A benign cutaneous histiocytoma in a pug

By Emma Scurrell

The following are cytological and histopathological specimens taken from a 1cm cutaneous mass on the pinna of a 7-year old pug.

Fig 1. This is a fine needle aspirate of the mass that reveals a mildly pleomorphic population of discrete round cells typical of a cutaneous histiocytoma. The round cells contain moderate amounts of often pale cytoplasm and single round to irregular nuclei with a fine chromatin pattern and indistinct nucleoli.

Fig 2. H&E section of the mass at low magnification. The dermis is infiltrated by sheets of neoplastic cells associated with irregular epidermal hyperplasia. The downgrowth of hyperplastic epidermis is called rete peg formation (arrows) and is a common feature associated with this tumour.

Fig 3. H&E section of the mass at high magnification. Sheets of neoplastic histiocytes are typical of this tumour.

Fig 4. H&E section. Another common feature of this tumour is the presence of lymphocytic infiltrates that typically begin at its base. It is these lymphocytic infiltrates (T lymphocytes) that are responsible for the so-called ‘spontaneous regression’ of these tumours.

Fig 5. Immunohistochemistry with E-Cadherin. E-Cadherin is an adhesion molecule present on keratinocytes and also on Langerhans cells that normally resides between the keratinocytes. It is the Langerhans cell from which the cutaneous histiocytoma arises. The positive immunoreaction in this case is characterised by the brown membranous staining on the neoplastic round cells (histiocytes) that infiltrate the dermis. Note that the normal keratinocytes that make up the epidermis also display a positive membranous immunoreaction.

Final Diagnosis

Cutaneous histiocytoma

Discussion

Cutaneous histiocytomas are common benign canine neoplasms. They arise from the epidermal Langerhans cell which is a type of antigen-presenting dendritic cell. They most commonly occur in young dogs however a fair proportion also occur in older dogs and therefore should remain on the differential list for a cutaneous mass, no matter what the age of the dog.

Lymphocyte-mediated regression of these tumours is common and this is why lymphocytes are commonly found in both aspirates and biopsies taken from these tumours. It is also the reason why treatment with immunosuppressive drugs is contraindicated because this may impair or delay the so-called ‘spontaneous regression’. In some cases of regressing histiocytomas, the number of lymphocytes will exceed the number of remaining neoplastic histiocytes, making the diagnosis a bit more difficult. Occasionally, some dogs may develop recurrent and/or multiple histiocytomas and these are usually associated with a lack of immune-response manifested histopathologically by a lack of lymphocytic infiltrate.

Ulceration and/or secondary bacterial infection is a common complication in cutaneous histiocytomas and this often necessitates surgical excision.