Primary histiocytosis X of the parieto-occipital lobe.

Immunohistochemical and ultrastructural evidence support the concept that histiocytosis X is the result of proliferation of pathological Langerhans' cells. Central nervous system involvement by histiocytosis X has been commonly described in multisystem disease and in association with lytic skull lesions. Unifocal brain involvement by histiocytosis X without concomitant osseous involvement is rare, with only 14 cases reported in the literature to date. Ten of these cases have involved the hypothalamus; the remaining four have involved the frontal lobe (two cases) and the temporal lobe (two cases). The fifth case of extrahypothalamic unifocal histiocytosis X, the first female case, and the first case with parieto-occipital lobe involvement, is reported. Pathology demonstrated infiltration of brain parenchyma by clusters of characteristic histiocytosis X cells with an admixture of morphologically related giant cells, eosinophils, and lymphocytes. Langerhans' granules were identified in the histiocytosis X cells by electron microscopy. Immunohistochemistry showed strong S-100 protein, HLA-DR, and T6 antigen positivity by the histiocytosis X cells. Therapy included complete surgical excision and postoperative radiation therapy for the incompletely excised lesion. Patients with unifocal extrahypothalamic histiocytosis X may have a better prognosis than patients with localized hypothalamic disease.