For the characterization of peripheral nerve sheath tumors (PNSTs) as benign or malignant in patients with neurofibromatosis type 1 (NF1), MRI-obtained apparent diffusion coefficient (ADC) and 18F-fluorodeoxyglucose (FDG)-PET/CT-obtained maximum standardized uptake values (SUVmax) are excellent imaging markers, according to study results published in Neurology.

Although it is well known that patients with NF1 are at increased risk for malignant tumors, there are no standardized clinical recommendations for monitoring, and frequently both FDG-PET and MRI are being used to identify premalignant and malignant lesions in people with NF1.

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The goal of the current study was to explore the utility of 2 quantitative metrics obtained from fMRI with diffusion-weighted imaging (DWI) and ADC mapping compared with FDG-PET/CT imaging for characterization of potential malignancy of PNSTs in patients with NF1.

The researchers retrospectively reviewed patients from the Johns Hopkins Comprehensive Neurofibromatosis Center and included patients with a definitive diagnosis of NF1, available MRI with DWI/ADC mapping and FDG-PET/CT performed within 2 months of one another, and 12 months of clinical or imaging data or a histologic diagnosis.

The study cohort included 20 patients (mean age, 29.5 years; 12 male) with 55 PNSTs, including 19 (35%) malignant and 36 (65%) benign PNSTs.

Benign PNSTs were smaller in size compared with malignant PNSTs (largest diameter, 4.3±1.3 vs 8.2±3.3 cm, respectively; P =.014). Benign PNSTs had higher ADCmin (1.6±0.4 vs 0.6±0.2 × 10−3 mm2/s, respectively; P <.0001) and lower SUVmax (3.2±1.8 vs 8.0±3.9, respectively, P <.0001) compared with malignant PNSTs.

Threshold for minimum ADC values ≤1 × 10−3 mm2/s or SUVmax <3.2 yielded 100% sensitivity with a specificity of 94% (81%–99%) for DWI and 83% (67%–93%) for FDG-PET/CT for distinguishing benign from malignant/premalignant PNSTs.

In PNSTs with elevated SUVmax (imaging marker for increased metabolic activity), the minimum ADC value tended to be lower (surrogate for increased cellularity; r = −0.58; 95% CI −0.72 to −0.38; P <.0001).

The researchers acknowledged several study limitations, including a relatively small sample size, retrospective design, and heterogenous magnetic resonance protocols.

“Both quantitative metabolic imaging and functional imaging offer high sensitivity for the characterization of PNSTs in NF1; however, DWI/ADC mapping offers increased specificity and may be a more useful modality,” conclude the researchers.

Reference

Ahlawat S, Blakeley JO, Rodriguez FJ, Fayad LM. Imaging biomarkers for malignant peripheral nerve sheath tumors in neurofibromatosis type 1. Neurology. 2019;93(11):e1076-e1084.