In dogs, iris melanocytic tumors arise from the anterior surface of the iris and are usually discrete, raised pigmented masses. As compared with ciliary body tumors, iris melanocytic tumors are easier to detect because of their anatomic location, thus allowing for earlier diagnosis in the course of the disease.2 In cats, flat, diffuse iris melanoma is more common than the nodular form (Figure 1) and usually starts as focal or multifocal areas of iris pigmentation, which may progress over time. Flat, diffuse iris melanoma cannot be differentiated from benign, diffuse iris melanosis based on clinical appearance alone (Figure 2; see Iris Freckles, Nevi, & Melanosis).6 Observation of clinical signs (eg, dyscoria, iris thickening, pigment dispersion, increased intraocular pressure) may help with the decision to enucleate an eye. In cats, some histologic and immunohistochemical parameters have been found to be helpful in determining the risk for metastasis of diffuse iris melanoma.7 Although these parameters may not be a practical tool for deciding whether to enucleate an eye, they may be used postenucleation to determine the metastatic risk in the individual cat and, when applicable, to form a tailored postoperative follow-up plan. Metastatic melanocytic tumors affecting the anterior uvea are rare but have been reported in dogs.8
For additional considerations regarding potential malignancy of diffuse iris pigmentary changes, see Iris Freckles, Nevi, & Melanosis.