Histopathological Investigation of Meningioma Capsule with Respect to Tumor Cell Invasion

No previous study has histopathologically investigated whether a meningioma capsule presents with tumor cells. We investigated which types of tumor capsules (TCs) included tumor cells to help intraoperatively determine those TCs that do not need to be removed and have a low recurrence risk. We investigated 22 specimens of 14 newly diagnosed meningiomas from February 2011 to June 2021. The capsules were classified into three types: TC, capsule-like thickened arachnoid membrane (CAM), and extended membrane (EM). Capsule properties were scored by hardness (soft = 1, medium = 2, hard = 3) and transparency (high = 1, medium = 2, low = 3). The hardness, transparency, and score sums were compared between capsules with and without tumor invasion in the CAM and EM types. The mean follow-up duration was 40.6 months, and there was only one recurrence in a remote location from the residual capsule. Nine capsules were classified as TC, seven as CAM, and six as EM. The tumor cells invaded 88.9% of TCs, 42.9% of CAMs, and 50% of EMs. The hardness, transparency, and score sums for CAMs with tumor invasion were lower than those for CAMs without tumor invasion, although not significant (P = 0.114, P = 0.114, P = 0.057, respectively). A thickened TC or soft and highly transparent CAM indicated a high risk for tumor cell invasion; thus, such cases require a careful and long-term follow-up. Hard and low transparent residual CAMs may have had a low risk for tumor invasion; therefore, leaving such capsules that tightly adhere to the eloquent cortex can be theoretically justified to avoid damaging the brain surface.