Preoperative grading of presumptive low-grade astrocytomas on MR imaging: diagnostic value of minimum apparent diffusion coefficient

Abstract

Background and purpose: Histopathologic grade of glial tumors is inversely correlated with the minimum apparent diffusion coefficient (ADC). We assessed the diagnostic values of minimum ADC for preoperative grading of supratentorial astrocytomas that were diagnosed as low-grade astrocytomas on conventional MR imaging.

Materials and methods: Among 118 patients with astrocytomas (WHO grades II-IV), 16 who showed typical MR imaging findings of low-grade supratentorial astrocytomas on conventional MR imaging were included. All 16 patients underwent preoperative MR imaging and diffusion-weighted imaging. The minimum ADC value of each tumor was determined from several regions of interest in the tumor on ADC maps. To assess the relationship between the minimum ADC and tumor grade, we performed the Mann-Whitney U test. A receiver operating characteristic (ROC) analysis was used to determine the cutoff value of the minimum ADC that had the best combination of sensitivity and specificity for distinguishing low- and high-grade astrocytomas.

Results: Eight of the 16 patients (50%) were confirmed as having high-grade astrocytomas (WHO grades III and IV), and the other 8 patients were confirmed as having low-grade astrocytomas (WHO grade II). The median minimum ADC of the high-grade astrocytoma (1.035 x 10(-3) mm(2) . sec(-1)) group was significantly lower than that of the low-grade astrocytoma group (1.19 x 10(-3) mm(2) . sec(-1)) (P = .021). According to the ROC analysis, the cutoff value of 1.055 x 10(-3) mm(2) . sec(-1) for the minimum ADC generated the best combination of sensitivity (87.5%) and specificity (79%) (P = .021).

Conclusion: Measuring minimum ADC can provide valuable diagnostic information for the preoperative grading of presumptive low-grade supratentorial astrocytomas.

Fig 1.

Relationship between minimum ADC and grading of astrocytic tumors. Minimum ADC values (10⁻³ mm² · sec⁻¹) of low-grade astrocytomas are higher that those of high-grade astrocytomas, though some overlap is noted (P = .021).

Fig 2.

A 44-year-old man with an anaplastic astrocytoma. A, Contrast-enhanced T1-weighted image shows a nonenhancing mass in the right frontal lobe. B, The mass shows increased signal intensity in the T2-weighted image with a focal hypointense area (arrows). C, On the DWI, the mass has relatively homogeneous increased signal intensity. D, The ADC map shows increased ADC with a region of interest placed in the focal area of low ADC within the mass. The lesion area has a minimum ADC of 0.902 × 10⁻³ mm² · sec⁻¹.

Fig 3.

A 44-year-old man with a grade II diffuse astrocytoma. A, The contrast-enhanced T1-weighted image shows a nonenhancing mass in the left parietal lobe. B, The mass has relatively homogeneous increased signal intensity in the T2-weighted image. C, The DWI shows increased signal intensities compared with the surrounding regions. D, The ADC map shows high a ADC value compared with surrounding brain parenchyma with a subtle low ADC area with a region of interest placed for the lowest ADC measurement. The measured minimum ADC within the mass is 1.185 × 10⁻³ mm² · sec⁻¹.