Case 4 - 170623

The patient was a 19-year-old, Arabian/Quarter horse mix breed, gelding

This patient presented to the ophthalmology department for evaluation of a mass located at the medial canthus of the right eye with associated increased tear production. This mass was first noticed 6 months prior to presentation. At that time, he was treated with topical antibiotics, which resulted in little improvement regarding tearing of the right eye. The patient did not seem to experience discomfort from his right eye. The mass recently grew in size prior to presentation. On physical examination additional small pigmented masses were identified on the underside of his tail base and in the perianal region, which were removed at same time as periocular mass, but not submitted.

One small sample of haired skin was received for histopathology.

Duplicates of two sections of sparsely haired skin and mucosa are on this slide and all have an approximately 10.5 mm subepithelial, partially pigmented, variably well demarcated, expansile, moderately cellular mass. The mass is composed of dissecting thin sheets and streams of cells that separate and isolate preexisting collagen fibers. The lateral and deep aspects of the mass are well demarcated while the superficial aspect has neoplastic cells that trickle into the overlying subepithelial stroma. These cells are ovoid to angular to spindloid with a moderate amount of fibrillar eosinophilic cytoplasm and an ovoid to elongated finely stippled nucleus with indistinct to one nucleolus. A majority of the neoplastic cells have cytoplasmic brown globular pigment which varies from scant amount to heavily pigmented. Two mitotic figures per 10 high power fields are identified. The superficial aspect of the pigmented mass blends with the superficial dermis and surrounds adnexa.

There is superficial dermal pallor (solar change) with wispy thin collagenous fibers and moderate numbers of infiltrating lymphocytes and plasma cells. Associated small caliber vessels are moderately reactive with plump endothelium. Regionally there is a moderate amount of thick bands of elastosis intermixed with dermal collagen and occasional hyalinized and smudged vessel walls. Predominantly overlying this region of solar change the epithelium extends into the superficial dermis/submucosa (junction) in thin short, anastomosing rete pegs and one region where small nests and individual atypical epithelial cells drop out and are isolated within the subjacent stroma. These isolated nests of atypical squamous epithelial cells rarely surround small keratin pearls. Mitotic count cannot be assessed based on small area. The epithelium is regionally ulcerated and overlain by a thin mat of fibrin and small numbers of erythrocytes.

The region of suspected atypical infiltrative neoplastic squamous cells are strongly immunoreactive for a pan-cytokeratin immunohistochemistry stain.

MEDIAL CANTHUS OF RIGHT EYE: MELANOMA (COMPLETELY EXCISED) MEDIAL CANTHUS OF RIGHT EYE: EPIDERMAL DYSPLASIA WITH FOCAL SQUAMOUS CELL CARCINOMA (COMPLETELY EXCISED) MEDIAL CANTHUS OF RIGHT EYE: MODERATE, CHRONIC, LYMPHOPLASMACYTIC CONJUNCTIVITIS WITH MODERATE, SOLAR ELASTOSIS, HYALINIZED VESSEL WALLS AND FOCAL ULCERATION (SEE COMMENT)

This case represented a spectrum of interesting lesions including multiple neoplastic processes. This horse was described as being gray, which are considered predisposed to multifocal melanomas, particularly around the tailbase. In this case, the melanoma was mainly underlying areas of pigmented epithelium. There is one segment of non-pigmented epithelium there is variable epithelial dysplasia and transition into squamous cell carcinoma, confirmed with immunohistochemistry for pancytokeratin. In association with the non-pigmented epithelium and squamous cell carcinoma are marked solar changes including solar elastosis, hyalinization of dermal vessels, epithelial dysplasia and collagenous pallor, which are considered UV-light induced and progress to squamous cell carcinoma.

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