February 2002: 29-year-old woman with a skull mass for 2 months.

A 29-year-old woman had a 2-month history of an enlarging lesion over her left frontal bone following minor trauma. CT scan showed an osteolytic lesion with an overlying soft tissue mass, thought to be an unhealed skull fracture with pseudomeningocele. Left frontal craniotomy revealed a soft tissue mass, which was resected. Histologic examination revealed multinucleated giant cells mixed with Langerhan's cells that showed the characteristic "coffee bean nuclei." Eosinophils were scant. Immunostaining for CD1a and S100 revealed strong positive staining primarily in the Langerhans' cells while giant cells and inflammatory cells were negative. Immunostaining for CD68, in contrast, stained the osteoclast-like giant cells and macrophages. Electron microscopy confirmed the presence Birbeck granules. The final diagnosis was Langerhans' cell histiocytosis (histiocytosis X) of the skull.