Histopathological features to define atypical meningioma: What does really matter for prognosis?

Atypical meningiomas are diagnosed in the presence of: (1) three or more of the following minor atypical criteria: increased cellularity, small cells with a high nuclear/cytoplasmic ratio, prominent nucleoli, sheeting, and foci of spontaneous or geographic necrosis; (2) mitotic count ≥ 4 mitoses per 10 HPF (high mitotic index); (3) brain invasion. The 5-year disease-free survival (DFS) is around 50%. Due to their heterogeneous behavior, the post-surgical treatment of atypical meningiomas is controversial. This study investigated the ability of histopathological features to predict recurrence risk of atypical meningiomas. Meningiomas classified as atypical only on minor atypical criteria had low recurrence risk. Brain invasion, high mitotic index and sheeting were significantly associated with shorter disease-free survival (DFS) (P = 0.001; P = 0.01; P = 0.01). The presence of brain invasion and the co-presence of sheeting and high mitotic index had the highest ability to identify recurring meningiomas (P = 0.0001) (sensitivity: 90.9%; specificity: 86.7%). Our results suggest reconsideration of classification of meningiomas as atypical based only on minor atypical criteria. The presence of brain invasion and the co-occurrence of sheeting and high mitotic count may be useful to identify high risk cases, which may benefit from adjuvant treatments.