Page 1416 - Clinical Small Animal Internal Medicine

1354 Section 11 Oncologic Disease

In dogs and cats, basal cell tumors (also called tricho­ people and animals. Lifestyle and behavior contribute to
VetBooks.ir blastomas), epitheliomas, sebaceous adenomas, epidermal sun exposure in all species of animals. Thus, white cats
living largely outdoors are particularly vulnerable to cuta­
inclusion and other cysts, lipomas, and papillomas are
the most prevalent benign tumors noted. Mast cell tumor
their head, including ear tips, nasal planum, lower eyelids,
is the most common malignant skin tumor in the dog, neous squamous cell carcinoma in sun‐exposed areas of
and is discussed in Chapter 155. and the scantly furred preauricular area of the skin located
Trichoblastomas, which are more commonly still between the ears and eyes on the lateral forehead. Siamese
termed basal cell tumors, are the most common skin and other color‐point breeds of cats are protected from
tumors of cats; approximately 15–26% of all skin tumors cutaneous squamous cell formation by the pigment that is
in cats are of this histology. Less than 10% of basal cell present in the exact facial areas that put white cats at risk.
tumors in cats are malignant in behavior. When malig­ Similarly, dogs are most affected in areas of glabrous
nant, these lesions display a striking tendency toward (nonhaired) skin that receive excessive sunlight exposure.
lymphatic metastasis, which can result in a “chain of This can occur when dogs “sunbathe” by lying on their
pearls” nodular progression up the draining lymphatics to back or sides in full sun. Outdoor dogs with no pigment in
the local lymph node. In dogs, poodles and setters are pre­ glabrous skin regions are most at risk. Thus, Dalmatians,
disposed to basal cell lesions, which may be described by pointers, pit bulls, bull terriers, and other dogs with white
their histologic subtypes of ribbon, medusoid, trabecular, or pale skin on the ventral abdomen, prepuce, and medial
spindle, granular cell, and clear cell types. These lesions thighs may suffer actinic dermal injury that can ultimately
are now believed to arise from the germinal cells of the transition to squamous cell carcinoma. Dermal hemangi­
hair follicles, and are generally benign in behavior. oma and hemangiosarcoma lesions may similarly arise in
Dogs are commonly affected by benign tumors of glan­ areas of excessive ultraviolet light exposure.
dular origin in the skin, although this is a rare occurrence In addition to physical carcinogenesis from ultraviolet
in cats. Sebaceous hyperplasia, sebaceous adenoma, and light exposure, there is also a viral etiopathogenesis
sebaceous epitheliomas are very common in dogs. These associated with causation of cutaneous squamous cell
lesions are most commonly benign and are rarely diag­ carcinoma. Papillomavirus infection in cats may cause
nosed as sebaceous gland carcinomas, which are gener­ the feline equivalent of a human syndrome called Bowen
ally locally aggressive and rarely metastatic. Benign disease, which is a term used to identify multicentric
lesions are often seen in female dogs (especially hyper­ cutaneous squamous cell carcinoma in situ. This form of
plasia), with peak incidence in dogs 7–13 years of squamous cell carcinoma occurs most frequently in
age. Miniature schnauzers, beagles, poodles, and cocker haired skin and has no predilection to arise in nonpig­
spaniels are breeds at increased risk, as are Nordic mented areas of the body. These lesions can be seen as
breeds, coonhounds, and terriers. crusty, scabbed dermal lesions of the head, neck or trunk
Apocrine gland adenomas and carcinomas arise from of affected cats. Papillomavirus is also associated with
sweat glands in both dogs and cats. These lesions are the development of feline fibropapillomas called feline
rarely malignant but can be aggressive when transformed sarcoids. Interestingly, cats have been found to be sus­
to carcinomas. Feline apocrine gland carcinoma lesions ceptible to human papillomaviruses 5, 21, and 38, which
are seen primarily on the head, while benign lesions may were discovered by polymerase chain reaction (PCR)
be present on the head, limbs or abdomen of older cats. analysis of papilloma lesions in several cats.
The front legs are the most common sites for canine Dogs also harbor papillomaviruses, which may be
apocrine gland carcinomas, with golden retrievers and involved in the causation of oral, footpad, and cutaneous
coonhounds being predisposed breeds. Lesions are papillomas, and potentially even in squamous cell carci­
locally invasive with a predisposition to lymphatic nomas. These lesions are particularly observed in dogs
spreading. Apocrine gland adenocarcinomas of the that are immunocompromised.
anal sac are discussed elsewhere. Breed association is noted in dogs affected by tumors
Eccrine gland adenomas and carcinomas arise from of adnexal skin structures. Infundibular keratinizing
specialized sweat glands of the footpads in dogs and cats acanthomas are common benign tumors of dogs.
and are exceedingly rare lesions. Synonyms for these tumors include intracutaneous
Squamous cell carcinoma is common in both dogs and keratinizing epitheliomas, keratoacanthomas, and intra­
cats. This tumor of the skin is frequently caused by chronic cutaneous cornifying epitheliomas. Breeds at risk for
ultraviolet light exposure in animals with white or fair these benign lesions include Norwegian elkhounds and
skin and scant hair or fur covering. As sunlight is most other Nordic breeds, German and Belgian shepherd
damaging close to the equator and at high altitudes, these dogs, various terriers, and the Lhasa apso. These lesions
geographic regions of the globe have the highest incidence are benign and generally cured by local excision, although
rates of cutaneous squamous cell carcinoma in both they are frequently seen as multiple lesions.