Epibulbar melanomas are relatively common in dogs, but significantly less frequent in cats.
These neoplasms are generally intensively pigmented, although occasionally amelanotic, and centrifugally extend from the limbal sclera as broad base smooth nodules with a tendency to invade the cornea. These tumors appear to act differently in younger dogs when compared to older dogs. Tumors arising in 2 to 4 year-old dogs are rapidly growing and invasive, whereas neoplasms arising in 8 to 11 year old dogs grow more slowly, are often detected as an incidental finding and tend to develop within the dorsolateral quadrant. Benign tumors are referred to as melanocytomas and they are comprised of two populations of cells such as spindloid neoplastic melanocytes and large round non neoplastic melanophages. The latter are large macrophages engulfing the melanin produced by the neoplastic cell and they form large part of the mass. Mitotic figures are inconspicuous. The malignant intraocular melanomas have different histologic features such as anaplasia and more than 1-2 mitotic figures per high power field and are less pigmented. Breed predisposition has been suggested for German Shepherds and Labrador and Golden Retrievers. Heavily pigmented dogs seem to be predisposed. Differential diagnoses include scleral coloboma, scleral staphyloma, conjunctival melanoma, transcleral extension of intraocular melanoma, metastatic melanoma and foreign body. Gonioscopy is paramount to differentiate epibulbar from intraocular melanomas and to evaluate their possible intraocular invasion, which may be cause of glaucoma, uveitis or hemorrhage.
Young dogs are frequently aggressively treated, whereas in older dogs periodic surveillance of the status of the lesion is suggested. Treatments include full thickness removal with homologous corneoscleral, third eyelid graft, polytetrafluoroethylene (PTFE) or porcine small intestinal submucosa (SIS), keratectomy, partial removal, cryosurgery, beta-radiation, photocoagulation, periodic evaluation only, various combination of the methods previously described, and enucleation.
Limbal melanoma has been infrequently reported in the cat. They are generally characterized by benign biologic behavior in cats older than 8 year, although younger cats have been recently found to be affected. The terminology "malignant melanoma with delayed metastasis" instead of "benign melanoma" has been proposed. In general, the tumors tend to localize in the dorsal part of the ocular globe and develop and differentiate from the limbal melanocytes.
Considering the long period of latency described in the literature, affected patients should undergo complete physical examination with particular focus on lymph nodes, spleen and liver, thoracic radiographic and abdominal ultrasound evaluation every 3-6 months starting 6 months following the initial diagnosis.
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