Page 177 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

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9.3 Melanocytic Tumours
VetBooks.ir Group of tumours arising from the melanocytes in the epidermis, dermis, or adnexa.

Clinical features
• Common neoplasms in dogs (5% of all skin tumours); less often observed in cats
(3%).
• They include a benign (melanocytoma) and malignant (melanoma) form, the latter
representing 19% and 33% of the total cutaneous melanocytic tumours in the
dog and cat, respectively.
• Age:
• Dogs: 7–12 years (melanocytoma), 10–13 years (melanoma).
• Cats: 4–13 years (melanocytoma), 8–12 (melanoma).
• Melanocytic tumours appear as variably sized and variably pigmented, circum-
scribed, raised cutaneous lesions. They are often alopecic and may be ulcerated,
inflamed and infiltrative, especially when malignant.
• Anatomical sites:
• Dogs: head (especially eyelid) and trunk for melanocytoma. Melanoma fre-
quently arises from the oral cavity, mucucutaneous junctions and nail bed.
• Cats: head (including pinnae, lips and nose) and back for both melanocyto-
ma and melanoma.
• The majority of melanocytomas in dogs are slow-growing masses. Surgical exci-
sion is curative.
• Melanomas are often rapidly growing and can be fatal. Metastases usually occur
to the draining lymph nodes, lung and occasionally to other internal organs.
• Over-represented canine breeds:
• Melanocytoma: Hungarian Vizsla, Miniature Schnauzer, Irish Setter, Schnauzer
and Australian Terrier.
• Melanoma: Schnauzer, Miniature Schnauzer, Giant Schnauzer, Chow Chow, Shar
Pei and Scottish Terrier.

Cytological features

• Cellularity is variable. Often medium-high especially in malignant forms.
• Background: variably haemodiluted, may contain free melanin granules.
• The aspirates are composed of nucleated cells that contain variable numbers of fine black-
green round or rice-shaped granules of melanin.
• Cells exfoliate singly, in cohesive clusters or in aggregates. They range from round to stellate
to spindloid in both benign and malignant forms.
• Nuclei are round to oval, central to paracentral. In melanocytomas, nuclei are often obscured
by the intracytoplasmic melanin granules. The chromatin is finely stippled to coarse. In melan-
omas, the chromatin is often clumped and nucleoli are usually prominent, multiple and
variably shaped.