Histological Grading of Meningioma Based on MIB-1 Immunoreactivity

Abstract

Atypical meningioma was added to the meningioma group in the revised World Health Organization classification, and thus meningioma was divided into three grades: histologically benign, atypical, and anaplastic. Several authors have defined the criteria of atypical meningioma, but there are still some practical problems in grading meningiomas histologically. We investigated the relationship between the histological features and proliferative ability assessed by immunoreactivity with MIB-1 monoclonal antibody and sought to apply this in grading of meningioma. In 73 meningioma specimens obtained from 49 cases, the following histopathological features and MIB-1 immunoreactivity were examined: mitotic index, prominent nucleoli, nuclear pleomorphism, small cells with high nuclear cytoplasmic ratios, hypercellularity, loss of architecture, brain invasion, and necrosis. Thirty-eight of 39 (97%) meningiomas with MIB-1 labeling index greater than 5.0 exhibited mitotic indices of more than 1. Necrosis, prominent nucleoli, loss of architecture, nuclear pleomorphism, hypercellularity, brain invasion, and small cells were observed in 92%, 64%, 62%, 56%, 56%, 21%, and 15% of these cases, respectively. Meningiomas with higher proliferative ability showed higher rates of presence of mitotic figures and necrosis. The mean MIB-1 labeling index of meningiomas with brain invasion was 19.80, and MIB-1 labeling indices correlated well with mitotic indices. Therefore, mitotic figures, necrosis, and brain invasion were more significant features for grading than other histological features. We propose a simplified diagnostic criteria: meningiomas exhibiting two or more of these three features are diagnosed as atypical meningioma. In addition, a MIB-1 labeling index of 5.0 in meningioma may be a border value between benign and atypical types.