[Role of MR imaging in the differentiation of benign and nonbenign intracranial meningiomas: the utility of contrast-enhanced T1-weighted images].

The purpose of this study was to develop useful criteria for distinguishing nonbenign (atypical and malignant) primary intracranial meningiomas from their benign counterparts by using magnetic resonance imaging (MRI). To determine useful MRI findings for this purpose, 12 benign and five nonbenign meningiomas were retrospectively evaluated according to the following items: 1) tumor signal intensity on plain T1-, T2- and proton density-weighted images, 2) degree of perifocal edema on T2-weighted images, 3) morphology of the tumor margin on contrast-enhanced T1-weighted images, 4) presence of irregular nodule and/or mushrooming pattern on contrast-enhanced T1-weighted images, 5) homogeneity of the tumor on contrast-enhanced T1-weighted images and 6) presence of marked skull destruction. Markedly irregular tumor margin, presence of irregular nodule and/or mushrooming pattern and markedly inhomogeneous enhancing pattern were significantly more frequent in nonbenign meningiomas. We defined these three MRI findings as nonbenign findings, and tried to categorize meningiomas by the number of nonbenign findings. It was found that 10 meningiomas with no or one nonbenign finding were benign lesions, of four meningiomas with two nonbenign findings two were benign lesions and two were nonbenign lesions, and three meningiomas with three nonbenign findings were nonbenign lesions. The two benign meningiomas with two nonbenign findings were accompanied by increased mitotic activity or brain invasion. Contrast-enhanced T1-weighted images were considered very useful in distinguishing benign and nonbenign meningiomas.