Melanocytoma are a benign neoplasia of pigmented epithelial melanocytes.
It is thought that melanocytomas develop as a result of chronic, inflammatory changes in melanocytes.
The differentiation of melanocytoma from malignant melanoma in dogs can be technically demanding to histopathologists. It is usually based histologically on the presence of nests of neoplastic melanocytes in the upper epidermis that appear to infiltrate the dermis or subcutaneous tissues. The presence of lymphatic or vascular invasion and more than 3 mitoses per 10 high-power fields is usually indicative of malignancy.
Some melanocytes are classified in a 'grey-zone', as melanocytic tumors of uncertain malignant potential and should be widely excised as a precautionary measure.
The metabolic difference between benign and malignant melanocytes appears to correlate well with COX-2 expression, with melanocytomas appearing to be less productive of this inflammatory mediator, suggesting that possible chronic inflammation may be an initiator in the transformation from benign to malignant status.
Care should be taken during clinical examination to distinguish benign melanocytomas (dark-pigmented solitary nodules) from malignant melanomas (raised, ulcerated and swollen lesions).
The majority of cutaneous melanocytomas are best monitored, and unless they involve the eye are of incidental interest. Any concern should warrant wde excision and submission of biopsied material for histopathological analysis.
- Cutaneous melanocytoma
The majority of these are found in pigmented skin as well as the oral mucosa, and mucocutaneous junctions such as mammary gland, prepuce, labia as well as ocular adnexa and the iris.
- Ocular melanocytoma
Benign ocular melanocytomas are a neoplastic proliferation of the uveal melanocytes and involve the iris, ciliary body, and, rarely, the choroid. They are usually benign in nature, nodular in appearance, and primarily contain large, heavily pigmented round-polygonal melanocytes and often less than 5% spindle shaped germinal cells. The cytological appearance or pattern of growth does not correlate with biologic behavior.
- Ocular melanosis
Ocular melanosis is also common in the dog eye and can appear as benign pigmental changes to the eye. These need to be distinguished both visually and histologically from benign uveal melanomas, which are also a common presentation. Both conditions carry a poor prognosis for the eye, due to development of secondary glaucoma. Uveal melanoma may also extend out of the globe and develop metastases.
- Balloon melanocytoma
Balloon cell melanoma, a variant of the traditional melanoma, have also been reported in dogs. These melanocytomas are composed of large round and epithelioid cells with abundant pale finely granular to foamy cytoplasm, with few melanin granules.
- Variant melanocytoma-acanthomas have also been reported which are also a benign epithelial keratinized growth.
- Martens AL et al (2007) Unusual presentation of an anterior uveal melanocytoma in a 3-year-old poodle. Can Vet J 48(7):748-750
- Uni of Pennsylvania
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