dog presented with a small, cutaneous mass located in the left axilla. The owner reported no noticeable changes in the dog’s behaviour, appetite, or activity level. Upon physical examination, the mass appeared raised, firm, mildly erythematous, and well-circumscribed, with no signs of ulceration. The dog did not seem bothered by the mass, and palpation elicited no apparent discomfort. No additional masses were observed, and the peripheral lymph nodes were unremarkable.

Cytologic Findings

A fine-needle aspirate (FNA) of the mass was collected and submitted for cytologic evaluation. Microscopic examination revealed a monomorphic population of round cells with uniform, round to reniform nuclei. The chromatin was finely stippled, and nucleoli were not clearly discernible. The cytoplasm of the cells was moderate in amount and pale blue in colour. These cytologic features suggested a round cell tumour, with characteristics most consistent with a histiocytoma.

The findings were suggestive of a round cell population, a broad category that includes tumours such as histiocytomas, mast cell tumours, lymphomas, and transmissible venereal tumours. However, based on the cytologic details and the patient’s age, a histiocytoma was considered the most likely diagnosis. Note the presence of a single mast cell and occasional neutrophils.

Discussion: Canine Histiocytomas

Histiocytomas are benign skin tumours that arise from Langerhans cells, which are specialised immune cells located within the skin’s epidermis. These tumours are common in younger dogs, generally appearing in animals under three years of age. Typically, histiocytomas are dome-shaped, small (usually less than 2 cm), and can occur on various parts of the body, including the head, ears, neck, and limbs, though they are not restricted to these areas and may also appear in other locations.

The typical course of a histiocytoma is benign and self-limiting. Most histiocytomas undergo spontaneous regression within one to three months, driven by the immune system’s recognition and clearance of the tumour. During this regression phase, the mass may become more erythematous or ulcerated before ultimately resolving on its own. When an FNA is obtained at this point, evidence of lymphocytic inflammation is often found. The cytologic appearance of histiocytomas can help distinguish them from other round cell tumours, such as lymphomas (which typically have larger nuclei and less cytoplasm), mast cell tumours (characterized by metachromatic granules), and plasma cell tumours (which often have greater pleomorphism and darker cytoplasm).

Prognosis for histiocytomas is excellent. Surgical removal is usually not required unless the tumour fails to regress spontaneously or is located in a problematic area that interferes with the dog’s movement or causes discomfort. However, some owners may opt for surgical removal for cosmetic reasons or if regression is prolonged. In cases where surgical removal is pursued, histopathology can be used to confirm the diagnosis.