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BACKGROUND Brain invasion has already been identified as an independent criterion of meningioma recurrence by Jääskaläinen’s in 1986. While it was only suggested that meningiomas harbouring brain invasion could be staged as grade II in the 2007 WHO Classification of Tumours of the Central Nervous System, brain invasion was clearly listed as a grade II atypical meningioma criterion for the first time in the 2016 version. Since 2007, brain invasion has been carefully screened during the histopathological characterisation of meningiomas in our centre thanks to specific training of the neuropathology department staff. Furthermore, neurosurgeons are asked to provide well-preserved and grossly orientable tumours borders whenever possible. The aim of the present study was to compare the characteristics of patients diagnosed with grade II meningiomas before and after this change of practices. MATERIAL AND METHODS We retrospectively reviewed the characteristics of 125 patients with grade II meningiomas diagnosed between 2011 and 2014 (group A) and compared them to those of 166 patients with grade II meningiomas diagnosed between 1998 and 2005 (group B). RESULTS Tumour location and socio-demographic parameters were comparable in group A and B with a median age of 62 years in both groups (p=0.18). There were more de novo meningiomas in group A (84% vs 68.7%, p=0.004). Brain invasion was found in 59% in group A and 9% in group B (p=0.00001) while median Ki67 labelling was of 8.4% in group A and 10.5% in group B (p=0.04). Consistently, tumour borders were irregular in 52% in group A and 29% in group B (p=0.0002). Progression free survival was similar in the two groups (65 vs 66 months, p=0.92) but grade progression was more frequently observed in group B (18.9% vs 0%, p=0.006). In group A, meningiomas that were classified as grade II exclusively based on brain infiltration (n=33, group A-bi) had a similar progression rate compared to meningiomas with other criteria of atypia (27% vs 25%, p=0.98) and a similar time to progression (32 vs 32 months, p=0.74). The median time to progression of group A-bi meningiomas was also comparable to that of the other 258 pooled meningiomas (32 vs 40 months, p=0.40). CONCLUSION In accordance with the 2016 WHO Classification of Tumours of the Central Nervous System, the outcome of meningiomas defined as atypical solely based on brain infiltration seems to be comparable to that of meningiomas with other criteria of atypia. Appropriate coordination between neurosurgeons and neuropathologists is mandatory to optimally assess this criterion.

OS8.4 Impact of brain invasion assessment on outcome in intracranial grade II meningiomas