Primary eosinophilic granuloma of frontal lobe. Diagnostic use of S-100 protein

Solitary eosinophilic granuloma of the frontal lobe of the brain is a rare lesion. A 30-year-old Jordanian man with a history of recurrent bilateral chronic middle ear infections and tuberculosis presented with focal seizures. A solitary right frontotemporal mass was diagnosed by brain biopsy as an eosinophilic granuloma. Immunoperoxidase stains of virtually all histiocytes in the lesion were strongly positive for S-100. Four cases of eosinophilic granuloma and one of Letterer-Siwe disease were also examined, and virtually all histiocytes demonstrated strong S-100 positivity. In comparison, 24 reactive or infectious granulomas were examined for S-100 staining and most showed complete negativity of histiocytes in granulomas for S-100. This is the first report in the literature that has examined the results of S-100 staining in a large number of reactive or infectious granulomas and compared it with S-100 staining in eosinophilic granuloma. This report illustrates that S-100 can be useful in distinguishing eosinophilic granuloma, generally considered to be part of the larger group of histiocytic disorders known as histiocytosis X, from reactive or infectious granulomatous conditions of similar or confusing histology.