Cutaneous histiocytomas are very common, benign but rapidly growing neoplasms of epidermal dendritic cells known as Langerhans cells. Dendritic cells are antigen-presenting cells. They process antigens and present them on the cell surface to T-lymphocytes, and so they act as a bridge between the innate and adaptive immune systems. There is debate about whether histiocytomas are true neoplasms or aberrant immune responses. This is because they are self-limiting lesions, most of which spontaneously regress. Nevertheless, they appear to be the result of uncontrolled, although transient, proliferation of a single cell type and occasional tumours will show spread to local lymph nodes which will resolve following tumour regression or excision.
Histiocytomas are unique to dogs and are typically found in young dogs less than four years of age, but they can develop at any age. Common locations include the head, particularly the pinnae. Clinically they develop rapidly as a raised, dome-shaped nodular mass in the dermis. They can become red and ulcerated and, as such, they are a major differential for a mast cell tumour. Most histiocytomas demonstrate spontaneous lymphocyte-induced regression over a period of several weeks, and the prognosis is good. Some require surgical excision because of ulceration and irritation. Mast cell tumours, however, require surgical excision and grading, with some needing adjuvant therapy, so it is very important to be able to differentiate between the two.
Fine needle aspirate is an ideal way of differentiating between the two tumours. Both are round cell tumours. Mast cell tumours exhibit granular cytoplasm in all but the most poorly differentiated lesions. Histiocytomas are large mononuclear round cells with moderate to abundant non-granular cytoplasm. This makes cytological differentiation unproblematic in most cases where good quality aspirates are available.
Local surgical excision is adequate for most histiocytomas and the prognosis in most cases is excellent following excision. Persistent and recurrent histiocytomas can occur. Therapy with immunosuppressive drugs is contraindicated for persistent and recurrent cutaneous histiocytomas, as it interferes with the possibility of lymphocyte-induced regression. A few cases develop as several, multiple histiocytomas, either synchronously or over time but the prognosis is the same: excellent with spontaneous regression or local surgical excision. A separate, rare condition called Langerhans cell histiocytosis involves the formation of numerous masses which can coalesce and spread to the viscera. This is a more aggressive condition which results in a poorer prognosis, but the cytological features of early Langerhans cell histiocytosis are the same as for the more common cutaneous histiocytoma so presenting clinical history is vitally important.
