CHAPTER | 14 Neoplastic and Nonneoplastic Tumors

Amy Leblanc *,

Intracutaneous Cornifying Epithelioma (keratoacanthoma, infundibular keratinizing acanthoma)

Feline Solar Dermatosis

Canine Solar Dermatosis

Squamous Cell Carcinoma

Bowen’s Disease/Multifocal Squamous Cell Carcinoma In Situ

Basal Cell Tumor/Carcinoma

Hair Follicle Tumors

Sebaceous Gland Tumors

Perianal Gland Tumors

Apocrine (epitrichial) Sweat Gland Cysts and Tumors

Fibropruritic Nodule

Fibroma

Fibrosarcoma

Nodular Dermatofibrosis

Hemangioma

Hemangiosarcoma

Hemangiopericytoma

Lipoma

Liposarcoma

Mast Cell Tumor

Nonepitheliotropic Lymphoma (lymphosarcoma)

Epitheliotropic Lymphoma (mycosis fungoides)

Cutaneous Plasmacytoma

Cutaneous Histiocytoma

Cutaneous Histiocytosis

Systemic Histiocytosis

Malignant Histiocytosis

Cutaneous Melanocytoma/Melanoma

Transmissible Venereal Tumor (TVT)

Collagenous Nevus

Follicular Cyst–Epidermal Inclusion Cyst (infundibular cyst)

Cutaneous Horns

Skin Tags (fibrovascular papilloma)

Calcinosis Circumscripta

Intracutaneous Cornifying Epithelioma (keratoacanthoma, infundibular keratinizing acanthoma)

Features

This is a benign neoplasm of hair follicle origin. It is uncommon in dogs. Solitary nodules can occur in any age or breed of dog; multicentric nodules occur most commonly in young male Norwegian elkhounds and Keeshonds.

This condition appears as single to multiple (as many as 40–50) firm to fluctuant, well-circumscribed, dermal or subcutaneous nodules ranging from 0.5 to 4 cm in diameter. Nodules may be partially alopecic, and most have a variably sized, dilated, central pore that opens directly to the skin surface; from this pore, gray-brown keratinaceous material can be expressed. Large pores may contain a hard, hornlike, keratin plug. Tumors deeply located in the dermis and subcutis may not have pores. Lesions may appear anywhere on the body but are most commonly found on the dorsal neck, back, and tail.

Diagnosis

1 Cytology (usually nondiagnostic): amorphous cellular debris and mature cornified squamous epithelial cells with cholesterol crystals.

2 Dermatohistopathology: lamellated keratin-filled cavity (which may contain a pore to the skin surface) lined with stratified epithelial cells. Focal rupture may release keratin into the dermis, inciting a pyogranulomatous reaction in surrounding tissue.

Treatment and Prognosis

1 Surgical or laser excision is curative if lesions are solitary to few in number. Cryotherapy may also be effective.

2 For multiple lesions, treatment with acitretin 0.5–2 mg/kg/day or isotretinoin 1–3 mg/kg/day PO may be effective in some dogs. A good response should be seen after 3 months of treatment. Those that respond usually require lifelong therapy to maintain remission. Vitamin A (8000–10,000 IU/10 kg/day) may be a less potent alternative.

3 After surgical removal, the prognosis for cure is good for dogs with a solitary lesion, but dogs with more than one tumor are likely to develop new tumors at other sites. The prognosis for resolving multiple lesions is fair to good with medical treatment. These tumors are benign and do not metastasize.

FIGURE 14-1 Intracutaneous Cornifying Epithelioma.

A small keratin nodule associated with underlying tumor. The keratin nodule can easily be mistaken for a crust.

FIGURE 14-2 Intracutaneous Cornifying Epithelioma.

An intracutaneous cornifying epithelioma and cutaneous horn on the lateral thorax of a young adult German shepherd.

Feline Solar Dermatosis

Features

This type of dermatosis is caused by actinic damage to white-haired skin. Initially, the skin becomes sunburned, but with repeated exposure to ultraviolet light, preneoplastic lesions (actinic keratoses, squamous cell carcinoma in situ) and squamous cell carcinoma may develop. The condition is common in older outdoor cats and indoor cats that like to sunbathe.

Initially, mild erythema, scaling, and alopecia of the white-haired skin may be observed. With continued exposure to sunlight, the skin becomes progressively erythematous and alopecic, crusted, ulcerated, and painful. The ear tips/margins are most commonly affected, but lesions may also occur on white-haired eyelids, nose/nasal planum, or lips.

Top Differentials

Differentials include dermatophytosis, trauma, autoimmune skin disease, vasculitis, hypersensitivity (flea, food, atopy), and squamous cell carcinoma.

