Inflammatory pseudotumors of the central nervous system.

Inflammatory pseudotumor is a disease with unsettled pathogenesis. The brain is a rare site of occurrence. The aim of this study is to investigate ALK-1 protein expression and IgG4-positive plasma cells detection in 4 intracranial inflammatory pseudotumors. Three dural-based and 1 intraventricular inflammatory pseudotumors were retrieved from the hospitals' archive. The data on clinical presentation, radiological findings, procedure undertaken, and patients' progress were collected. Sections from the excised lesions were examined under hematoxylin and eosin, histochemical, and immunohistochemical staining including ALK-1 and IgG4. All 4 cases displayed typical histological features of inflammatory pseudotumor with dense lymphoplasmacytic infiltrate admixed with small number of benign-looking spindle cells in a collagenous stroma. Three cases exhibited high density of IgG4-positive plasma cells per high-power field. ALK-1 was negative. ALK expression was not found in any of our cases. On the contrary, the detection of significant number of IgG4-positive plasma cells in 3 inflammatory pseudotumors suggests that a considerable proportion of intracranial inflammatory pseudotumor may belong to the IgG4-related subgroup. Hence, a trial of corticosteroid after histological confirmation may be valid to avoid unnecessary risk-taking neurosurgical procedures or in cases with incomplete tumor removal.