Diffusion-Weighted Imaging in Meningioma: Prediction of Tumor Grade and Association with Histopathological Parameters1,2

OBJECTIVES: To analyze diffusion-weighted imaging (DWI) findings of meningiomas and to compare them with tumor grade, cell count, and proliferation index and to test a possibility of use of apparent diffusion coefficient (ADC) to differentiate benign from atypical/malignant tumors. METHODS: Forty-nine meningiomas were analyzed. DWI was done using a multislice single-shot echo-planar imaging sequence. A polygonal region of interest was drawn on ADC maps around the margin of the lesion. In all lesions, minimal ADC values (ADC(min)) and mean ADC values (ADC(mean)) were estimated. Normalized ADC (NADC) was calculated in every case as a ratio ADC(mean) meningioma/ADC(mean) white matter. All meningiomas were surgically resected and analyzed histopathologically. The tumor proliferation index was estimated on Ki-67 antigen-stained specimens. Cell density was calculated. Collected data were evaluated by means of descriptive statistics. Analyses of ADC/NADC values were performed by means of two-sided t tests. RESULTS: The mean ADC(mean) value was higher in grade I meningiomas in comparison to grade II/III tumors (0.96 vs 0.80 x 10(-3) mm(2)s(-1), P = .006). Grade II/IIImeningiomas showed lower NADC values in comparison to grade I tumors (1.05 vs 1.26, P=. 015). There was no significant difference in ADC(min) values between grade I and II/III tumors (0.69 vs 0.63 x 10(-3) mm(2)s(-1), P = .539). The estimated cell count varied from 486 to 2091 (mean value, 1158.20 +/- 333.74; median value, 1108). There were no significant differences in cell count between grade I and grade II/III tumors (1163.93 vs 1123.86 cells, P = .77). Themean level of the proliferation index was 4.78 +/- 5.08%, the rangewas1% to18%, andthemedianvaluewas2%. The proliferation index was statistically significant higher in grade II/IIImeningiomas in comparison to grade I tumors (15.43% vs 3.00%, P = . 001). Ki-67 was negatively associated with ADC(mean) (r = -0.61, P < .001) and NADC (r = -0.60, P < .001). No significant correlations between cell count and ADC(mean) (r = -0.20, P = .164) or NADC (r = -0.25, P = .079) were found. ADC(min) correlated statistically significant with cell count (r = -0.44, P = .002) but not with Ki-67 (r = -0.22, P = .129). Furthermore, the association between ADC(min) and cell count was stronger in grade II/III tumors (r = -0.79, P = .036) versus grade I meningiomas (r = -0.41, P = . 008). An ADC(mean) value of less than 0.85 x 10(-3) mm(2)s(-1) was determined as the threshold in differentiating between grade I and grade II/III meningiomas (sensitivity 72.9%, specificity 73.1%, accuracy 73.0%). The positive and negative predictive values were 33.3% and 96.8%, respectively. The same threshold ADC(mean) value was used in differentiating between tumors with Ki-67 level >= 5% and meningiomas with low proliferation index (Ki-67 <5%). This threshold yielded a sensitivity of 70.6%, a specificity of 81.2%, and an accuracy of 77.6%. The positive and negative predictive values were 66.6% and 83.9%, respectively. CONCLUSIONS: Grade II/III tumors had lower ADC(mean) values than grade I meningiomas. ADC(mean) correlated negatively with tumor proliferation index and ADC(min) with tumor cell count. These associations were different in several meningiomas. ADC(mean) can be used for distinguishing between benign and atypical/malignant tumors.