Diagnosis

1 Usually based on signalment, history, and clinical findings.

2 Dermatohistopathology: in early lesions, epidermal hyperplasia and superficial perivascular dermatitis may be observed. Vacuolated epidermal cells, dyskeratotic keratinocytes, and basophilic degeneration of elastin (solar elastosis) may be seen. In advanced lesions, the epidermis may be dysplastic, without invasion through the basement membrane (actinic keratosis), or the dermis may be invaded by nests of dysplastic epidermal cells (squamous cell carcinoma).

Treatment and Prognosis

1 Affected cats should be kept indoors and prevented from sunbathing between 9 am and 4 pm.

2 If some sun exposure is unavoidable, a waterproof sunscreen (titanium dioxide) with a sun protection factor (SPF) of at least 30 can be applied twice daily to protect ears, but this is not recommended for use around the eyes, nose, or mouth in cats.

3 Treatment with β-carotene 30 mg/cat PO every 12 hours may be effective in resolving preneoplastic lesions. It is not effective if squamous cell carcinoma has developed.

4 Treatment with the synthetic retinoid, acitretin (5–10 mg/cat PO q 24 hours), may be effective in the treatment of nonneoplastic actinic lesions in some cats (monitor liver function). Vitamin A may be used as a less potent alternative.

5 Surgical excision, laser ablation, therapy for carcinoma in situ (see therapy for squamous cell carcinoma), or cryotherapy may be curative.

6 The prognosis is good if further sunlight exposure can be avoided before invasive squamous cell carcinomas develop.

FIGURE 14-3 Feline Solar Dermatosis.

Multifocal erythematous papular lesions on the preauricular region of the white cat are typical of actinic dermatitis. Note that the crusting obscures the actual dermatitis.

FIGURE 14-4 Feline Solar Dermatosis.

Same cat as in Figure 14-3. The crusts have been removed, revealing the erythematous papular lesions.

FIGURE 14-5 Feline Solar Dermatosis.

Alopecia, erythema, erosions, and crusting on the ear pinna. As the disease progresses, papules will develop, with erosion and ulceration that suggest progression to squamous cell carcinoma.

FIGURE 14-6 Feline Solar Dermatosis.

The lesion on the distal ear margin of this cat has progressed to squamous cell carcinoma, destroying the normal ear architecture.

FIGURE 14-7 Feline Solar Dermatosis.

Erythema and crust formation on the ear pinna of an aged cat.

FIGURE 14-8 Feline Solar Dermatosis.

Multiple erythematous papular lesions on the preauricular area of a white cat. Note that the focal area of erosion may have progressed to squamous cell carcinoma.

Canine Solar Dermatosis

Features

Canine solar dermatosis is caused by actinic damage to lightly pigmented or nonpigmented, sparsely haired skin on the nose or trunk. With repeated exposure to ultraviolet light, preneoplastic lesions (actinic keratoses, squamous cell carcinoma in situ) may develop. Nasal solar dermatosis is uncommon in dogs, with the greatest incidence in outdoor dogs. Truncal solar dermatosis is also uncommon in dogs, with the highest incidence noted in outdoor dogs that are avid sunbathers or are kept in unshaded areas. Predisposed breeds for truncal solar dermatosis include White Boxers and Bull terriers, American Staffordshire terriers, Beagles, Dalmatians, and German shorthaired pointers.

Nasal Lesions

Initially, the nose and adjacent nonpigmented, sparsely haired skin become erythematous and scaly (sunburned). Continued exposure to sunlight leads to alopecia, crusting, erosions, ulceration, and scarring.

Truncal Lesions

Initially, affected skin becomes erythematous and scaly (sunburned). With continued sun exposure, erythematous macules, papules, plaques, and nodules develop. These lesions may be crusted, eroded, and ulcerated. Palpable irregular thickenings of what appears to be visually normal skin may be detected. The ventral and lateral aspects of the abdomen and inner thighs are most frequently affected, but lesions may also develop on the flanks, tail tip, or distal extremities. Secondary pyoderma is common.

Diagnosis

1 Usually based on history of prolonged sun exposure, clinical findings, and ruling out other differentials.

2 Dermatohistopathology: in early lesions, epidermal hyperplasia and superficial perivascular dermatitis are seen. Vacuolated epidermal cells, dyskeratotic keratinocytes, and basophilic degeneration of elastin (solar elastosis) may be noted. In advanced lesions, the epidermis may be hyperplastic and dysplastic, with no invasion through the basement membrane (actinic keratosis, carcinoma in situ).

