Pigmented Paget disease--a diagnostic pitfall mimicking melanoma.

Pigmented mammary and extramammary Paget disease are rare entities in both males and females that mimic melanoma both clinically and histologically. Furthermore, Paget disease can be associated with increased number of benign melanocytes, thus resulting in additional diagnostic difficulty, especially when only hematoxylin and eosin-stained sections are examined and a limited immunohistochemical study is performed. Using standard hematoxylin and eosin-stained sections and routine immunohistochemical studies, we describe and characterize 7 cases of pigmented extramammary and mammary Paget disease. In all cases, malignant epithelial cells showed intracytoplasmic pigment, along with an immunohistochemical epithelial phenotype. In 2 of the cases, immunohistochemistry revealed numerous dendritic processes positive for melanocytic markers, thus resulting in an initial diagnosis of melanoma. Careful analysis confirmed that the immunolabeling corresponded to cytoplasmic labeling of melanocyte dendrites surrounding tumor cells. The correct diagnosis of pigmented Paget disease can be reached after close histologic examination and detailed evaluation of immunohistochemical studies. The latter are especially important in some extraordinary cases in which there may be an associated intraepithelial melanocytic hyperplasia.