A conceptual shift in the grading of meningiomas.

Grading schemes for meningiomas have traditionally designated tumors as "meningioma," "atypical meningioma," or "anaplastic (malignant) meningioma," depending upon the presence of histopathologic features thought to indicate aggressive behavior. In the past, most systems have considered brain invasion by tumor as the best evidence of malignancy. Perry et al. have recently investigated the significance brain invasion as a prognostic feature in meningiomas. The authors studied a series of 116 patients who had been diagnosed previously with "malignant meningioma" due to the presence of brain invasion, histologic anaplasia, or metastasis. On the basis of a multivariate analysis of histopathologic features and their relationship to tumor recurrence and patient survival, the authors concluded that brain invasion should be considered one of the diagnostic features of atypical meningioma. Accordingly, the diagnosis of malignant meningioma should be reserved for those tumors that are frankly anaplastic and/or contain (> = 20 mitoses per 10 high-power fields (HPF). Due in large part to the strength of evidence in this study, the World Health Organization (WHO) has adopted a grading scheme for meningiomas that incorporates many of the authors' proposals. New diagnostic criteria will result in improved reproducibility with fewer diagnoses of malignant meningioma (WHO grade III).