Squamous Cell Carcinomas
(Epidermoid carcinomas, Prickle cell carcinomas) |  |
| Thought to arise from either the epidermis or the epithelium of the superficial (infundibular) regions of the outer root sheath of the hair follicle, squamous cell carcinomas have been recognized in all domestic animals. Although most arise without antecedent cause, in many species, especially in white cats, prolonged exposure to sunlight is a major predisposing factor. The grooming habits of cats also expose them to particulate carcinogens from cigarette smoke and flea collars. In
addition, a unique form of feline squamous cell carcinoma associated with papilloma virus infection has been described (see below). |
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In dogs, these are the most frequently diagnosed carcinomas arising in the skin. Two forms are recognized—cutaneous and subungual.
Cutaneous squamous cell carcinomas are tumors of older dogs, with Bloodhounds, Basset Hounds, and Standard Poodles at greatest risk. Lesions commonly arise on the head, distal extremities, ventral abdomen, and perineum. Most cutaneous squamous cell carcinomas appear as firm, raised, frequently ulcerated plaques and nodules; sometimes they can be extremely exophytic and have a surface reminiscent of a wart. The etiology of most of these tumors is undefined; however, some
are induced by prolonged solar injury. These usually develop on ventral abdominal, preputial, scrotal, and inguinal skin in white-skinned, shorthaired breeds such as Dalmatians, Bull Terriers, and Beagles. They develop in a ventral location because the poorly haired skin offers minimal shielding from ultraviolet radiation, many animals sun themselves lying on their backs, and perhaps because solar radiation reflects from the ground. Before a carcinoma develops, animals acquire focal
zones of lichenification, hyperkeratosis, and erythema known as
solar keratosis
(solar dermatosis,
actinic keratosis,
senile keratosis). |
|
Subungual squamous cell carcinomas are most commonly found in Giant and Standard Schnauzers, Gordon Setters, Briards, Kerry Blue Terriers, and Standard Poodles. Generally, all are darkhaired breeds, and a dark coat color has been associated with the development of subungual squamous cell carcinomas arising on multiple digits, often on different extremities. Females have a slight predilection and both fore- and hindlimbs are equally predisposed to tumor development. |
| In cats, cutaneous squamous cell carcinomas most commonly develop in conjunction with chronic solar injury. Consequently, they usually develop on the pinnae, frontal ridges, eyelids, nose, or lips of cats that have white skin in these regions. There is no breed or sex predilection. As in dogs, solar keratosis or carcinoma in situ (early superficial stage), often precedes development of a malignant tumor. Recently, coat-associated particulate carcinogens from exposure to cigarette
smoke and flea collars have been identified as risk factors for cats with oral squamous cell carcinoma. Lesions not caused by sun exposure most commonly develop on the digits, but subungual forms are uncommon. |
| Cutaneous squamous cell carcinomas are the most common malignant neoplasm in horses. They generally develop in adult or aged horses with white or part-white coats; breeds at risk include Appaloosa, Belgian, American Paint, and Pinto. Although they can arise anywhere on the body, these tumors most commonly arise in nonpigmented, poorly haired areas near mucous membranes. Thus, the periorbital regions, lips, nose, anus, and external genitalia (especially the penile sheath) are sites
most likely to be affected. |
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In cattle, these tumors are most common in breeds with white hair and poorly pigmented skin (especially Holsteins and Ayrshires) and, as in horses, develop around the mucous membranes, usually at the mucocutaneous junctions, particularly the periocular and vulvar regions. In India, squamous cell carcinomas of the horn core are common in aged bullocks. The most common cause is actinic injury. Solar keratoses often precede development of an
invasive tumor; genetic factors, immunodeficiency, and viruses may also play a role. |
| In sheep, squamous cell carcinomas are of economic significance in some parts of the world. In a study in Australia, they were responsible for more than one-third of all condemnations before slaughter. The Merino breed is most at risk, and females more so than males. The most common sites are the poorly haired skin of the ears, lips, muzzle, and the vulvar lips after they have been externalized by Mules operation to prevent fly strike. Tumors at these sites develop in conjunction
with solar injury, which is heightened when animals ingest photosensitizing plants. Tumors of the ears also occur more frequently after a procedure such as ear tagging. Squamous cell carcinomas can develop from follicular cysts on sites not commonly exposed to sunlight. |
