Pilar Cyst Pigmented Epithelial Remnants: A Potential Diagnostic Pitfall

Pilar cysts are common cutaneous cysts of follicular origin. They are easy to diagnose clinically and pathologically. Histologic diagnostic difficulties might arise in certain situations, however. Acute inflammation with total destruction of the cyst wall due to rupture with replacement by an abscess formation, foreign body giant cell reaction, and fibrosis could obscure their recognition. Cysts with hybrid lining epithelium could be confused with other cutaneous cysts. Epithelial remnants of the basal layer with loss of the squamous epithelium and shelled out cyst contents might mimic other epithelial cysts and vascular lesions. Few studies focused on the phenomenon of epithelial remnants or epithelial separation of pilar cysts. We report a case of a scalp cyst composed of a single layer of pigmented cuboidal lining epithelium. The initial differential diagnosis was hidrocystoma, solid-cystic hidradenoma, arteriovenous malformation, and lymphangioma. The intraepithelial pigment was melanin. The lining epithelium was positive for cytokeratin cocktail, CK5/6, CK8, CK19, p63, and D2-40 with scattered S-100 protein and melan-A positive melanocytes. Being unaware of the phenomenon of epithelial split in pilar cysts, it was mislabeled as a melanin-pigmented eccrine hidrocystoma. Surgical pathologists should be aware of pilar cysts' epithelial remnants to avoid potential diagnostic pitfalls. An attention to certain histologic hints and knowledge of the immunoprofile of the basal layer should help pathologists avoid this pitfall.