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A 36-year-old Korean man presented with a nodular lesion on the buttock. It was a walnut-sized, tender, soft, and movable nodule covered by normal skin which had been enlarging steadily over a period of one year ( Fig. 1). Surgical exploration revealed the tumor to be a cyst containing serous, brownish fluid. It was totally excised. Histological examination showed a well-demarcated, 1.3 × 0.9 cm tumor with cyst formation in the center and surrounded by a zone of connective tissue in the middle and lower dermis, without any connection to the overlying epidermis ( Fig. 2a). The cystic wall was composed of tumor cells. Variably sized tumor lobules protruded into the cystic space. The tumor nests consisted of two cell types: smaller, darker poroid cells and larger, paler cuticular cells. Mitotic figures were not present. Vacuolar spaces were seen within the cytoplasm of some cuticular cells, which were thought to represent early stages of ductal differentiation. Ductal structures lined by eosinophilic cuticle were present in some tumor lobules ( Fig. 2b). Peripheral pallisading was occasionally observed, but it was not a prominent feature] the cells at the edges of the lobules tended to be rather flattened. Periodic acid-Schiff stain with diastase demonstrated variable amounts of glycogen within the cytoplasm of some tumor cells. Carcinoembryonic antigen (CEA) immunostaining was positive in the luminal border of the cystic space, which appeared as a sensitive marker of ductal differentiation. Epithelial membrane antigen also showed a positive reaction in luminal borders of the cystic space. Tumor cells were positive for vimentin, but negative for S-100 protein] however, many dendritic cells scattered throughout the neoplasm were positive for S-100 protein.

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Figure 1. A  walnut-sized, tender, soft, movable nodule covered by normal skin is seen on the buttock.

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Figure 2. (a)  A well-demarcated tumor with cyst formation in the center is surrounded by a zone of connective tissue in the middle and lower dermis, without any connection to the overlying epidermis. Solid parts protrude into the cystic space (hematoxylin and eosin; original magnification × 40). (b)  Ductal structure lined by eosinophilic cuticle is surrounded by cuticular tumor cells (hematoxylin and eosin; original magnification × 200).