Background:
Meningioma, the most frequent intracranial, extra-axial tumor, constitutes %13-26 of all intracranial neoplasms. Less than %10 of all meningiomas account for grade II or III (atypical-anaplastic) based on WHO criteria. Preoperative estimation of tumor grade might play a directive role for neurosurgeons. Higher Ki-67 index is associated with increased risk of tumor recurrence affecting the prognosis.
 Objectives:
We aimed to investigate the efficiency of diffusion weighted imaging properties in preoperative estimation of tumor grade and correlation between ADC values and Ki-67 proliferative index. To our knowledge this is the largest single tertiary center study series in English literature investigating association between DWI and histopathologic features of meningiomas.
 Materials and Methods:
Pre-operative brain MR examinations of 206 consecutive patients were evaluated retrospectively. ADC values of 6 ROI’s in two consecutive slices were measured and the average ADC value was calculated for each lesion. Proliferation activity of the tumors which were identified with ki-67 index was recorded. The meningiomas were classified into low grade (WHO grade I) and high grade (WHO grade II/III) groups.
 Results:
Median ADC value in low and high grade lesions were 826.23 ± 134.567 × 10-6 mm2s-1 and 732.48 ± 150.846 × 10-6 mm2s-1 respectively. Threshold of 750.5 × 10-6 mm2s-1 for ADC value, yielded the best sensitivity (%74), specifity (%72) and accuracy (%73.79) in discriminating low and high grade meningiomas. The area under curve in ROC analysis was 0,742. Determination of 750.5 × 10-6 mm2s-1 as threshold ADC value, led to 95.04% and 27.69% negative and positive predictive values, respectively. There was also a significant negative correlation between ADC value and Ki-67 index level (r = - 0,26 , p < 0.001 )
 Conclusion:
ADC value is an important parameter in characterization of meningioma with implications for prognosis and management. ADC values can also predict tumor proliferative index providing the advantage of proper surgical approach and further use of fractionated radiotherapy or stereotactic single-dose radiosurgery to decrease the possibility of tumor recurrence.