Page 454 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

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Cancer of the Gastrointestinal Tract

SECTION A: ORAL TUMORS Malignant Melanoma
In comparison to other malignant oral tumors, MM tends to
occur in smaller body weight dogs. Cocker spaniel, minia-
JULIUS M. LIPTAK ture poodle, Anatolian sheepdog, Gordon setter, Chow Chow,
and golden retriever are overrepresented breeds. The mean
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Incidence and Risk Factors age at presentation is 11.4 years. MM occurs in cats, but is
11
uncommon. 29
Oral tumors are common in both cats and dogs, with cancers of MM can present a confusing histopathologic picture if the
the oral cavity accounting for 3% to 12% and 6% of all tumors tumor or the biopsy section does not contain melanin (Fig.
in these species, respectively. 1–5 Oropharyngeal cancer is 2.6 23.1A), and amelanotic melanomas (see Fig. 23.1B) represent
times more common in dogs than in cats, and male dogs have up to 38% of cases. A histopathologic diagnosis of undifferen-
30
a 2.4 times greater risk of developing oropharyngeal malignancy tiated or anaplastic sarcoma or even epithelial cancer should be
compared with female dogs. A male sex predisposition has also viewed with suspicion for possible reclassification as melanoma.
6,7
been reported for dogs with malignant melanoma (MM), tonsil- Several immunohistochemical antibodies can be applied to biopsy
lar squamous cell carcinoma (SCC), and peripheral odontogenic specimens to help differentiate melanoma from other poorly dif-
fibromas, and a female sex predisposition has been reported for ferentiated tumors and an immunodiagnostic cocktail using anti-
8,9
dogs with axial osteosarcoma (OSA). Dog breeds with the high- bodies against PNL2, Melan A, TRP-1, and TRP-2 was found to
10
est risk of developing oropharyngeal cancer include the cocker have 100% specificity and 94% sensitivity and may be helpful in
spaniel, German shepherd dog, German shorthaired pointer, Wei- differentiation. 11,31
maraner, golden retriever, Gordon setter, miniature poodle, Chow Melanoma of the oral cavity is a highly malignant tumor with
Chow, and boxer. 3,8,11 In one study, German shepherd dogs and frequent metastasis to the regional lymph nodes (LNs) and then
boxers had a decreased risk of developing oral melanoma. 11 the lungs. 9,30–68 There is a small subset of dogs with well-differen-
In dogs, the most common malignant oral tumors are, in tiated oral melanomas and these may have a more benign biologic
descending order, MM, SCC, and fibrosarcoma (FSA), 12–24 behavior. 41,69 The metastatic rate is site, size, and stage dependent
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although in other studies SCC is more common than MM. and reported in up to 80% of dogs. 12,20,30–68 The World Health
SCC is the most common oropharyngeal cancer in cats, followed Organization (WHO) clinical staging system for oral tumors in
by FSA, which accounts for 13% of feline oral tumors. Other dogs may have prognostic significance in dogs with oral mela-
5
malignant oral tumors in dogs include OSA, chondrosarcoma, noma (Table 23.2). 30–68,70 MM is a highly immunogenic tumor,
anaplastic sarcoma, multilobular osteochondrosarcoma (MLO), and molecular and immunomodulatory approaches to treatment
intraosseous carcinoma, myxosarcoma, hemangiosarcoma, lym- are active areas of research and treatment. 56–68 A review of the
phoma, mast cell tumor, and transmissible venereal tumor. 12–28 biology and molecular mechanisms of canine melanoma develop-
Tumors or tumor-like lesions of unusual sites, types, and biologic ment and progression is provided in Chapter 20. 71,72
behavior (e.g., peripheral odontogenic fibroma, acanthomatous
ameloblastoma [AA], odontogenic tumors, tonsillar SCC, tongue Squamous Cell Carcinoma
tumors, malignancy of young dogs, viral papillomatosis, canine
and feline eosinophilic granuloma complex, and nasopharyngeal SCC is the most common oral tumor in cats (Fig. 23.2) and the
polyps) will be covered at the end of this chapter. A general sum- second most common in dogs. 1,4,5,20–24 There are five different
mary of the common oral tumors is found in Table 23.1. histologic subtypes of SCC in dogs: conventional, papillary, basa-
loid, adenosquamous, and spindle cell. Papillary SCCs typically
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Pathology and Natural Behavior occur in the rostral oral cavity of dogs less than 9 months old,
although cases in older dogs have also been reported. 73–77 The
The oral cavity is a very common site for a wide variety of metastatic rate for nontonsillar SCC in dogs is 5% to 29%, 43,76–85
malignant and benign cancers. Although most cancers are fairly but the metastatic risk is site dependent, with the rostral oral cav-
straightforward histologically, some have confusing nomenclature ity having a low metastatic rate and the caudal tongue and tonsil
or extenuating circumstances that warrant discussion. having a high metastatic potential. 77

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