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).
43.2 Meningiomas on the sphenoid ridge and olfactory groove.
These meningiomas were an incidental postmortem finding.
MENINGIOMAS
The incidence of meningiomas rises with age.
Asymptomatic meningiomas are often found at necropsy in elderly people.
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term angioblastic meningioma, previously used for angiomatous meningiomas, hemangiopericytomas, and hemangioblastomas, is obsolete.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).
43.3 Meningioma.
(a) A contrast-enhanced MR scan showing a large sphenoid wing meningioma. The even enhancement is typical. (Courtesy of Dr J S Millar, Wessex Neurological Center.) (b) A midline tumor has compressed the inferior part of the frontal lobes.
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.
43.5 Formalin-fixed meningiomas.
(a) The tumor has a lobular appearance. Note the entrapped dura. (b) In this pediatric case, meningioma encircles optic nerve.
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
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).
43.8 Meningothelial meningioma.
(a) Oval cells with prominent small nucleoli characterize the cytology of this meningothelial meningioma in a smear preparation. (b) Typical meningothelial meningioma containing a sheet of cells with indistinct borders and nuclear pseudo-inclusions. (c) Rudimentary whorls in a meningothelial meningioma. (d) This tumor has infiltrated the fibrous capsule of the optic nerve. (e) Meningothelial meningioma with sparse reticulin.
Fibrous meningiomas consist of spindle-shaped cells associated with variable amounts of pericellular collagen (Fig. 43.9).
43.9 Fibrous meningioma.
(a) A smear preparation of this meningioma consists of elongated cells, many of which contain a single nucleolus. (b) This is the tumor from which the smear in (a) was prepared. (c) Spindle cells in a fibrous meningioma. (d) Abundant collagen in one region of a fibrous meningioma. (e) This tumor is characterized by fibrous nodules. (f) Fibrous meningiomas contain more reticulin than do other variants, but not around every cell.
Meningothelial and fibrous areas are combined in transitional meningiomas, which also contain widespread whorls and scattered psammoma bodies (Fig. 43.10).
43.10 Transitional meningioma.
(a) This typical example is composed of whorls and cords of neoplastic cells. (b) This contains prominent whorls and psammoma bodies. (c) Note the combination of whorling and fascicular patterns. (d)
