Clear cell squamous cell carcinoma in the third eyelid of a horse

Squamous cell carcinoma (SCC) is the most common ocular and periocular neoplasm of horses, typically arising from the limbus, third eyelid mucosa or mucocutaneous junctions of the upper and lower eyelids. Tumours are locally invasive, prone to recurrence and may rarely metastasize. Histological evaluation is required for definitive diagnosis and atypical SCC subtypes may be diagnostically challenging. Here we describe a clear cell SCC in the third eyelid of a 16-year-old female Quarter Horse. The red and invasive mass was surgically excised for histological evaluation. The lesion consisted of a poorly demarcated, exophytic proliferation of neoplastic cells arranged in lobules supported by a scant stroma. Neoplastic cells had round, mostly clear cytoplasm with distinct cell borders and round to oval nuclei with coarse chromatin and a prominent nucleolus. Squamous differentiation was observed throughout. The mitotic count was 23 in 2.37 mm² (equivalent to 10 FN22/ × 40 fields). There was one area of vascular infiltration by neoplastic cells. Dysplastic epithelial plaques were observed in the adjacent third eyelid mucosa. Most neoplastic cells had no cytoplasmic staining with periodic acid-Schiff reaction and were negative for mucus and lipid using Mayer mucicarmine and Sudan IV stains, respectively. A mild degree of submucosal elastosis was observed using Verhoeff-van Gieson stain. Neoplastic cells had widespread, robust cytoplasmic labelling for pancytokeratin (AE1/AE3). Ocular or periocular clear cell SCCs have not been previously reported in domestic animals.