Malignant meningiomas: CT and histologic criteria, including a new CT sign.

Histologic features that could be correlated with malignancy were assessed by reviewing the microscopic slides of 167 meningiomas. Six tumors had shown two or more recurrences. In three having three or more recurrences, the number of mitoses counted under high power was higher than in those meningiomas showing clinically benign behavior. The radiologic and histologic features of seven meningiomas showing malignant clinical behavior and/or malignant histologic features were also evaluated and correlated. On computed tomography (CT), most of the malignant meningiomas were moderately hyperdense before contrast enhancement, but showed no or minimal calcification. Marked perifocal edema was common. Indistinct tumor margins or, occasionally, deeply extending fringes of tumor interdigitating with brain substance, marked bone destruction, or prominent pannus or tumor, extending well away from the globoid mass, termed "mushrooming," is described for the first time and seems to be the most useful correlate of histologic or clinical malignancy. This sign occurred in five of the seven cases and was absent in about 250 benign meningiomas reviewed. It was visible only at surgery in one additional case.