| In goats, squamous cell carcinomas develop most frequently in females, in which tumors develop on the perineal and vulvar regions and on the skin of the teats and udders. Both males and females can develop sun-induced tumors on the ears. Although Angoras are most at risk, Saanan goats occasionally develop squamous cell carcinomas on the udder in association with papillomas. The role papilloma viruses play in tumor progression is undefined. |
| Squamous cell carcinomas are extremely uncommon in swine. |
| Most squamous cell carcinomas are solitary lesions; however, multiple tumors may develop in conjunction with solar injury. They appear as endophytic or exo-endophytic lesions, the former as raised, irregular dermal masses with an ulcerated surface, and the latter as raised, irregular dermal masses covered by a papillated epidermis. Cats initially exhibit small crusting facial sores that do not heal. The lesions are often allowed to persist for months before defects appear on the
ear tips, nares, and eyelids. Subungual squamous cell carcinomas of dogs are first identified by lameness or malformation, an infection that mimics chronic osteomyelitis, or loss of the claw of the affected digit. In cattle with involvement of the horn, the first sign is distorted growth. |
| Squamous cell carcinomas are characteristically invasive into adjacent soft and bony tissues. Infrequently, in cattle, they regress spontaneously. In small animals, longterm survival and the likelihood of metastasis are correlated with histologic differentiation. Well-differentiated tumors are slowly progressive or remain localized; undifferentiated tumors are more likely to metastasize or recur within 20 wk of excision. In general, failure of treatment is due to late diagnosis and
lack of control of local disease rather than metastasis. |
| For dogs and cats, surgical excision, such as amputation of the involved digit or pinnae or nosectomy, is the treatment of choice, and margins of at least 2 cm are recommended. One review of 117 digit masses in dogs found that 25% of the lesions were squamous cell carcinomas and 66% were subungual lesions. These had a 95% 1-yr survival after amputation; however if the lesion originated in other parts of the digit, the 1-yr survival was 60%. Excision may be combined with radiation
or chemotherapy. Feline squamous cell carcinomas are more radiosensitive than their canine counterparts. Still, the 1-yr survival rate is <10% for invasive neoplasms. Cryosurgery and hyperthermia may be helpful for local therapy especially in early (carcinoma in situ) lesions, but controlled studies have not been done to determine their effectiveness. Intralesional implant chemotherapy with 5-fluorouracil, cisplatin, or carboplatin along with retinoids and photodynamic therapy has
been used with variable success. Intratumoral injection of nasal plane squamous cell carcinomas in cats using carboplatin in a water-sesame seed oil emulsion resulted in a 70% general response with 1-yr progression-free survival rate of ~50%. In dogs with multiple ventral actinic keratoses, topical dinitrochlorbenzene or 5-fluorouracil (5%) may be of benefit. Limiting exposure to ultraviolet radiation may help prevent solar-induced squamous cell carcinomas in dogs and cats. This may
be accomplished by using UV window screens, sunscreen, and keeping the animals indoors during hours of peak sunlight. Tattoos, magic markers, and sunscreen are used with variable success. In horses, radiotherapy using surface or interstitial brachytherapy is the treatment of choice for squamous cell carcinomas. Other options include 90Sr or 192Ir implants, wide surgical excision (especially for neoplasms of the third eyelid,
penis, and prepuce), and cryosurgery. Immunotherapy, with either an autogenous vaccine made from the tumor tissue suspended in Freund’s adjuvant, or nonspecific immunomodulation using
Corynebacterium
parvum
, has had some success in treating ocular or horn core squamous cell carcinomas in cattle. |
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Feline multicentric squamous cell carcinoma in situ
(feline Bowen’s disease) is a disease of aged (>10 yr old) cats may be associated with immunosuppression. There is no defined breed or sex predilection. Clinically, lesions appear as multiple discrete, erythematous, black or brown hyperkeratotic plaques and papules. Lesions are nonpruritic, and ulceration is uncommon. Their development is associated with the presence of a papilloma virus. The term in situ refers to a
malignant proliferation of epidermal and follicular outer sheath cells that are not invasive into the underlying dermis. Unfortunately, lesions may progress over time into an invasive carcinoma. Metastasis is extremely uncommon. These lesions usually develop in systemically ill or immunosuppressed cats and are believed to be virally induced; they have not been amenable to therapy. |
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