Faecal Occult Blood Test

Case of the Month (Emma Scurrell September 2010)

A benign cutaneous histiocytoma in a pug

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 which 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.

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 which typically begin at its base. It is these lymphocytic infiltrates (T lymphocytes) which 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 which normally reside 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) which infiltrate the dermis. Note that the normal keratinocytes which make up the epidermis also display a positive membranous immunoreaction

Final Diagnosis

Cutaneous histiocytoma


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 histologically by a lack of lymphocytic infiltrate. 

Ulceration is a common complication in cutaneous histiocytomas and often necessitates surgical excision which should be curative.

Previous Monthly Cases

Hypertensive Retinopathy January 2010
FIP in a cat - uncommon presentation February 2010
Phaeohyphomycosis    March 2010 
Scleral Rupture April 2010
Canine and feline epulides May 2010
Erythema multiforme in a labrador retreiver June 2010
Keloidal fibrosarcoma in a labrador retreiver July 2010
Cutaneous epitheliotropic lymphoma  August 2010

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Faecal Occult Blood Test