Squamous cell carcinoma with clear cell differentiation in an equine eyelid.

A 15-y-old Miniature horse mare had a 6-mo history of an ulcerated mass on the right lower eyelid. An incisional biopsy and a subsequent excisional biopsy were submitted to the Michigan State University Veterinary Diagnostic Laboratory for microscopic evaluation. Histologically, the incisional biopsy was composed of sheets of large neoplastic vacuolated polygonal cells. A few regions contained poorly differentiated neoplastic round-to-basaloid cells that rimmed the sheets of highly vacuolated polygonal cells. Both vacuolated and basaloid cells exhibited strong perimembranous and cytoplasmic immunoreactivity for E-cadherin and cytokeratin 5/6, respectively. Vacuolated polygonal cells were histochemically negative for periodic acid-Schiff, mucicarmine, and oil red O, consistent with a diagnosis of poorly differentiated carcinoma. Within the excisional biopsy specimen, there were anastomosing cords and nests of neoplastic squamous epithelial cells that merged with sheets of similar vacuolated polygonal cells. These findings are consistent with a squamous cell carcinoma with clear cell differentiation. In addition, in the adjacent dermis, there was solar elastosis suggestive of ultraviolet (UV) damage. A clear cell variant of squamous cell carcinoma is a rare entity in humans that previously has not been described in animals, to our knowledge, and is often associated with chronic UV exposure.