Author: Assoc Prof Patrick Emanuel, Dermatopathologist, Auckland, New Zealand, 2013.
Proliferating epidermoid cyst has been poorly defined in the literature. Regular epidermoid cyst should be seen in at least part of the lesion in addition to an epidermal proliferation.
Histology of proliferating epidermoid cyst
Sections show a cyst in the dermis with a proliferating epidermal component (figures 1, 2). Characteristically, the proliferative areas are made up of bland squamousepithelium with striking squamous eddies (figures 2, 3, 4). These eddies are whorles of maturing squamousepithelium and are exactly the same as those seen in irritated seborrheic keratoses or inverted follicular keratoses.
Differential diagnosis of proliferating epidermoid cystpathology
HPV-related epidermal cysts – These have a hyperplastic lining with virapathic nuclear and cytoplasmic changes
Cystic squamous cell carcinoma – Must be considered if there is nuclear atypia and adjacent infiltration into the surrounding dermis. This can be a challenging differential when cysts have partially ruptured or there is extensive proliferation.
Pathology of the Skin (Fourth edition, 2012). McKee PH, J. Calonje JE, Granter SR