Reconsideration of So-Called "Invasive" Meningioma —Definition and Surgical Strategy

For the purpose of histopathological diagnosis, so-called "invasive meningioma" is defined as a meningioma that has infiltrated the cerebral parenchyma, and such cortical infiltration raises the risk of recurrence. However, as the definition of "invasive" remains far from clear, we have at- tempted to redefine it. Tumors that were completely removed from 32 patients between April 2005 and September 2015 and that had been diagnosed as so-called "invasive meningioma" were classified by World Health Organization grade (Group I-III), and then further classified as: true invasion, involving invasion of the brain with breakdown of the pia mater; and pseudo-invasion, involving encroachment into the brain with the pia mater still intact. We then investigated recur- rence rates in the brain or dura mater for each group. Rate of recurrence in the brain or dura ma- ter was significantly higher for Group I meningioma classified as true invasion compared to that of the same group classified as pseudo-invasion. We redefined so-called "invasive meningioma" as Group I meningioma exhibiting true invasion, and considered that, when possible, wide resection of the areas of adhesion to the dura mater in addition to tumor extirpation along with the cerebral parenchyma increased the success rate for curative treatment.