Epidermal inclusion cyst with melanoma-like melanophagic proliferation.

A 26-year-old male had had a painless right gluteal swelling for the past 10 years. It measured 5 cm64 cm and was soft and non-£uctuant. The swelling was excised. The surgical specimen measured 4 cm63 cm63 cm and appeared as an irregular mass of adipose tissue. The cut surface revealed a bilocular cystic lesion (Fig. 1). The larger cyst (A) measured 2 cm in diameter and had a thick ¢brous wall with a smooth, pearly white inner lining. This showed a focal black pigmented area of 0.660.4 cm. The smaller cavity, (B), was collapsed, irregular and completely surrounded by black tissue. Microscopically, the larger cyst was lined by keratinizing strati¢ed squamous epithelium with marked hyperpigmentation of the keratinocytes. Melanocytes arranged in the basal layer showed enlargement, but no nuclear atypia. The pigment also stained the luminal keratin £akes. The focal pigmented area in the wall included a collection of heavily pigmented cells, obscuring the cytologic morphology (Fig. 2). The pigment was identi¢ed as melanin by the bleaching reaction. The cells were seen to be ovalto spindle-shaped, with moderate cytoplasm and round-to-oval vesicular nuclei. Immunohistochemistry showed no evidence of S100 or HMB 45 positivity in these cells, indicating that they were melanophages. The smaller cavity, which lacked an epithelial lining, was lined by a very prominent foreign body giant cell reaction. This was surrounded by similar oval-to-fusiform pigmented cells. The giant cells also showed melanin within the cytoplasm. A few of the sections showed a tract of communication lined by granulomatous tissue between the two cavities.