A histiocytoma on the ear of a boxer dog. Image by Joel Mills (own work) [GFDL, CC-BY-SA-3.0 or CC-BY-SA-2.5-2.0-1.0], via Wikimedia Commons.
Histiocytomas are common, benign skin tumours that begin in the immune cells. They usually present as small, firm, dome or button-shaped masses on the skin surface, which are fast growing, non-painful, and usually solitary.

Common sites include the head, nose, ear edges, and limbs. Occasionally, multiple skin nodules or plaques are present. Rarely, an autoimmune blistering is possible that may ulcerate.

Histiocytomas can also occur internally – in the stomach, lungs and intestines.

Retrievers, bull terriers, cocker spaniels, and great Danes are more susceptible to histiocytomas. The masses often spontaneously regress over two to three months.

The most common problem that arise from histiocytomas is when they become pruritic, triggering self-excoriation and infection. Excision is then required.

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