Treatment and Prognosis

1 Further exposure to sunlight, especially between 9 am and 4 pm, should be prevented.

2 If some sun exposure is unavoidable, sun block (zinc oxide) or sunscreen (titanium dioxide) should be applied to susceptible areas twice daily. For sunscreens, waterproof products with a sun protection factor (SPF) of at least 30 should be used.

3 If lesions are secondarily infected, appropriate systemic antibiotics should be administered for 2 to 3 weeks.

4 Treatment with vitamin A (8000–10,000 IU q 24 hours) or acitretin (0.5–1 mg/kg PO q 24 hours) may be effective in resolving lesions in some dogs with truncal solar dermatosis.

5 The prognosis is variable, depending on lesion chronicity. With sun avoidance, early cases of nasal solar dermatosis usually heal completely. However, chronic, ulcerative nasal lesions often heal by scarring, and with continued sun exposure, squamous cell carcinoma may develop. In early cases of truncal solar dermatosis, the prognosis is good if further exposure to sunlight is avoided. With continued exposure to sunlight, the likelihood of developing squamous cell carcinoma is high. Sun-damaged truncal skin is also predisposed to the development of hemangioma or hemangiosarcoma.

FIGURE 14-9 Canine Solar Dermatosis.

Generalized alopecia and erythema covering the face and trunk of a Bull terrier.

FIGURE 14-10 Canine Solar Dermatosis.

Alopecia and erythema with papular dermatitis on the muzzle.

(Courtesy D. Angarano.)

FIGURE 14-11 Canine Solar Dermatosis.

Close-up of the lateral thorax of a dog with solar dermatitis. The skin is erythematous with moth-eaten alopecia. The pigmented areas may appear depressed or atrophied, which is an illusion caused by swelling of the surrounding unpigmented skin.

FIGURE 14-12 Canine Solar Dermatosis.

A focal area of solar dermatitis demonstrating alopecic erythematous skin. Note that the hair follicles are partially occluded, forming comedones, which may become inflamed or secondarily infected.

FIGURE 14-13 Canine Solar Dermatosis.

Generalized solar dermatitis on the ventrum of a Boxer. The skin is erythematous and edematous with focal areas of scarring. Several ulcerated nodules, which may be squamous cell carcinoma, are apparent.

FIGURE 14-14 Canine Solar Dermatosis.

This focal area of ulceration on the scrotum of a Boxer had progressed to squamous cell carcinoma.

FIGURE 14-15 Canine Solar Dermatosis.

This large, erythematous plaque on the abdomen was a combination of chronic solar dermatitis and squamous cell carcinoma. The pigmented areas of skin were protected and thus were not affected.

FIGURE 14-16 Canine Solar Dermatosis.

Close-up of the abdomen of a dog with solar dermatitis. Note that the pigmented regions seem depressed or atrophied. This is an illusion caused by swelling of the unpigmented skin.

FIGURE 14-17 Canine Solar Dermatosis.

A focal nodule with ulceration and drainage. This lesion had progressed to form a squamous cell carcinoma.

FIGURE 14-18 Canine Solar Dermatosis.

Severe erythematous dermatitis with a coalescing papular rash caused by sun exposure.

FIGURE 14-19 Canine Solar Dermatosis.

Severe erythema affecting only the nonpigmented areas of skin on the ventrum of a Boxer.

FIGURE 14-20 Canine Solar Dermatosis.

Severe infiltrative dermatitis that only affects the nonpigmented skin. The pigmented areas often appear too thin, but it is actually the nonpigmented skin that is thickened by the abnormal cellular infiltrate.

Squamous Cell Carcinoma

Features

Squamous cell carcinoma is a malignant neoplasm of keratinocytes that accounts for 15% of cutaneous tumors in cats and 5% of cutaneous tumors in dogs. It most often occurs in thinly haired, nonpigmented, sun-damaged skin and may be preceded by actinic (solar) keratosis. Recently, papillomavirus infection has been implicated in tumor development in dogs, as papillomavirus antigen can be demonstrated in up to 50% of canine squamous cell carcinomas. Squamous cell carcinoma is common in dogs, with highest incidence in older dogs. Sunlight-induced tumors on the flank and ventrum occur most commonly in lightly pigmented dog breeds such as Dalmatians, Beagles, Whippets, and White English Bull terriers. The condition is common in cats, with highest incidence reported in older white cats. The incidence of solar-induced squamous cell carcinoma is highest in geographic areas with intense sunlight.

Dogs

Squamous cell carcinoma appears usually as single, but possibly multiple, proliferative or ulcerative lesions on the trunk, legs, digits, scrotum, nose, and lips. Proliferative tumors often have a cauliflower-like appearance, vary in size, and may ulcerate and bleed easily. Crater-like ulcerative lesions begin as crusted-over, shallow erosions that deepen. Nail bed tumors usually involve one digit, but multiple digits may be involved, especially in large, black-coated dogs such as black Labradors, German shepherds, and Standard Poodles. Affected digits are typically swollen and painful, and have a misshapen or absent nail.

