Case 180504-1

Eighteen-year-old, female, Appaloosa horse

A mass was noted on the vulva by the referring veterinarian, who performed repeated liquid nitrogen treatments over the course of several years. The mass continued to recur, and an additional, smaller mass also developed. A punch biopsy performed in December 2017, resulted in a diagnosis of an incompletely excised squamous-cell carcinoma. The masses were surgically excised in February 2018, and were submitted for histopathology.

An 8 x 3 x 2.5 cm, irregularly shaped, tan to dark red, thickened, superficially ulcerated piece of haired and non-haired skin with subjacent subcutaneous tissue and skeletal muscle was received.

One section of sparsely haired skin, dermis, and skeletal muscle is examined in which the histologic appearance of the epidermis varies throughout the section. The epidermis is normal, near the margins of the sample. Towards the center of the tissue, the epidermis becomes hyperplastic with thickening and vacuolation of the stratum spinosum. Further towards the center of the tissue, the basal layer of the epidermis forms rete pegs into the underlying dermis and exhibits increasing levels of cellular and nuclear atypia, characterized by anisocytosis, anisokaryosis, and multiple prominent nucleoli. The basement membrane in this region is intact. Near the center of the tissue, the epidermis is expanded by nests, packets, and lobules of polygonal epithelial cells supported by a fine fibrovascular stroma that extend along the follicles. The cells have variably distinct cell borders, moderate amounts of finely vesiculated, lightly amphophilic cytoplasm, and round to oval nuclei with finely stippled chromatin and one to three prominent, basophilic nucleoli. Anisocytosis is mild, and anisokaryosis is marked. Seventeen mitotic figures are seen in ten 400x fields. The neoplastic cells occasionally undergo squamous metaplasia. Numerous isolated, necrotic cells are scattered throughout this region. The overlying superficial epidermis is multifocally eroded and ulcerated, and is overlain by a serocellular crust. The superficial and periadnexal dermis is densely infiltrated with lymphocytes and plasma cells. Occasionally the glands are dilated, contain spicules of mineral and cholesterol clefts, and are surrounded by variable numbers of macrophages.

N/A

Vulva: Basaloid squamous cell carcinoma

Vulva: Severe, chronic, multifocal to coalescing, superficial and periadnexal, lymphoplasmacytic dermatitis and multifocal, granulomatous adenitis with intraluminal mineralization and cholesterol clefts

This case exhibits a wide spectrum of epidermal proliferative changes, including hyperplasia, dysplasia and carcinoma in situ, with eventual progression to carcinoma. A diagnosis of a basaloid squamous cell carcinoma was made based on the basal orientation and basaloid cytologic appearance of the neoplastic cells with rare squamous differentiation. However, a basal-cell carcinoma is a possible differential diagnosis as well. The inflammation adjacent to the tumor is likely secondary to chronic ulceration and irritation of the mass.

UV-induced epidermal neoplasms are a well-documented phenomenon in lightly pigmented horse breeds such as the Appaloosa. Sarcoids and squamous cell carcinomas are the most prevalent types of skin neoplasms in horses, whereas equine basal cell carcinomas are much less common. Cryotherapy with liquid nitrogen, as was performed in this case prior to surgical resection, is one of many treatment options for cutaneous neoplasms. Histologic changes associated with cryotherapy include homogenization and edema of the epidermis and superficial dermis, epidermal hyperplasia, elastosis, and hypopigmentation due to the increased susceptibility of melanocytes to freezing. Mild dermal elastosis and glandular pigmentation is seen, but solar and cryotherapy effects are not a major feature of this case, despite the presumed UV-induced nature of the tumor and the previous treatments.

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