ABSTRACT
Intraosseous epidermal inclusion cyst is a rare benign, cystic lesion. It is thought to result from traumatic implantation of epidermal elements into bone. Radiologic findings of intraosseous epidermal inclusion cysts are well-defined, lytic lesions. It is difficult to diagnose intraosseous epidermal inclusion cyst without pathologic diagnosis. We experienced a 43-year-old man with a history of trauma followed by painless expansion of his left thumb. Radiographs demonstrated a severe expansile, ill-defined lytic lesion with cortical destruction in the distal phalanx of left thumb, mimicking neoplastic bone lesion or infectious lesion. An intraosseous epidermal inclusion cyst was confirmed by pathologic diagnosis, which was lined by stratified squamous epithelium, containing keratinized cellular debris.