Solid Iridociliary Carcinoma in a Dog

Abstract

Background: Iridociliary epithelial tumors (ICETs) originate from the iris epithelium or ciliary body. They comprise ciliary body adenoma, carcinoma, pleomorphic adenocarcinomas, medulloepitheliomas, and other primitive neuroectodermal tumors. They are the second most common primary intraocular tumors in dogs and have already been reported in sheep and humans. In dogs, they occur more frequently in middle-aged to elderly animals, and the Labrador and Golden Retriever seem to be more predisposed breeds. This study aimed to describe the clinical and pathological aspects of solid iridociliary carcinoma in a dog.

Case: A 3-year-old Poodle bitch was treated for discomfort in the left eyeball region, increased intraocular pressure and moderate buphthalmia. A direct ophthalmological examination was performed without equipment, and a mass was visualized in the posterior chamber, distorting the pupillary cleft. We opted for unilateral enucleation and forwarded the material for histological analysis. Macroscopically, the eyeball measured 3.4 cm (anteroposterior) x 2.6 cm (vertical), with a brownish mass that occupied the entire anterior chamber and part of the posterior chamber. Histologically, there was a neoformation in the ciliary body and iris pigment epithelium, partially well-delimited and densely cellular. The neoplasm was organized into predominantly solid formations interspersed with a discrete amount of blood vessels, rare bundles of fibrous stroma, and amorphous eosinophilic material forming membranes that were positive for PAS. Sections of the neoplasm were subjected to immunohistochemistry using anti-cytokeratin AE1/AE3, anti-S100 protein, anti-vimentin, and anti-Ki-67. Positive cytoplasmic immunostaining for cytokeratin and S-100 was observed. Only 45.6% of cells were positive for Ki-67 (500 cells). No immunostaining was observed for vimentin.

Discussion: The diagnosis of solid iridociliary carcinoma was based on the histological features and positive immunostaining for cytokeratin AE1/AE3 and protein S100. Iridociliary carcinomas present positive immunostaining for cytokeratin, whereas adenomas and normal iridociliary epithelium do not present this immunostaining. Moreover, the high rate of cell proliferation was indicative of malignant neoplasia, as observed by the high mitotic count and high positivity for Ki-67. The S100 protein helped in the diagnosis of ICETs, as the iridociliary epithelium showed positive staining for this protein. Some histological features are important to consider in the diagnosis of iridociliary tumors in dogs, such as noninvasive growth in the posterior chamber, pigment epithelium, and thick homogeneous membranes on the cell surface. Furthermore, the presence of positive PAS membranes favors the diagnosis of iridociliary epithelial tumors. ICETs must be differentiated from melanocytomas, anterior uveal melanoma, medulloepitheliomas, and metastatic and pleomorphic carcinomas. The histological characteristics, especially the presence of PAS-positive membranes, associated with the immunohistochemical profile of neoplasm cells, help differentiate the ICETs from these tumors. In general, the prognosis is poor for eyeball and vision maintenance in canine iridociliary tumors, and scleral invasion is associated with a higher recurrence rate.

Keywords: neoplasm, eye, immunohistochemistry, cytokeratin, S100 protein.