Cats

Squamous cell carcinoma manifests as proliferative, crusting, or ulcerative lesions that may bleed easily. They most commonly involve nonpigmented ear pinnae, nose, and eyelids.

Treatment and Prognosis

1 The treatment of choice is early, complete surgical excision that includes amputation of digital tumors. Excised tissues should be examined histopathologically for completeness of excision (margin assessment). Laser ablation may be appropriate for superficial lesions.

2 Cryotherapy or laser ablation may be appropriate for small, superficial lesions.

3 For nonresectable or partially resectable lesions, adjuvant radiotherapy (especially electron beam radiation) or strontium-90 (a form of superficial radiotherapy) may be effective.

4 Alternatively, for nonresectable lesions, intralesional chemotherapy (cisplatin, carboplatin, 5-fluorouracil), local hyperthermia, or photodynamic therapy may be effective in some cases. Systemic chemotherapy is less consistently effective for treatment of squamous cell carcinoma.

5 To prevent new solar-induced lesions from developing, future ultraviolet light exposure should be avoided. (See “Solar Dermatosis.”)

6 The prognosis for dogs is variable, depending on the degree of differentiation and the site of the lesion. Most tumors are locally invasive and slow to metastasize, although squamous cell carcinoma of the digit tends to be more aggressive and may metastasize more readily. The prognosis in cats depends on the size and degree of differentiation, with smaller, well-differentiated tumors having a better prognosis than large or poorly differentiated ones.

FIGURE 14-21 Squamous Cell Carcinoma.

A small, ulcerated tumor on the nonpigmented nasal planum of a cat.

FIGURE 14-22 Squamous Cell Carcinoma.

Erythema, ulceration, and crusting on the nose of an adult white cat. The initial lesions were typical of solar dermatosis.

FIGURE 14-23 Squamous Cell Carcinoma.

The radical surgical excision was necessary to remove the entire tumor. Surgical correction would have been much easier, if it had been performed earlier.

(Courtesy R. Seamen.)

FIGURE 14-24 Squamous Cell Carcinoma.

Severe tissue destruction and tumor proliferation on the face and periocular tissue of a cat.

(Courtesy S. McLaughlin.)

FIGURE 14-25 Squamous Cell Carcinoma.

Necrosis and crusting of the distal ear margin of an adult white cat.

FIGURE 14-26 Squamous Cell Carcinoma.

Severe tissue destruction of the entire distal ear pinna caused by progression of the squamous cell carcinoma. Early detection and therapeutic intervention provide better cosmetic outcomes.

FIGURE 14-27 Squamous Cell Carcinoma.

The squamous cell carcinoma has progressed beyond the ear pinna. Surgical resection of this tumor will be difficult and will require extreme reconstructive surgery.

FIGURE 14-28 Squamous Cell Carcinoma.

Amputation of this cat’s ear pinna was performed to remove the tumor. Early detection and therapeutic intervention provide better cosmetic outcomes.

FIGURE 14-29 Squamous Cell Carcinoma.

A large, ulcerated tumor on the hip of an aged Basset hound.

FIGURE 14-30 Squamous Cell Carcinoma.

Close-up of the dog in Figure 14-29. This raised tumor has a deep ulcer, with tissue destruction forming a central crater.

FIGURE 14-31 Squamous Cell Carcinoma.

Focal area of carcinoma in situ on the ear pinna of a white cat. The ear pinnae have been illuminated from behind to demonstrate focal erythematous lesions.

FIGURE 14-32 Squamous Cell Carcinoma.

A carbon dioxide (CO₂) laser is used to ablate the focal neoplastic tissue. This technique allows for well-demarcated borders and minimal bystander tissue injury.

FIGURE 14-33 Squamous Cell Carcinoma.

Same cat as in Figure 14-32. The tumor has been ablated, leaving a focal area of ulceration.

FIGURE 14-34 Squamous Cell Carcinoma.

Same cat as in Figure 14-32. Three weeks after treatment, the focal area has healed and the hair is regrowing. Early detection and therapeutic intervention provide better cosmetic outcomes.

FIGURE 14-35 Squamous Cell Carcinoma.

Multifocal squamous cell carcinoma on the preauricular area of a white cat.

FIGURE 14-36 Squamous Cell Carcinoma.

Same cat as in Figure 14-35. The tumors were ablated with a carbon dioxide (CO₂) laser and were allowed to heal. Early detection and therapeutic intervention provide better cosmetic outcomes.