Diffusion-weighted MR imaging: diagnosing atypical or malignant meningiomas and detecting tumor dedifferentiation

Abstract

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/mm(2)) 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) mm(2)/s) was significantly lower compared with benign meningiomas (0.88 +/- 0.08 x 10(-3) mm(2)/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) mm(2)/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.

Fig 1.

Patient 34, atypical meningioma in a 73-year-old man. A, Enhanced coronal T1 image shows an en plaque meningioma (arrow) over the left frontoparietal region, with ill-defined margins. B, DW MR image demonstrates the mass to be hyperintense. C, ADC map shows decreased signal intensity compared with normal white matter (absolute ADC = 0.63 × 10⁻³ mm²/s; NADC ratio = 0.88).

Fig 2.

Patient 18, benign meningioma in a 37-year-old woman. A, DW MR image showing the tumor in the left frontal region to be hyperintense. B, ADC map showing increased signal intensity compared with normal white matter (absolute ADC = 0.98 × 10⁻³ mm²/s; NADC ratio = 1.46). C, Axial T2-weighted image shows that the hyperintensity in A was probably due to T2 shinethrough effect.

Fig 3.

Box and whisker plot comparing ADC values between groups. A, Absolute intratumoral ADC values in benign and atypical/malignant meningiomas showing some overlap between groups. A single outlier (patient 44) in the atypical/malignant group had a recurrent malignant meningioma of the rhabdoid variant. B, There is no overlap in NADC ratios between benign and atypical/malignant meningiomas.

Fig 4.

Patient 12, right sphenoid wing meningioma in a 79-year-old man. A, Before first surgery, contrast-enhanced axial T1 image shows an extra-axial enhancing mass lesion at the right sphenoid wing causing cavernous sinus obstruction. B, Corresponding DW MR image demonstrates the mass to be isointense. C, ADC map shows isointensity (absolute ADC = 0.90 × 10⁻³ mm²/s; NADC ratio = 1.23). Histologic examination showed benign meningothelial meningioma. D, Follow-up contrast-enhanced axial T1-weighted image before second resection shows the recurrent mass to be more extensive. E, Corresponding DW MR image of the tumor before second resection (arrow) is now hyperintense. F, ADC map shows hypointensity (absolute ADC has decreased to 0.60 × 10⁻³ mm²/s, and NADC ratio decreased to 0.77). Histologic examination revealed dedifferentiation to atypical meningioma.