Signalment:  
Gross Description:  
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Morphologic Diagnosis:  
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Contributor Comment:  
The differential diagnosis for the gross presentation included nodular iris melanoma and a uveal cyst. On histopathologic examination, the neoplastic melanocytes diffusely infiltrated iridial stroma, ciliary body, and iridocorneal drainage angle and protruded into the anterior chamber resulting in the grossly visible nodule. Melanocytic tumors are the most common primary intraocular neoplasms in cats,(2,7) with intraocular sarcomas and ciliary epithelial neoplasms also occurring with lesser frequency.(3)
Diffuse iris melanoma generally begins as an asymmetrical abnormal pigmentation of the iris that may be clinically noted several years prior to the development of neoplasia.(2) This area of pigmentation may remain static or progress to nodular iridial irregularities and diffusely infiltrate the sclera, ciliary body, and posterior segment.(2) Neoplastic cells originate from melanocytes that line the anterior border of the iris.(9) Rarely, atypical melanomas, not associated with diffuse iris melanoma, may arise multifocally within the limbus or choroid. Little is known about their origin, behavior, and prognosis.(2,4)
Three morphologically distinct neoplastic cells are often found in feline diffuse iris melanoma. These different morphologic cell types may occur in any combination within diffuse iris melanoma, and no prognostic significance has yet been attached to these various combinations.(2,9)
1) Pleomorphic round cells (most common type), consist of cells with abundant eosinophilic cytoplasm and a primarily centralized round nucleus. These cells may occasionally exhibit cytomegaly, cytoplasmic invaginations forming pseudo-inclusions, and variable amounts of pigmentation.
2) Spindle cells (2nd most common).
3) Balloon cells, consist of cells with abundant eosinophilic to clear cytoplasm with a high cytoplasm to nuclear ratio and a small often centralized round nucleus. No prognostic significance has been associated with the morphologic type of neoplastic cells.
Poorly differentiated intraocular melanomas may exhibit a wide variety of morphoglogic and pigmentation characteristics. This often results in a struggle to develop a definitive morphologic diagnosis on H&E alone. Common immunohistochemical stains include HMSA-5, S-100, and tyrosinase.(1)
Feline diffuse iris melanomas are considered more likely to metastasize than canine uveal melanomas, but due to the slow progression of the tumor, the overall risk that an animal will die from the melanoma is less than 20%.(9) In cats, metastasis associated with diffuse iris melanomas has been documented affecting lungs, lymph nodes, skeletal system and abdominal viscera.(6) Predictors of metastasis include scleral invasion, invasion into posterior iris epithelium, and overall tumor size.(9)
Retroviral antigen has been identified in both intraocular sarcoma and diffuse iris melanoma. (1,3,8) In a retrospective study by Stiles et. al. in 1999, 3 of 36 intraocular melanomas tested positive for FeLV-FeSV DNA by nested PCR.7 Another retrospective study in 2002 by Cullen et al failed to find viral DNA sequences in the intraocular melanomas examined in 10 cats.(3)
JPC Diagnosis:  
Conference Comment:  
The contributor mentions uveal cysts, or pigmented uveal nodules, as a gross differential diagnsosis. Uveal cysts are fluid-filled cysts which may be congenital or arise secondary to trauma or inflammation. They are non-neoplastic, non-progressive and considered an incidental finding.
References:
2. Dubielzig RR: Tumors of the eye. In: Tumors in Domestic Animals, 4th ed., Meuten DJ, ed., pp. 744-749. Iowa State Press, Ames, IA, 2002
3. Grahn BH, Peiffer RL, Cullen CL, Haines DM: Classification of feline intraocular neoplasms based on morphology, histochemical staining, and immunohistochemical labeling. Vet Ophthalmol 9:395-403, 2006
4. Harris BP, Dubielzig RR: Atypical primary ocular melanoma in cats. Vet Ophthal 2:121-124, 1999
5. Michau TM, Proulx DR, Rushton SD, Olivry T, Dunston SM, Gilger BC, Davidson MG: Intraocular extramedullary plasmacytoma in a cat. Vet Opthalmol 6:177-181, 2003
6. Njaa BL, Wilcock BP. The eye and ear. In: Zachary JF, McGavin MD, eds. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO: Elsevier; 2012:1215-20, 1228-9.
7. Planellas M, Pastor J, Torres MD, Pe+�-�a T, Leiva M: Unusual presentation of a metastatic uveal melanoma in a cat. Vet Ophthalmol 13:391-394, 2010
8. Stiles J, Bienzle D, Render JA, Buyukmihci NC, Johnson EC: Use of nested polymerase chain reaction (PCR) for detection of retroviruses from formalin-fixed, paraffin-embedded uveal melanomas in cats. Vet Ophthalmol 2:113- 116, 1999
9. Wilcock B, Dubielzig RR, Render JA: Histological Classification of Ocular and Otic Tumors of Domestic Animals. 2nd Series, Vol. IX, pp.18-19, 24-25. Armed Forces Institute of Pathology, Washington, DC, 2002
10. Wilcock, BP. Eye and ear. In: Maxie MG, ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. 5th ed., vol. 1, Philadelphia, PA: Elsevier Saunders; 2007:514-5, 538-42.