MACROSCOPIC APPEARANCES

Meningiomas arising around the cerebral convexities are generally spherical because they can displace underlying brain (Fig. 43.3). The shape of other meningiomas is constrained by their location, e.g. optic nerve meningiomas are fusiform. Meningiomas occasionally envelop rather than displace adjacent CNS structures (Fig. 43.4). Some meningiomas, particularly those on the sphenoid ridge, grow as plaques within the dura (en plaque meningioma).

Most meningiomas have a smooth or slightly lobulated surface and a rubbery consistency (Fig. 43.5). They are usually solid on sectioning, but may have a soft, granular texture, and there may be obvious interlacing bands of fibrous tissue. Rarer variants may have specific macroscopic features, e.g. the microcystic meningioma has a gelatinous texture and the psammomatous meningioma feels gritty.

The attached dura should be inspected for separate nodules of tumor, and the relationship between dura and tumor should be examined histologically (Fig. 43.6). Invasion of adjacent bone may be evident at operation, and pieces of bone may be submitted for examination (Fig. 43.7). The infiltrated bone often shows hyperostosis, a feature that can, however, occur adjacent to meningiomas even in the absence of bony invasion.

MICROSCOPIC APPEARANCES

Meningiomas exhibit a wide range of histologic patterns. Furthermore, a single meningioma may show a combination of patterns. The meningothelial, transitional, and fibrous variants predominate, forming a histologic continuum that should be regarded as prototypical of this neoplasm because their features appear, to a greater or lesser extent, in other variants as well.

Meningothelial meningiomas consist of sheets or lobules of oval cells that form rudimentary meningothelial whorls in a few places. Cytologic borders are indistinct. Single small nucleoli and nuclear pseudoinclusions formed by invaginations of the nuclear membrane are common (Fig. 43.8).

Fibrous meningiomas consist of spindle-shaped cells associated with variable amounts of pericellular collagen (Fig. 43.9).

Meningothelial and fibrous areas are combined in transitional meningiomas, which also contain widespread whorls and scattered psammoma bodies (Fig. 43.10).

The criteria for diagnosing these variants of meningioma are subjective and a clear distinction is not always possible. This is also true for other variants of meningioma. For example, psammomatous meningioma is not defined by a specific density of psammoma bodies.

Focal cytologic pleomorphism in association with large hyperchromatic nuclei and in the absence of an increased mitotic count may be seen in classic meningiomas. It does not indicate a worse prognosis, being regarded as a degenerative phenomenon.

Foci of foamy (xanthomatous) cells are frequently found in meningiomas (Fig. 43.11), and should not be mistaken for micronecrosis.

Other variants of meningioma (Table 43.2) have distinctive architectural and cytologic features which are usually combined with the prototypical histology described above (Figs 43.12–43.17).

It is clear that some histologic variants have a more aggressive behavior than others, even when a meningioma’s site and the completeness of its resection have been taken into account. This variation is reflected in three levels of WHO grade (see Table 43.1). WHO grade 2 is assigned to atypical, clear cell and chordoid meningiomas (Table 43.3, Figs 43.18–43.20), which show an increased frequency of recurrence, as do grade 3 anaplastic, papillary and rhabdoid meningiomas (Figs 43.21–43.23), which are also more likely to exhibit malignant behavior, such as invasion of local structures or metastasis. Atypical and anaplastic meningiomas lie on a histologic continuum, but in the current WHO classification have been divided mainly on the basis of mitotic count (see Table 43.3). This nomenclature does not apply to meningeal sarcomas (see Chapter 45); while anaplastic meningiomas may contain sarcomatoid areas, they retain histologic features that identify them as meningiomas. The term ‘meningosarcoma’ is obsolete.

Invasion of the brain is an adverse prognostic feature (Fig. 43.24), but should be diagnosed with care. Tumor cells must breach the pia, invading brain parenchyma and eliciting reactive changes, such as astrocytosis. Superficial invasion along the subarachnoid space is not considered to be brain invasion. Brain invasion can be a feature of meningiomas that, on the basis of their cytological features, would be classified as WHO grade I, II, or III. However, when present in a meningioma with grade 1 cytological features, brain invasion is associated with an increased rate of recurrence, and such a tumor is classified as WHO grade II.

Meningiomas are immunoreactive for vimentin (Fig. 43.25), and most meningiomas, particularly those with epithelial qualities, immunolabel with antibodies to epithelial membrane antigen (EMA) (Fig. 43.26). Fibrous meningiomas tend to show only focal labeling for EMA. Approximately 25% of meningiomas show focal immunoreactivity for cytokeratins. The neoplastic cells surrounding

the characteristic globules in secretory meningiomas label strongly with antibodies to cytokeratin (see Fig. 43.15d). Some meningiomas contain scattered cells that are immunoreactive for S-100 protein. There is no immunoreactivity for GFAP, except in some rhabdoid tumors.

The proportion of neoplastic cells labeled by Ki-67 antibodies and other cell cycle specific markers is greater in atypical and anaplastic meningiomas than in prototypical classic meningiomas. In some series, Ki-67 labeling indices have correlated with rapidity of recurrence, but controversy surrounds the value of the Ki-67 labeling index as an independent prognostic indicator and its use in the individual case.

Ultrastructural features of meningiomas include interdigitating processes, intermediate filaments, desmosomes, and hemidesmosomes. Intercellular collagen deposition is found in fibrous variants.