The role of magnetic resonance imaging in the diagnostic evaluation of malignant peripheral nerve sheath tumors

Malignant peripheral nerve sheath tumors (MPNSTs) are the leading cause of mortality in patients with neurofibromatosis type-1 (NF1)); however, they may also arise sporadically. Differences in magnetic resonance imaging (MRI) features between MPNSTs arising in NF1 subjects versus non-NF1 subjects have not been studied before. The accuracy of MRI in distinguishing MPNSTs from benign peripheral nerve sheath tumors (BPNSTs) has also been debated. The objective of this study was to determine the potential differentiating MRI features between (a) NF1-related and non-NF1-related MPNSTs and (b) MPNSTs and BPNSTs. We retrospectively evaluated the MRI studies of 21 patients (12 NF1 subjects and nine non-NF1 subjects) with MPNSTs and 35 patients with BPNSTs. In all studies, the lesions were assessed in terms of size, margins, T1 and T2 signal characteristics, internal architecture, pattern of contrast enhancement, invasion of adjacent structures and necrosis/cystic degeneration as well as for the presence of tail-, target- and split-fat signs. MPNSTs of NF1 subjects occurred at an earlier age and displayed a higher incidence of necrosis/cystic degeneration compared with MPNSTs of non-NF1 subjects. Compared with BPNSTs, MPNSTs were significantly larger at the time of diagnosis and demonstrated a higher incidence of ill-defined margins (specificity 91%, sensitivity 52%) and invasion of adjacent structures (specificity 100%, sensitivity 43%). Differences exist between NF1-related and non-NF1-related MPNSTs regarding the age of occurrence and MRI appearance. In the MRI evaluation of peripheral nerve sheath tumors, the presence of ill-defined tumor margins and/or invasion of adjacent structures are highly specific for malignancy.