Primary intraocular malignant melanoma in a Siamese cat.

Abstract

Cutaneous malignant melanoma is common in domestic animals, particularly dogs and horses. Melanoma has been the most commonly reported intraocular tumor in man and animals [3]. Reports of intraocular tumors, however, are not numerous. Thus, intraocular melanoma is an unusual finding in any animal. There are few reports of intraocular malignant melanomas in cats. One paper reported nine new cases and five previous ones [2]. None of these tumors has been reported in the Siamese cat. A 14-year-old intact male Siamese cat was presented at an animal hospital. The cat had a “swollen eye” that had been untreated for 6 months. The cornea was ruptured and there was severe panophthalmitis. Two months later the eye was enucleated. The cat’s physical condition had begun to deteriorate but he lived in apparent good health for more than 20 months after surgery and seemed well until a few days before death. Consent for necropsy was not granted, and the cause of death was not determined. Histologic examination of the enucleated eye showed a malignant melanoma of the iris, ciliary body and choroid. The tumor seemed to originate in, and virtually replace, the iris and ciliary body. The choroid was infiltrated for a short distance. There were anterior and posterior synechiae and corneal rupture with a prominent polymorphonuclear leukocyte response. Keratitis was characterized by edema, deep vascularization and hemorrhage; there was early invasion of sclera and cornea by neoplastic tissue. The tumor was composed mostly of plump epithelioid cells with indistinct cytoplasmic boundaries and oval, vesicular nuclei, with large, prominent nucleoli (fig. 1). There was considerable pleomorphism with many large bizarre cells. Multinucleate cells were not seen. There were up to 15 mitotic figures per high power field; many were abnormal. Many tumor cells scattered throughout in clumps had well defined granules of brown, isotropic, argyrophilic pigment. Reticulum fibers were scant. The number of inflammatory cells in the neoplasm was moderate; the cells were mainly at the periphery of the tumor. Generally, the infiltrate was a mixture of polymorphonuclear cells, lymphocytes and plasma cells. Neoplasms of the weal tract to be considered are malignant melanoma (melanotic and amelanotic) and tumors arising from uveal epithelial cells that may or may not contain pigment. In addition to melanoma of the uveal tract, there are medulloepitheliomas, adenomas and carcinomas which, unless poorly differentiated, have a more characteristic pattern [3, 5-71. The biological behavior of malignant melanoma varies and the rate of metastasis is unpredictable. Intraocular melanoma often extends locally into the orbit or