Conventional and Functional MR Imaging of Peripheral Nerve Sheath Tumors: Initial Experience

Abstract

BACKGROUND AND PURPOSE Differentiating benign from malignant peripheral nerve sheath tumors can be very challenging using conventional MR imaging. Our aim was to test the hypothesis that conventional and functional MR imaging can accurately diagnose malignancy in patients with indeterminate peripheral nerve sheath tumors. MATERIALS AND METHODS This institutional review board–approved, Health Insurance Portability and Accountability Act–compliant study retrospectively reviewed 61 consecutive patients with 80 indeterminate peripheral nerve sheath tumors. Of these, 31 histologically proved peripheral nerve sheath tumors imaged with conventional (unenhanced T1, fluid-sensitive, contrast-enhanced T1-weighted sequences) and functional MR imaging (DWI/apparent diffusion coefficient mapping, dynamic contrast-enhanced MR imaging) were included. Two observers independently assessed anatomic (size, morphology, signal) and functional (ADC values, early arterial enhancement by dynamic contrast-enhanced MR) features to determine interobserver agreement. The accuracy of MR imaging for differentiating malignant from benign was also determined by receiver operating characteristic analysis. RESULTS Of 31 peripheral nerve sheath tumors, there were 9 malignant (9%) and 22 benign ones (81%). With anatomic sequences, average tumor diameter (6.3 ± 1.8 versus 3.9 ± 2.3 mm, P = .009), ill-defined/infiltrative margins (77% versus 32%; P = .04), and the presence of peritumoral edema (66% versus 23%, P = .01) were different for malignant peripheral nerve sheath tumors and benign peripheral nerve sheath tumors. With functional sequences, minimum ADC (0.47 ± 0.32 × 10−3 mm2/s versus 1.08 ± 0.26 × 10−3 mm2/s; P [H11021] .0001) and the presence of early arterial enhancement (50% versus 11%; P = .03) were different for malignant peripheral nerve sheath tumors and benign peripheral nerve sheath tumors. The minimum ADC (area under receiver operating characteristic curve was 0.89; 95% confidence interval, 0.73– 0.97) and the average tumor diameter (area under the curve = 0.8; 95% CI, 0.66 – 0.94) were accurate in differentiating malignant peripheral nerve sheath tumors from benign peripheral nerve sheath tumors. With threshold values for minimum ADC ≤ 1.0 × 10−3 mm2/s and an average diameter of ≥4.2 cm, malignancy could be diagnosed with 100% sensitivity (95% CI, 66.4%–100%). CONCLUSIONS Average tumor diameter and minimum ADC values are potentially important parameters that may be used to distinguish malignant peripheral nerve sheath tumors from benign peripheral nerve sheath tumors.

Multimodal Imaging in Neurofibromatosis Type 1-associated Nerve Sheath Tumors

“…DCE-MRI constitutes a quantitative method for evaluating tumor profusion, for which significantly different imaging patterns have been described particularly when it comes to differentiating benign and malignant soft tissue tumors [34]. In a current article, Demehri et al observed that when DCE-MRI was used to examine 9 MPNST and 22 nerve sheath tumors, 50 % of the MPNST exhibited an early arterial contrast medium enhancement, which, however, was discovered in only 11 % of the benign peripheral nerve sheath tumors [35]. To put it simply, diffusion-weighted imaging is based on the limited diffusion (random thermal motion of water molecules) owing to the high cell density in tumors [36].…”

“…While a sensitivity of 100 % and a specificity of 77 % MPNST were identified at a minimum ADC value of < 1.0 × 10 −3 mm 2 /s, even this limited specificity was observed only in tumors with a diameter of ≥ 4.2 cm. An even lower specificity must be assumed for smaller lesions [35]. In another study involving 29 patients, MPNST exhibited in diffusion tensor imaging (DTI) a significantly lower diffusivity than benign tumors (0.900 ± 0.25 versus 1.848 ± 0.40 × 10 −3 mm 2 /s; p < 0.001), with this difference not being significant for ADC values from DWI sequences [37].…”

“…However, irregular contrast medium enhancement appears also with plexiform neurofibromas [31 -33], which constitute histologically heterogeneous tumors. Constituting another malignancy criterion, peritumoral edema is present in 29 -66 % of cases of MPNST, yet can also be present with benign nerve sheath tumors [32,35]. Intratumoral cystic changes in T2w as signs of, e. g., cystically degenerated infarctions with large neurofibromas were likewise associated with MPNST [32].…”

“…Furthermore, this is primarily a result, however, of the often late diagnosis. Poorly defined demarcation with surrounding tissue is another malignancy criterion [31,33,35]. However, this is likewise a relatively advanced sign of a malignant transformation, since in this case growth exceeding the capsule must be present, which -given the genesis of MPNST from plexiform neurofibromas -can be expected to appear late.…”

“…In addition, criteria described in individual articles as being significantly associated with MPNST have not been reproducible in all studies. In some cases, characteristics ascribed to MPNST such as, for example, an irregular tumor shape were associated more with benign tumors in several studies [31,35]. A characteristic criterion of peripheral nerve sheath tumors is the presence of what is referred to as target sign ( • " Fig.…”