Meningiomas

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43

Meningiomas

MENINGIOMAS

Meningiomas arise from meningothelial (arachnoid) cells in the leptomeninges. These cells have both epithelial and mesenchymal characteristics, which are shown by meningiomas in a spectrum of diverse histologic appearances. The World Health Organization (WHO) classification recognizes several variants of meningioma (Table 43.1). Differentiation towards the epithelial end of the histologic spectrum is represented by the meningothelial and secretory variants. Differentiation towards the mesenchymal end of the spectrum is represented by the fibrous and metaplastic variants.

Table 43.1

WHO classification (2007) of meningiomas

WHO grade 1

Meningothelial

Fibrous (fibroblastic)

Transitional (mixed)

Psammomatous

Angiomatous

Microcystic

Secretory

Lymphoplasmacyte-rich

Metaplastic

WHO grade 2

Atypical

Chordoid

Clear cell

WHO grade 3

Anaplastic

Papillary

Rhabdoid

Meningiomas occur throughout the central nervous system (CNS), but have a predilection for certain sites (Fig. 43.1). Unusual sites include the ventricles and the epidural space. Depending on its location, a meningioma can grow for a prolonged period and to a considerable size before producing symptoms and signs. It may lead to increased intracranial pressure, loss of CNS function, or epilepsy. Some meningiomas are incidental post-mortem findings (Fig. 43.2).

The site of a meningioma and any involvement of local structures are major prognostic factors because they affect surgical accessibility and determine whether complete removal can be achieved. Approximately 15% of meningiomas that seem to have been completely resected recur.

Histologic assessment of a meningioma provides some information about its biologic behavior. An increased tendency to recur and a shorter interval to recurrence are properties of meningiomas that show malignant architectural and cytologic features. Such features are present in atypical and, to a greater extent, in anaplastic meningiomas, and develop progressively in meningiomas that recur several times. Infiltration of the brain is the most important histologic predictor of a high likelihood of local recurrence (see below). Other features are less reliable in predicting behavior. Even cytologically bland meningiomas can behave in a locally aggressive manner.

Not all meningeal neoplasms are meningiomas. Metastatic neoplasms (see Chapter 46); hemangiopericytomas, hemangioblastomas, melanocytic neoplasms, and various mesenchymal neoplasms (see all in Chapter 45), may be located in the meninges. The term angioblastic meningioma, previously used for angiomatous meningiomas, hemangiopericytomas, and hemangioblastomas, is obsolete. Meningeal hemangiopericytomas/solitary fibrous tumors and hemangioblastomas are morphologically, immunophenotypically, and genetically distinct from meningiomas.

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).