Peripheral nerve sheath tumors

Peripheral nerve sheath tumors (PNSTs) are a group of primary neurogenic tumors that arise from nerve sheaths outside of the central nervous system. The vast majority are benign, however, malignant transformation is seen particularly in large tumors and those associated with neurofibromatosis type 1 (NF1).

Classification

Their imaging appearances, demographics, treatment and prognosis vary greatly and these are therefore discussed separately.

Peripheral nerve sheath tumors may be classified as follows:

• benign
  • schwannoma
  • neurofibroma
    • localized neurofibroma
      • localized cutaneous neurofibroma
      • localized intraneural neurofibroma
    • diffuse cutaneous neurofibroma
    • plexiform neurofibroma
  • perineurioma
  • hybrid nerve sheath tumor
• malignant
  • malignant peripheral nerve sheath tumor

Radiographic features

Imaging of a solitary peripheral nerve sheath tumor, in most cases, cannot reliably distinguish between the different histological subtypes, and a presumptive diagnosis must take into account the patient demographics, including pre-existing conditions (such as neurofibromatosis type 1 or neurofibromatosis type 2), the location and size of the tumor, and evidence of rapid growth. As such these are discussed separately in each of the aforementioned tumors. A number of features are, however, shared by localized neurogenic tumors of peripheral nerves.

MRI

As a group, localized peripheral nerve sheath tumors demonstrate the following features:

• fusiform-shaped mass with tapered ends, with nerve seen leading into and out of the mass
• split-fat sign
• fascicular sign
• target sign
• denervation changes in muscles supplied by the involved nerve