Abstract
A case of monostotic fibrous dysplasia of the left parietal bone in a 22-year-old woman is described. The patient was hospitalized because of a soft, painful mass in the left parietal region. The lesion had increased gradually in size during the previous month. The neurological examination and laboratory data were normal. Skull X-rays showed an osteolytic, well circumscribed lesion in the left parietal bone. A computed tomography scan revealed an area of lysis of the outer table with expansion of the diploic space. There was a soft-tissue mass contiguous to the bone lesion. Angiography disclosed abnormal vascularity. At surgery, the enlarged bone was excised to the extent possible and the cystic lesion, which contained dark green, viscous fluid, was curetted. There was no evidence of dural invasion. Microscopically, the excised bone lesion was confirmed to be typical fibrous dysplasia. No sarcomatous changes were found. The cyst wall was composed of multinucleated giant cells, hemosiderin-laden macrophages, and extravasated red blood cells. Although there were macroscopic differences, the specimen closely resembled an aneurysmal bone cyst microscopically. Its rapid growth may have been secondary to the cyst formation and hemorrhage that occur in fibrous dysplasia. The diagnosis and surgical treatment of this case are discussed.
