Faecal Occult Blood Test

Case of the Month (Emma Scurrell December 2011)

Malignant transformation of a viral papilloma in a dog

The following are H&E sections of a solitary, exophytic papilliform mass which was surgically resected from the skin on the dorsum of a 1-year-old male Labrador Retriever.

 Fig 1. Subgross view of the H&E section of the exophytic cutaneous mass. Most of the mass consists of papillomatous fronds of squamous epithelium associated with marked hyperkeratosis. If you look at the base of the mass however, you will see nests of cells infiltrating the dermis (black arrow). 

Fig 2.  Close-up view of the solid nests of neoplastic squamous cells infiltrating the dermis (as indicated by the black arrow in Fig 1), indicative of a squamous cell carcinoma (H&E, x100)


Fig 3.  . Close-up view of the more superficial component of the papillomatous mass. Cytopathic effects consistent with a viral aetiology in this case include hypergranulosis (increased/clumping of basophilic keratohyaline granules) and koilocytosis in which altered keratinocytes contain clear cytoplasm and pyknotic nuclei (blue arrow).

Final Diagnosis

Squamous cell carcinoma (ex-viral papilloma).


Cutaneous viral papillomas in dogs usually occur in dogs under two years of age and are caused by a papillomavirus. The majority of these tumours undergo regression over a course of weeks to months. In rare cases, malignant transformation to a squamous cell carcinoma can occur. In this case, although the clinical appearance was typical of a benign papilloma, the presence of invasive nests of squamous cells at the base of the tumour is indicative of malignancy. From a practical point of view, biopsy specimens of papillomatous lesions should therefore ideally include a deep margin so that this can be properly examined for any evidence of invasive behaviour/malignancy .

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 
Benign cutaneous histiocytoma  September 2010 
Iridociliary adenoma October 2010
Proliferative thrombovascular necrosis of the pinnae November 2010
Signet-ring cell carcinoma in the stomach of a dog December 2010
Cutaneous infiltrative lipoma in a labrador January 2011

Tonsillar squamous cell carcinoma February 2011
Canine orbital meningioma March 2011
"Lung-digit" syndrome in a cat April 2011
Hepatic amyloidosis in a cat May 2011
Haemophagocytic histiocytic sarcoma June 2011
Alternaria spp. infection in an immunocompromised dog July 2011
Ocular manifestation of lymphoma in a cat  August 2011
Kerion dermatophytosis in a Cocker spaniel September 2011
Subcuticular coenurus in a rabbit  October 2011
Optic nerve glioma in a dog November 2011

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Faecal Occult Blood Test