Squamous cell carcinomas (SCC) are malignant tumors originating from epithelial cells. They occur most frequently in unpigmented, sun-damaged skin and the most common locations are the nose, ears, eyelids and lips. SCC can also occur in the skin of cats with colored hair coats or in the mouth. SCCs are more common in cats who have feline immunodeficiency virus. Although the clinical presentation differs for each location and sub type, most SCCs are locally invasive, slow to metastasize, and occur in older cats. Diagnosis for all types is based on biopsy of the lesion(s) for histopathologic examination. Neoplastic diseases of squamous epithelial cell origin can be subdivided into groups based on depth of invasion and location of lesions.

• In actinic keratosis affected areas are reddened with variable degrees of hair loss, scaling, ulceration, crusting, pain, and pruritis. Sites commonly affected include the nose, ear margins, and eyelids. This is a precancerous condition that may lead to carcinoma in situ or squamous cell carcinoma if further sun exposure is allowed.
   

• Squamous cell carcinoma in situ is a cluster of cancerous epithelial cells confined to the most superficial skin layer. Lesions are generally located in haired, pigmented skin of the head, neck, shoulders, forelimbs and digits. Single or multiple tumors may occur and usually have crusts that leave an ulcer when removed. This cancer may invade the basement membrane and the dermis to become invasive squamous cell carcinoma with the potential for local and distant metastasis.
   

• Cutaneous squamous cell carcinoma is one of the most common skin tumors in cats. It is thought to be sunlight induced and it occurs most often in thin-haired, nonpigmented, sun-damaged skin. These tumors become locally invasive with the potential to cause disfiguration and loss of function. White cats have a much greater risk of developing cutaneous SCC than cats of other coat colors.
   

• Oral squamous cell carcinoma is the most common malignant oral tumor seen in domestic felines. It can hide anywhere in the mouth. One of the common locations is under the tongue. Oral SCC frequently infiltrates bone and muscle prior to diagnosis. Clinical signs may include decreased food intake, excessive salivation, oral hemorrhage and pain. This tumor carries a poor prognosis because it is locally aggressive and is frequently not amenable to surgical excision.