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AJNR - American Journal of Neuroradiology

Published ahead of print on March 20, 2008
doi: 10.3174/ajnr.A0996

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American Journal of Neuroradiology 29:1147-1152, June-July 2008
© 2008 American Society of Neuroradiology

BRAIN

Diffusion-Weighted MR Imaging: Diagnosing Atypical or Malignant Meningiomas and Detecting Tumor Dedifferentiation

V.A. Nagar^a, J.R. Ye^a, W.H. Ng^b, Y.H. Chan^c, F. Hui^a, C.K. Lee^b and C.C.T. Lim^a,d

^a Department of Neuroradiology, National Neuroscience Institute, Singapore
^b Neurosurgery, National Neuroscience Institute, Singapore
^c Department of Biostatistics, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
^d Diagnostic Radiology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore

Please address correspondence to C.C. Tchoyoson Lim, Department of Neuroradiology, National Neuroscience Institute, 11 Jalan Tan Tock Seng, Singapore 308433; e-mail: tchoyoson_lim{at}nni.com.sg

BACKGROUND AND PURPOSE: Atypical and malignant meningiomas are uncommon tumors with aggressive behavior and higher mortality, morbidity, and recurrence compared with benign tumors. We investigated the utility of diffusion-weighted (DW) MR imaging to differentiate atypical/malignant from benign meningiomas and to detect histologic dedifferentiation to higher tumor grade.

MATERIALS AND METHODS: We retrospectively compared conventional and DW MR images (b-value 1000 s/mm²) acquired on a 1.5T clinical scanner between 25 atypical/malignant and 23 benign meningiomas. The optimal cutoff for the absolute apparent diffusion coefficient (ADC) and normalized ADC (NADC) ratio to differentiate between the groups was determined by using receiver operating characteristic (ROC) analysis.

RESULTS: Irregular tumor margins, peritumoral edema, and adjacent bone destruction occurred significantly more often in atypical/malignant than in benign meningiomas. The mean ADC of atypical/malignant meningiomas (0.66 ± 0.13 x 10^–3 mm²/s) was significantly lower compared with benign meningiomas (0.88 ± 0.08 x 10^–3 mm²/s; P < .0001). Mean NADC ratio in the atypical/malignant group (0.91 ± 0.18) was also significantly lower than the benign group (1.28 ± 0.11; P < .0001), without overlap between groups. ROC analysis showed that ADC and NADC thresholds of 0.80 x 10^–3 mm²/s and 0.99, respectively, had the best accuracy: at the NADC threshold of 0.99, the sensitivity and specificity were 96% and 100%, respectively. Two patients had isointense benign tumors on initial DW MR imaging, and these became hyperintense with the decrease in ADC and NADC below these thresholds when they progressed to atypical and malignant meningiomas on recurrence.

CONCLUSIONS: ADC and NADC ratios in atypical/malignant meningiomas are significantly lower than in benign tumors. Decrease in ADC and NADC on follow-up imaging may suggest dedifferentiation to higher tumor grade.

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