Meningioma sampling: how much is enough for the accurate grading of atypical meningiomas?

Meningioma grading relies on several pathological criteria (brain invasion, mitotic count, sheeting, small cell foci, necrosis, macronucleoli and hypercellularity) and histopathological subtypes. Regardless of histopathological subtype, the presence of these pathological parameters can be focally present and not present on each slide of a meningioma. We performed (1) a retrospective work comparing the frequency of parameters used for meningioma grading between two periods with different sampling techniques, and (2) we calculated the probability of presence of each criterion on resected meningiomas entirely processed included and examined. First, we compared two time periods: between 2002-2008 where meningiomas were not all entirely sampled, and between 2012-2018 where all meningiomas were entirely sampled. The frequency of tumour grades was not significantly different between the two periods (p=0.17). Mitosis ≥4/1.6mm2, small cell foci, macronucleoli and hypercellularity were more frequently found when meningiomas were entirely sampled (p<0.05). Second, we focused on 59 grade 2 meningiomas entirely sampled to highlight the distribution of histopathological parameters used for meningioma grading. We have shown that a correct grading of more than 95% of meningiomas can be achieved when at least six slides are examined. Our work suggests that meningioma sampling might be an issue and the sampling system must be specified in research works on grading.