Introduction

Meningiomas are slowly growing neoplasms thought to arise from meningothelial cells found within arachnoid granulations. Concentrated in the walls of the major venous sinuses, these structures, which contain “arachnoid cap cells,” account for the dural localization of most meningiomas within the cranium and spinal cord.1 Since the original classification of meningiomas in 1931,2,3 there has been a gradual yet significant change in our views regarding the pathology of these tumors. The collective experience from the literature has shown that the vast majority of meningiomas do not behave in a biologically aggressive fashion, with tumors that exhibit aggressive behavior representing only ~10% of cases. Intriguingly, not all studies have demonstrated a correlation of histology with survival. Although metastases are uncommon and usually restricted to ana-plastic examples, even tumors judged benign are capable of metastasis. Detailed correlative clinicopathological studies by Perry and others have greatly improved consistency in predicting clinical outcome and contributed to our understanding of the biology of meningiomas.2–7 The recently revised World Health Organization (WHO) classification has incorporated these advances into a more precise and objective classification system.8 Nevertheless, challenges remain in identifying reliable biomarkers that will lead to the refinement of prognosis and define treatment responsiveness.

Localization

The majority of meningiomas are supratentorial, with a large number located along the convexities. Approximately 17 to 25% occur in a frontobasal location; however, only about 10% occur in the posterior fossa. Within the frontobasal region, the olfactory grooves, tuberculum sellae and parasellar region, and the petrous bone are preferred sites. Approximately 5% occur along the cerebellar convexity, 2 to 4% at the tentorium cerebelli, and 2 to 4% within the cerebellopontine angle.9

Uncommonly, meningiomas are found within the ventricular system or arise within the optic nerve sheath, where they produce diffuse circumferential thickening of the nerve sheath rather than a focal mass ( Fig. 5.1 ). Notably, meningiomas are the most common trigonal intraventricular mass in adults.10 Atypical locations, including those within the posterior fossa, brain parenchyma, and ventricles, are more frequent in the pediatric population.11

Second only to nerve sheath tumors in this location, spinal meningiomas account for ~10% of meningiomas. They are most frequently found anterolaterally or dorso-laterally in the thoracic spine and show greater proclivity for psammomatous change. Extracranial meningiomas are very rare tumors and have been described in ectopic locations such as intraosseous, scalp, paranasal sinuses, parotid, parapharyngeal space, mediastinum, lung, or adrenal gland. Histologically, these lesions are indistinguishable from ordinary intracranial meningiomas, with most cases diagnosed as benign variants. Multicentric meningiomas account for 1 to 10% of meningiomas and tend to present at a younger age, with most examples confined to a single hemicranium. Metastases are uncommon and may be seen with benign or malignant meningiomas.

Gross Features

Grossly, meningiomas manifest as tan, well-demarcated solitary masses with a firm, broad-based dural attachment and smooth or bosselated surfaces. Falcine or tentorial meningiomas may have a bilobed, dumbbell shape. Except for aggressive examples, meningiomas are readily separated from underlying brain or spinal cord. Similarly, hemorrhage and necrosis are usually restricted to malignant tumors. Rare meningiomas cover the dura in a flat, carpetlike growth and are referred to as meningioma en plaque. This variant often elicits hyperostosis and is more common along the sphenoid ridge or within the cavernous sinus, where they become enmeshed with sinus contents. Most meningiomas compress and protrude into the underlying brain without true invasion. True invasion implies penetration of the pial membrane.

Specimens vary from soft and gelatinous to firm and calcified. On sectioning, mineralization due to the development of psammoma bodies imparts a gritty consistency. The cut surface is translucent and pale gray or homogeneously reddish brown, particularly in tumors with increased vascularity or hemorrhage. Tumors with a high lipid content (xanthomatous change) show yellow streaks, and, exceptionally, bone or cartilage may be detected. Cysts, either within or surrounding the tumor, are reported in only 1.6 to 10% of meningiomas and may be visible grossly.

Histopathology

As a group, meningiomas probably display the greatest variety of patterns of any primary brain tumor ( Table 5.1 ), which sometimes poses diagnostic challenges. The three most common histologic subtypes are the meningothelial (syncytial), transitional, and fibroblastic meningiomas; however, meningiomas may display more than one histologic pattern in a tumor. According to WHO criteria, meningiomas are designated as benign (grade I), atypical (grade II), or anaplastic/malignant (grade III). The following sections briefly describe the morphological subtypes associated with each grade.

WHO Grade I

Meningothelial

This variant of meningioma exhibits a syncytial growth pattern and is composed of polygonal cells with abundant eosinophilic cytoplasm and indistinct cell borders Fig. 5.2 Syncytial growth pattern in meningothelial meningioma. ( Fig. 5.2 ). The sheetlike growth of the cells is interrupted by vascularized connective tissue trabeculae that divide the tumor into lobules. The nuclei are round to oval with pale chromatin. A diagnostically useful cytologic feature is the presence of nuclear pseudoinclusions or clear nuclear vacuoles ( Fig. 5.3 ). These nuclear features are more commonly seen in this subtype and arise from invaginations of cytoplasm into the nucleus.

Table 5.1 2007 World Health Organization Meningioma Classification

Meningothelial meningioma	WHO grade I
Fibrous (fibroblastic) meningioma	WHO grade I
Transitional (mixed) meningioma	WHO grade I
Psammomatous meningioma	WHO grade I
Angiomatous meningioma	WHO grade I
Microcystic meningioma	WHO grade I
Secretory meningioma	WHO grade I
Lymphoplasmacyte-rich meningioma	WHO grade I
Metaplastic meningioma	WHO grade I
Chordoid meningioma	WHO grade II
Clear cell meningioma	WHO grade II
Atypical meningioma	WHO grade II
Papillary meningioma	WHO grade III
Rhabdoid meningioma	WHO grade III
Anaplastic (malignant) meningioma	WHO grade III

Microcystic

Microcystic meningiomas are composed of loosely arranged clusters of cells with long, thin processes containing variably sized intercellular vacuoles that either appear empty or contain pale mucinous material, xanthomatous cells, and hyalinized blood vessels ( Fig. 5.5 ). Pleomorphic-appearing cells may be noted in this variant but are not of prognostic significance. Whorls, lobules, and psammoma bodies are not common in this variant.12–14

Psammomatous

Psammomatous meningiomas contain many psammoma bodies to the extent that in extreme examples only sparse remnants of tumor cells are present in highly calcified examples ( Fig. 5.6 ). Although psammoma bodies are found in a variety of meningiomas, most densely calcified tumors are transitional. Bone-related proteins, including osteopontin produced by CD68-positive macrophages, are thought to play a role in psammoma body formation.15

Related posts:

3 Anatomy and Biology of the Leptomeninges 4 Epidemiology of Meningiomas 1 Meningiomas: A Personal Perspective 12 Risk Evaluation and Anesthesia for Intracranial and Spinal Meningiomas 27 Cerebellar Convexity Meningiomas 33 Meningiomas of the Third Ventricle and Pineal Region

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