May-June 2017
Peripheral Nervous System

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

Ahlawat pic

Shivani Ahlawat

Fayad pic

Laura M. Fayad

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.

However, people with neurocutaneous syndromes such as neurofibromatosis type 1 (NF-1) can develop both benign and malignant neoplasms at increasing frequency throughout life.3 The most common NF-1-related tumors are nodular and plexiform PNSTs, but an estimated 30–50% of patients with NF-1 develop plexiform neurofibromas, which can transform into MPNSTs.4 NF-1 carries an increased risk of MPNST, especially in those under 50 years of age, with a rate that is approximately 2.5 to 4 times higher than that of the general population and an overall lifetime risk of 10%.4–6

The recommendations for ongoing assessment and periodic surveillance of asymptomatic patients with NF-1 are limited and provide little guidance on how to evaluate new or progressive PNSTs. Screening comprising physical exams alone is especially difficult for deep or internal PNSTs that would manifest later, potentially with malignant conversion. Hence, early detection of malignancy remains particularly challenging in this patient population. Because both benign and malignant NF-1-related PNSTs can be large tumors that cross anatomic planes, whole body imaging is an efficient method of tumor detection in a single session. As such, 18F-FDG-PET/CT has traditionally been used for whole-body PNST detection and characterization until the advent of whole-body MR imaging. Although the interplay between these techniques has not been robustly studied, we have incorporated functional imaging into our whole-body MR imaging protocol.

References

  1. Fayad LM, Jacobs MA, Wang X, et al. Musculoskeletal tumors: how to use anatomic, functional, and metabolic MR techniques. Radiology 2012;265:340–56, 10.1148/radiol.12111740
  2. Del Grande F, Ahlawat S, Subhangwong T, et al. Characterization of indeterminate soft tissue masses referred for biopsy: what is the added value of contrast imaging at 3.0 Tesla? J Magn Reson Imaging 2017;45:390–400, 10.1002/jmri.25361
  3. Seminog OO, Goldacre MJ. Risk of benign tumours of nervous system, and of malignant neoplasms, in people with neurofibromatosis: population-based record-linkage study. Br J Cancer 2013;108:193–98, 10.1038/bjc.2012.535
  4. Evans DGR, Baser ME, McGaughran J, et al. Malignant peripheral nerve sheath tumours in neurofibromatosis 1. J Med Genet 2002;39:311–14,
    10.1136/jmg.39.5.311
  5. Sørensen SA, Mulvihill JJ, Nielsen A. Long-term follow-up of von Recklinghausen neurofibromatosis. N Engl J Med 1986;314:1010–15, 10.1056/NEJM198604173141603
  6. Walker L, Thompson D, Easton D, et al. A prospective study of neurofibromatosis type 1 cancer incidence in the UK. Br J Cancer 2006;95:233–38, 10.1038/sj.bjc.6603227

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