Cutaneous squamous cell carcinoma manifesting as follicular isthmus cysts in a cat

CASE SUMMARY A 9-year-old spayed female domestic shorthair cat was examined for swelling of the right upper lip. The cat had been receiving oral ciclosporin A for eosinophilic plaques. The swelling appeared clinically and cytologically consistent with an abscess; exudate was cultured and treatment consisted of antibiotic therapy and surgical curettage. Five months of antibiotic therapy with three separate surgical treatments resulted in minimal improvement; three separate biopsy samples demonstrated epithelial cysts with severe dermal inflammation. Swelling and drainage of purulent material from the affected lip persisted and progressed to involve the left upper lip. Euthanasia was elected 13 months after initial examination due to disease progression. On necropsy, histopathology demonstrated multiple isthmus cysts intermixed with squamous cell carcinoma (SCC). RELEVANCE AND NOVEL INFORMATION The clinical and histopathologic features were unusual for feline cutaneous SCC. The cystic nature and lack of epidermal involvement suggest the tumor arose from non-epidermal squamous cells such as follicular isthmus or ductal epithelium. There is a pattern of SCC recognized in human renal transplant patients with features of epidermal inclusion cysts. These features have not been previously reported in SCC from a cat.