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Nonmelanoma Skin Cancers
Barbara E. Kitchell, DVM, PhD, DACVIM (Oncology)

VCA Veterinary Care Animal Hospital and Referral Center, Albuquerque, NM, USA

Etiology/Pathophysiology important defensive mechanism is its self‐renewal and
repair functions, which are dependent on the presence of
The skin is the largest organ in the body, and has a both lineage‐committed replicating cells and pluripotent
complex tissue structure made up of many cell types. stem cells. These replicating cells are subject to mutation
Any of these cell types could give rise to a tumor. Some as a consequence of repeated noxious environmental
cells are more commonly affected by malignant transfor­ exposures, which may ultimately result in carcinogenesis.
mation than others.
The skin is composed of layers, referred to as the epi­
dermis and dermis. The epidermis contains stratified Epidemiology
epithelium, while the dermis is largely composed of con­
nective tissue, including collagen, extracellular matrix, All cancers arise as a consequence of genetic predisposi­
fibrocytes, and fibroblasts. Within the dermis also reside tions and environmental exposures to carcinogens. The
adnexal structures such as sebaceous or eccrine glands, skin is at particular risk for environmental injury. As a
and the various cells that make up the hair follicles. tissue with cells that continuously replicate or that can be
Below the dermis, subcutaneous tissues largely contain recruited to replicate, mutagenic induction followed by
adipocytes. Interspersed throughout the complex organ tumor cell promotion through repeated rounds of cell
we call skin are cells of the immune system, which replication result in skin tumors of various kinds. Thus,
are present as either transient or permanent residents cutaneous tumors are the most common type of neopla­
( lymphoid cells, macrophages, mast cells, Langerhans sia in both humans and dogs, although fortunately not all
dendritic cells). Panniculus skeletal muscle, neurons skin tumors are malignant.
and associated peripheral nerve sheath cells, vascular Genetic contributions to skin carcinogenesis are evi­
epithelium and hemangiopericytes, and the specialized denced by human familial hereditary syndromes, as well
neuroendocrine cells we know as melanocytes are all as by canine breed predispositions to specific tumor
important to normal skin structure. types. The skin is the most prevalent location for tumors
Thus, the number of potential histopathologic diagno­ in dogs, with an estimated 20–30% of these lesions being
ses for tumors of the skin is quite expansive. Skin tumors malignant. It is estimated that 25% of tumors of cats arise
are generally classified as being of epithelial, mesenchymal, in the skin, and that 50–65% of these lesions are malig­
melanotic, or round cell type. Mesenchymal, melanotic, nant. Excluding the common basal cell tumor of cats,
mast cell tumors, and cutaneous lymphoma will be dis­ which is generally benign, the incidence of malignancy in
cussed in other chapters. feline cutaneous tumors is estimated to be 70–80%.
The functions of this complex organ are also varied, and The estimated annual incidence of skin tumors in dogs
include prevention of water and electrolyte loss, ther­ and cats is 90.4 and 34.7 per 100 000 animals at risk,
moregulation, respiration, and sensory perception. A crit­ respectively, while for humans the annual incidence is
ical skin function is to provide a physical barrier to reportedly 5.2/100 000 people at risk. Nonmelanoma
environmental insults, whether mechanical, chemical, cutaneous tumors are the most common malignant
osmotic, thermal, or microbiologic. As the front line of neoplasms affecting people in the United States, but
defense against environmental insults, the skin’s most fortunately are not the most fatal types of malignancy.

Clinical Small Animal Internal Medicine Volume II, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical