Malignant transformation of a viral papilloma in a dog

The following are histopathology (H&E) sections of a solitary, exophytic papilliform skin mass that was surgically resected from 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 squamous 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) and associated with a desmoplastic (fibrous) response (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 indicates malignant transformation. 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.