"Malignancy" in meningiomas: a clinicopathologic study of 116 patients, with grading implications

Due to the rarity of malignancy in meningiomas, prior studies have been limited to small series. Controversies regarding the definition of malignant meningioma have complicated matters further. Although histologic anaplasia and extracranial metastasis are established criteria, the former is difficult to define and the latter represents a clinical finding. Traditionally, brain invasion has also been accepted, although this has recently been debated. In a prior series, the authors were unable to prove that 23 meningiomas that had invaded the brain were more aggressive than atypical meningiomas.

The authors expanded their analysis to include 116 patients diagnosed with "malignant meningioma" due to brain invasion, frank anaplasia (20 mitoses per 10 high-power fields or histology resembling carcinoma, sarcoma, or melanoma), and/or extracranial metastasis. Patients were followed until death or for a median of 3.7 years.

Survival time was highly variable, ranging from 10 days to 24 years. In multivariate analysis, histologic anaplasia (P=0.0035), subtotal resection (P=0.0038), 20 mitoses per 10 high-power fields (P=0.0071), and nuclear atypia (P=0.0068) were associated with poor survival. Of the 89 cases of meningioma that had invaded the brain, 23% were otherwise benign, 61% were otherwise atypical, and 17% were frankly anaplastic. Those without anaplasia behaved similarly to atypical meningiomas from the authors' prior study. In contrast, anaplastic meningiomas were usually fatal, associated with a median survival of 1.5 years.

Based on these findings, the authors suggest that brain invasion constitutes an additional criterion for the diagnosis of atypical meningioma (World Health Organization [WHO] Grade II), whereas frank anaplasia indicates high grade (WHO Grade III-IV) malignancy.