Immunohistochemical Identification of Canine Melanocytic Neoplasms and Prognostic Evaluation of Ki67 Expression

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In dogs, melanoma is a highly aggressive tumor, diagnosis and prognosis are still based onhistological signs of malignancy. The purposes of the present study were to evaluate the sensitivity andspecificity of several melanocytic markers, and, to investigate the relation between tumor growth fractionand classical histological signs of malignancy. Twenty-two dog melanoma samples (16 cutaneous tumorsand 6 oral) were selected, and ten non melanocytic tumors were the negative control. Tumor histologicalmalignancy criteria were compared with the growth phase, which was expressed by Ki67 immunolabelingand measured by quantitative computerized analysis. Of the 22 tumors, 16 were positive to Melan A, 20 toVimentin and all of them to S100. When refer to specificity, Melan A did not stain the negative controlsamples, Vimentin stains seven, and S100 all of them. The most melanocytic neoplasms, regardless of thelocation, shown clear signs of malignancy, they were anaplastic, invasive tumors, with high mitotic index,and extended necrosis. Growth phase index was also very high, but it was not correlated with any of thehistological features of malignancy, and neither to histological type. The only positive correlation wasthose between mitotic index and the extension of necrotic areas (r=0.5, p<0.05). These findingsrecommend Melan A because of its very high specificity. However, it has relatively low sensitivity,especially in amelanotic tumors. The present study did not find any correlation between growth phase andclassic histological features of malignancy.