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Trichoblastoma and squamous cell carcinoma: a case report of collision tumor in the skin of a dog

The simultaneous occurrence of two or more different neoplasms in the same anatomic site is a rare finding in veterinary pathology. Melanoma is the most commonly reported component of canine collision tumors, followed by different types of testicular neoplasms occurring in the same testicle. The literature survey yielded one finding of trichoblastoma and melanoma in a rabbit and two findings of squamous cell carcinoma and melanoma in dogs. The combination of trichoblastoma and squamous cell carcinoma in the same skin biopsy of a dog, to the authors knowledge, has not been described yet. A nine year-old, male, mixed-breed dog was admitted to a veterinary clinic because of a rapidly growing tumor approximately 2 cm in size located on the right side of the head. Three months after its removal, the tumor recurred and was now approximately 3 cm in size, oval- shaped, dark brown on the surface, and firm. Histopathological analysis showed demarcated, nonencapsulated multilobular neoplasm composed of cuboidal and polygonal basal cells arranged in trabeculae, cords, and islands surrounded by a thick collagenous and fibrous stroma. Borders between neoplastic cells were partly visible, cells had scant eosinophilic cytoplasm and oval nuclei with predominantly one nucleolus. Mitoses were moderately frequent, 1/HPF. This finding corresponded to trichoblastoma. In the same biopsy, in the superficial epidermis, a focal proliferation of epithelial cells with different stages of squamous differentiation and formation of keratin pearls was found. This finding was consistent with squamous cell carcinoma. Trichoblastoma and squamous cell carcinoma are common skin tumors in dogs when occurring separately, however, our finding of their simultaneous occurrence in the same biopsy is a new and unusual finding.

Trichoblastoma, squamous cell carcinoma, collision tumor, dog

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