ABSTRACT
A 10-year-old female mongrel dog presented with a swelling in the skin on the right chest. The lesion was clinically pigmented; therefore, melanoma was suspected. Gross examination revealed, the lesion to be encapsulated, well demarcated, and multicystic. Histopathological examination showed that it was not composed of melanocytes. The tumor comprised apocrine cells with intracytoplasmic granules, which stained positive for iron, Sudan black B, and Periodic acid Schiff (PAS), in addition to small papillary growths containing PAS-positive fluid within the cyst lumen. Immunohistochemistry showed that, the cyst wall comprised α-SMA-positive spindle cells and apocrine gland epithelial cells, which were negative for Ki-67. These findings are consistent with apocrine hidrocystoma in humans and cats; therefore, the case was diagnosed as pigmented apocrine hidrocystoma. This may be the first reported case of a pigmented apocrine hidrocystoma in canines and may be of clinical relevance for differentiating such lesions from other pigmented tumors, including melanoma.