A diffuse, dense follicular lymphoplasmacytic infiltration dominated the histological picture (Fig 3A and 3B; single arrow). The small spaces which remained between the extensive lymphoplasmacytic infiltrations were taken up by round-oval cells with large nuclei containing finely dispersed chromatin, some with small nuclear vacuoles. These cells showed morphological similarity to meningothelial cells but morphologically it was difficult to rule out the possibility of dendritic cells. There was no apparent lobular or fascicular pattern, whorl formation. A pattern typical for meningothelial meningioma (region surrounded by arrows on Fig 3B), was rarely detected in the biopsy specimen. There were no psammoma bodies. The tumor was richly vascularized.
An immunohistochemical analysis showed that the follicles consisted of CD20 positive B lymphocytes (Fig 3C) and CD21 positive follicular dendritic cells (Fig 3D). The remaining lymphoid cell population consisted of CD3 positive T lymphocytes and numerous polyclonal plasma cells. The cells between the dense lymphoplasmacytic infiltrations were immunoreactive for the epithelial membrane antigen (EMA) (Fig 4A) and vimentin (Fig 4B) and corresponded to meningothelial cells. Cells of the germinal centers showed a high proliferation activity as indicated by a high MIB-1 labelling index (Fig 4C), and were bcl-2 negative (Fig 4D).