Atypical primary ocular melanoma in cats.

The clinical and pathologic features of six cases of atypical primary ocular melanoma in cats are described. In contrast to diffuse iris melanomas, these atypical tumors originated multifocally from any portion of the uvea and in five of the six cases were very advanced at the time of clinical presentation. Two of the advanced cases involved primarily the ciliary body, while three others showed extensive involvement of the choroid. Histologically, these tumors consisted primarily of round, heavily pigmented cells lacking the anaplastic nuclear features characteristic of advanced diffuse iris melanomas. The advanced stage of the tumors at presentation may have been due to either the posterior uveal origin of these tumors or to more rapid progression. Additional cases with long-term follow-up need to be studied in order to accurately assess the biological behaviour of these tumors.