Page 374 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

PART IV Specific Malignancies in the Small Animal Patient

VetBooks.ir 19

Tumors of the Skin and Subcutaneous

Tissues

MARLENE L. HAUCK AND MICHELLE L. OBLAK

Incidence Etiology

The overall incidence of tumors of the skin and subcutaneous tis- Physical Factors
sues of dogs and cats is difficult to determine because of inconsis-
tency of reporting, particularly with tumors of the subcutaneous Ionizing radiation and thermal injury are reported to increase the
tissues. If tumors determined to be of “skin” origin are considered, risk of skin cancer in many species; however, a recent epidemio-
the percentage of biopsy specimens has been reported to be 26% logic study on the incidence of cancer in human burn victims
3,4
to 43%. 1–6 Of these, between 20% and 40% are malignant. demonstrated no increased risk of the development of skin cancer
16
In a survey of neoplasms in Alameda and Contra Costa counties compared with the general population. Ultraviolet (UV) radia-
performed from 1963 to 1966, the estimated incidence of skin tion has long been known to cause neoplastic transformation in
and connective tissue tumors in dogs was 150.4/100,000; this the skin and is a major contributor to rising rates of skin cancer of
17
decreased to 90.4/100,000 when melanocytic skin tumors were all subtypes in people. Evidence for the role of UV irradiation
5
excluded. The incidence of skin and connective tissue tumors in in the development of skin tumors in cats and dogs is primarily
5
cats was 51.7/100,000. In one report, skin tumors represented epidemiologic 18–20 and supported by case reports on dogs diag-
10% of all feline biopsy or necropsy accessions, but skin tumors nosed with a spectrum of sunlight-induced lesions. 21,22
6
accounted for 30% of all cancer cases. Other studies report a The association of SCC development with solar exposure of
similar percentage of tumors arising from the skin, ranging from skin in light-colored cats has been established epidemiologically.
2
7
19% to 21%. Disregarding basal cell tumors (BCTs), the per- White cats in California had a 13.4-fold increased risk of devel-
centage of malignant skin tumors is higher in cats than dogs, oping SCC, and 143 of the 149 cases of nonoral SCC occurred
ranging from 70% to 82%. 6 on the head or neck. Similar results were found in a case series
2
18
The relative prevalence of the most common tumor types of nasal planum or pinnal SCC in cats; 58 of the 61 cats were
in dogs and cats can be determined from prevalence studies. white or partially white in color, and all but three cats spent time
23
In dogs, the numbers are based on a large number of surveys outdoors.
of skin tumor types from across the globe, totaling almost 9000
skin tumor submissions to various pathology services. 1,3,7–14 The Viral Factors
data for the prevalence of the most common tumors of dogs are
presented in Table 19.1. The overall prevalence of lipomas and The ability to induce neoplastic transformation in mucosal infec-
sebaceous adenomas is likely higher than reported because of the tions of papillomaviruses (PPVs) of people is well-established.
bias present in samples submitted for histopathologic evaluation. PPVs are only able to replicate in terminally differentiated cells,
The data on the prevalence of feline tumors is compiled from therefore infection of the keratinocyte can stimulate increased
four studies with a total of 1225 skin tumors and is presented proliferation and terminal differentiation. Neoplastic transfor-
24
in Table 19.2. 2,6,9,15 In cats, the top four tumor types of the skin mation arises from the viral effects on cell proliferation, integra-
and subcutaneous tissues are consistently BCTs, mast cell tumors tion into the genome, and interaction of papilloma viral proteins
(MCTs), squamous cell carcinoma (SCC), and fibrosarcoma; with cellular proteins, particularly the destabilization of p53 by
25
these account for approximately 70% of all feline skin tumors. viral protein E6 and the inhibition of pRB by viral protein E7.
The remainder of this chapter focuses on SCC, BCTs, glandular This disruption in p53 can result in increased levels of p16 pro-
26
skin tumors, and additional assorted primary tumors of the skin, tein, which is detectable with immunohistochemistry (IHC).
ears, and digits. Other tumor types are discussed in specific chapters: The association of PPV infection with cutaneous SCC in people is
melanomas (Chapter 20), MCTs (Chapter 21), soft tissue sarco- primarily epidemiologic: organ transplant recipients and immu-
mas (Chapter 22), cutaneous lymphoma (Chapter 33, Sections A nosuppressed individuals have an increased rate of cutaneous
and B), and hemangiosarcoma (Chapter 34, Section A). PPV infection and an increased risk of SCC development. 27,28
352