Report of a rare case of atypical lymphoplasmacyte-rich meningioma in the tentorium mimicking idiopathic hypertrophic pachymeningitis.

A lymphoplasmacyte-rich meningioma (LPRM) is an extremely rare variant of meningioma. Here, we report a case of atypical LPRM with increased mitosis in a 55-year-old man. Preoperative magnetic resonance imaging suggested meningioma with brain invasion in the left tentorium cerebelli. Histological examination revealed sclerotic fibrosis and dense lymphoplasmacytic infiltration; based on these findings, the differential diagnosis included LPRM and fibroinflammatory lesions of the dura, such as idiopathic hypertrophic pachymeningitis (IHP), IgG4-related disease (IgG4-RD), and Rosai-Dorfman disease. Epithelial membrane antigen (EMA) immunostaining highlighted sheets of meningothelial cells, which strongly supported the diagnosis of meningioma. Although brain invasion was suspected in radiologic image, no histologic evidence of brain invasion was found. However, the mitoses were observed to be 8/10 high power fields, along with increased Ki-67 labeling index with focal spontaneous necrosis, and the final pathologic diagnosis was atypical LPRM. IgG4-RD was ruled out, because IgG4 counts and the IgG4:IgG ratio of plasma cells did not meet the diagnostic criteria for IgG4-RD. To date, only one case of LPRM with brain invasion has been reported as atypical LPRM. This case is therefore the second case of atypical LPRM with increased mitosis that histologically mimicked IHP.