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Elizabeth Rustemeyer May, DVM, DACVD, University of Tennessee
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Although most skin tumors encountered in veterinary patients are benign, malignant tumor types do occur. Because many are erythematous and pruritic, they can be easily confused with commonly diagnosed pruritic conditions (eg, scabies, allergic disease). The following tumors are commonly misdiagnosed based on their gross appearance.
Figure 1 Nodular sebaceous hyperplasia Benign sebaceous gland tumor on the lateral thorax of a dog. Sebaceous gland tumors develop anywhere sebaceous glands are present, but occur primarily on the limbs and trunk; they can cause discomfort if they become secondarily infected. Sebaceous gland tumors are often observed on canine skin and infrequently on feline skin. These tumors have a recognizable wart-like appearance, but unlike papillomatous skin lesions, do not have a known viral cause. Although most sebaceous gland tumors are classified as the nodular sebaceous hyperplasia type, other types of sebaceous gland tumors—including sebaceous epithelioma, sebaceous adenoma, and sebaceous adenocarcinoma—are differentiated by distinctive histologic features.
Figure 2 Cutaneous papilloma, ventral surface of tailSolitary, raised, keratinized mass on the tail of a dog. This papilloma has an irregular fronded appearance and a whitish gray-to-yellowish color. Its location (the underside of the tail of a long-haired dog) is considered unusual for development of a cutaneous papilloma, which are more frequently encountered on the head or limbs of dogs. Because of its similarity in clinical appearance to other neoplastic skin lesions, particularly squamous cell carcinoma (SCC), this lesion was surgically removed and submitted for histopathology for definitive diagnosis.
Figure 3 Canine oral papillomaGrayish, shiny, solitary pedunculated mass originating from the lip margin of a young dog. Note the classic cauliflower appearance of this mass caused by a papillomavirus as compared with the appearance of the tail mass shown in Figure 2. This benign skin tumor occurs frequently in young (ie, <2 years of age) dogs and usually spontaneously regresses within a few months. Surgical excision may be necessary if multiple masses are present. If lesions become ulcerated, surgical removal and histopathology should be considered to prevent transformation into SCCs. While considered uncommon, progression of papilloma to SCC can occur and has been previously reported.1
Figure 4 Interdigital cystDorsal (A) and plantar (B) appearance of a large keratinized interdigital cyst on the foot of a boxer. Especially important for successful treatment is recognizing that the problematic cyst is located interdigitally on the plantar surface of the foot, whereas the connecting draining tract and much of the observable inflammation and subsequent discomfort is present on the dorsal aspect. Treating the draining tract without addressing the primary problem by completely removing the cyst from the plantar aspect of the foot could result in recurrence of clinical signs.
FIGURE 4 Interdigital cystDorsal (A) and plantar (B) appearance of a large keratinized interdigital cyst on the foot of a boxer. Especially important for successful treatment is recognizing that the problematic cyst is located interdigitally on the plantar surface of the foot, whereas the connecting draining tract and much of the observable inflammation and subsequent discomfort is present on the dorsal aspect. Treating the draining tract without addressing the primary problem by completely removing the cyst from the plantar aspect of the foot could result in recurrence of clinical signs.
Figure 5 Feline squamous cell carcinoma in situ, Bowen diseaseSCC in situ in a cat. SCCs are malignant cutaneous neoplasms routinely diagnosed in dogs and cats. Lightly pigmented dogs and cats can develop neoplastic skin lesions resulting from ultraviolet light exposure. However, SCCs in situ (ie, Bowen disease) are less common in cats and develop regardless of ultraviolet light exposure. Note that this lesion, located above the left eye, is plaque-like and pigmented, which is atypical of SCC neoplasms. Lesions consistent with those of Bowen disease are multiple in number and frequently located on the head but can also be located on the skin of the neck, dorsal trunk, and limbs. As lesions progress, they can bleed easily with minimal manipulation.
Figure 6 Fibrovascular papillomas, multiple (skin tags)Benign tumors in a dog. In this patient, multiple skin tags (fibrovascular papillomas) are located on the skin of the sternum and ventral thorax. The tags are more numerous on the sternum and thus are likely associated with chronic friction or pressure. The pedunculated and pigmented appearance may resemble papilloma-induced viral pigmented plaques, which have been reported in pugs and miniature schnauzers. In contrast, fibrovascular papillomas are not associated with papillomavirus infection, despite the nomenclature, and do not cause clinical problems beyond altered cosmetic appearance.
Figure 7 Fibrovascular papilloma, single (skin tag) Unusual fibrovascular papilloma in a dog. This skin tag is focal and not located in an area of known friction or trauma. The narrow stalk and solitary nature of the lesion made it easy to remove via CO2 laser excision. There was no evidence of regrowth or development of new lesions 2 years later.
Figure 8 MiliaGroups of milium cysts (ie, milia) in a dog. These cysts develop when keratin is trapped beneath the skin surface and accumulates into thin-walled cystic structures. Development of milia in dogs is consistent with steroid excess, similar to that associated with spontaneous or iatrogenic hypercortisolemia, and is particularly common after application of topical corticosteroids. In this dog, multiple very large milia are present in a locally extensive area because of repeated application of a potent topical steroid spray. The well demarcated affected area of skin suggests previous application of a product to this location. In addition, note the large dilated comedones and evidence of cutaneous atrophy, as demonstrated by prominent blood vessels and a crepe-paper appearance of the skin.
Figure 9 Epitheliotropic lymphoma, felineCutaneous lymphoma located along the medial aspect of the left thoracic limb in a cat. These lesions are pruritic and improve with corticosteroid therapy, making them easily mistaken for eosinophilic plaques, which are steroid-responsive pruritic lesions common in cats. Surface cytology of cutaneous lymphoma neoplasms is not predominantly eosinophilic in nature and, therefore, is not consistent with cytology of an eosinophilic plaque. Findings should prompt the clinician to consider other diagnostic differentials and pursue skin biopsy.
Figure 10 Epitheliotropic lymphoma, canineCutaneous lymphoma lesions located in the inguinal region of a dog. Patients with this type of lesion are intensely pruritic, and lesions can be very similar in clinical appearance to lesions caused by staphylococcal pyoderma, demodicosis, or dermatophytosis. Note the erythematous borders of these lesions along with the circular rims of scale along the periphery and varying degrees of hyperpigmentation. These features are similar to a bacterial skin infection secondary to allergic dermatitis. Epitheliotropic lymphoma is more common in dogs more than 10 years of age; however, the pruritus associated with this disease may be steroid-responsive, and progression of the disease is slow, which may complicate diagnosis.
Figure 11 Mast cell tumor, felineCutaneous mast cell tumor (MCT) in a cat. Note the discrete raised masses above both eyes, one of which has surface crust formation. MCTs in cats occur much more commonly on the skin of the head and neck; although discrete masses are more obvious, some patients present with areas of indistinct swelling or lesions that appear more like sebaceous gland tumors when near the eye itself.
Figure 12 Mast cell tumor, canineCutaneous MCT in a dog. On initial examination, the MCT on the lateral aspect of the head has a very similar clinical appearance to that of pyotraumatic dermatitis. However, cytology findings were not primarily neutrophilic with intracellular and extracellular cocci, as would be expected with a diagnosis of pyotraumatic dermatitis. Instead, the mast cells were poorly differentiated without granules, making cytologic diagnosis challenging when compared with findings for most canine cutaneous MCTs.
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