Meningiomas

aggressive forms of meningiomas, such as atypical and anaplastic variants. A recent paper, however, suggests that malignant meningiomas in young patients may not be quite as frequent as was previously thought [9]. Meningio-angiomatosis, a reactive perivascular proliferation of fibroblasts and meningothelial cells which can trap islands of gliotic cortex, is occasionally associated with meningiomas in younger patients, and may be mistaken for brain invasion [9].

The association of meningiomas and head injury has been addressed by many authors, although the reports are mainly anecdotal [1]. In a rather large review, Inskip et al. [10] did not find a significant increase in the incidence rate of meningiomas, gliomas or neurilemmomas in association with head trauma. It is more likely that a portion of the many asymptomatic meningiomas in the general population is simply detected incidentally by routine neu-roimaging following head trauma.

Unlike trauma, radiation as a cause of menin-gioma development has been well documented [1]. In such cases, the meningiomas must meet certain criteria to be considered radiation-induced, such as the tumor location in the field of radiation, pathology distinct from the original neoplasm or condition under treatment, and occurrence after an appropriately long "latent period" following the radiation exposure [1]. Meningiomas have been reported following low-dose radiation exposure for tinea capitis and following high-dose radiation treatment for other central nervous system (CNS) or head and neck neoplasms. The male:female ratio seems to be more equal for radiation-induced menin-giomas, in contrast to the female predominance for sporadic intracranial meningiomas in the general population.

Multiple meningiomas are rare. Only 1-9% of all intracranial meningioma patients have multiple lesions [11]. The patient age and tumor locations do not differ significantly from those with single meningiomas, yet there is a distinction among patients with familial syndromes such as neurofibromatosis (NF) who are prone to developing multiple meningiomas at a much younger age. Intracranial meningiomas are far more common in NF-2 than in NF-1, but they have been reported in both syndromes. It is questionable, however, whether meningiomas occur at a greater rate in NF-1 than in the general population. In NF-2, it has been estimated that 50% of all patients develop meningiomas, and 30% of these patients have multiple meningiomas.

Malignant (i.e. anaplastic) meningiomas account for only approximately 1-3% of all meningiomas. One large series of 936 primary intracranial meningiomas revealed that 94.3% were grade I (benign), 4.7% were grade II (atypical) and 1.0% were grade III (anaplastic). Additionally, anaplastic meningiomas occurred in 12% of males and in only 4% of females in that study. The precise definition of "malignant meningioma", however, has recently been a source of debate. Perry et al. found that frank anaplasia and high mitotic activity (20 or more mitoses per high-powered field) were most closely associated with malignant meningiomas. Extracranial metastases from meningiomas have also been considered to be one of the strong indicators of malignancy and have been shown to occur in 11-23% of patients with frankly anaplastic meningiomas. Interestingly, brain invasion, a traditional criterion of malignant meningiomas, has recently been considered as a diagnostic feature that is more common in atypical meningiomas than in anaplastic types.

Spinal meningiomas occur approximately one-tenth as frequently as intracranial menin-giomas. Of all intradural extramedullary spinal tumors, meningiomas account for 25% and are second only to schwannomas, which account for nearly 30%. The peak age of presentation for spinal meningiomas occurs between the 5th and 7th decades, but they may be found in patients of any age. The male:female ratio ranges from 1:3 to 1:6, and 80% of all spinal meningiomas are found in the thoracic region [12].

Ectopic meningiomas are exceedingly rare and likely arise from rests of arachnoid tissue trapped in ectopic locations during development. These must be distinguished from the equally rare extracranial metastases of malignant meningiomas that occur in less than 0.1% of all meningiomas. Primary cutaneous meningiomas are the most common and typically occur in the scalp of frontal and occipital regions. Other reported locations of primary ectopic meningiomas include: paranasal sinuses, eyelids, parotid gland, temporalis muscle, temporal bone and zygoma, as well as some distant sites, such as the lungs, mediastinum and adrenal gland.

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