Although benign peripheral nerve sheath tumors (PNSTs) are more prevalent than malignant peripheral nerve sheath tumors (MPNSTs), they can share an overlapping clinical presentation and imaging appearance, particularly on conventional MR imaging. As a result, many of these PNSTs are definitively diagnosed via percutaneous or surgical biopsies, which are invasive and painful procedures that often require anesthesia. Noninvasive advanced functional MR imaging sequences, namely quantitative DWI with ADC mapping, are increasingly utilized methods that improve diagnostic accuracy for the characterization of indeterminate soft-tissue masses, including neurogenic neoplasms.1,2 On DWI/ADC mapping, restricted diffusion of water or low ADC values have been observed in tumors, including MPNSTs, and have been attributed to tumoral hypercellularity that restricts water motion. The combination of conventional MR imaging sequences (average tumor diameter > 4.2 cm) and functional MR imaging sequences (ADC value ≤ 1.0 × 10-3 mm2/s) provides useful metrics for the highly accurate characterization of MPNSTs. At a recent meeting of the Society of Skeletal Radiology in Santa Barbara, California in March of 2017, we discussed the role of a “target sign” as identified by DWI with ADC mapping to further characterize the cellularity of PNSTs and detect malignancy.
Our group at Johns Hopkins University has routinely added advanced MR imaging sequences to conventional sequences as part of a comprehensive “tumor protocol” used to evaluate all soft-tissue tumors. This alteration in clinical practice has led to overall improvement in the diagnostic accuracy of noninvasive imaging-based characterization of PNSTs and prevents unnecessary biopsies in patients with benign PNSTs. Sporadic, benign PNSTs are notably encountered far more commonly in clinical practice and, consequently, do not always present a diagnostic dilemma.