Plexiform neurofibroma

Last revised by Yuranga Weerakkody on 15 May 2024

Plexiform neurofibroma is an uncommon variant of neurofibroma, a benign tumor of peripheral nerves (WHO grade I), arising from a proliferation of all neural elements. Plexiform neurofibromas are essentially pathognomonic of neurofibromatosis type 1 (NF1). Unlike small sporadic localized neurofibromas and diffuse cutaneous neurofibromas (both discussed separately), these tumors are at significant risk of eventual malignant transformation. 

There is variable use and some confusion about the distinction between plexiform neurofibroma and diffuse cutaneous neurofibroma, with some sources not clearly distinguishing between the two. Generally, plexiform neurofibromas are deeper lesions affecting nerves and plexus. The two may, however, co-exist 5

Importantly it appears that diffuse cutaneous neurofibromas may not be as closely associated with neurofibromatosis type 1 (NF1), nor have the same risk of malignant transformation 1-4. The distinction is thus important. 

Plexiform neurofibromas are usually diagnosed in early childhood. They are found in approximately 30% of patients with NF1. 

Clinically, it presents as a subcutaneous mass which feels like a "bag of worms". Most of the time, it is a superficial cutaneous/subcutaneous lesion, but it can occur almost anywhere in the body. Symptoms may be related to local mass effect.

Plexiform neurofibromas diffusely involve long nerve segments and its branches, often extending beyond the epineurium into the surrounding tissue. 

Immunohistochemistry demonstrates findings in keeping with a neurogenic origin, including 6

  • S100: positive (fewer reactive cells than in schwannoma)
  • SOX10: positive (fewer reactive cells than in schwannoma)
  • neurofilament: only positive in entrapped axons

Non-specific infiltrative subcutaneous lesions.

Reported signal characteristics include:

  • T1: hypointense
  • T2: hyperintense +/- hypointense central focus (target sign)
  • T1 C+: mild enhancement

Although generally benign tumors, there is a significant potential for malignant transformation, which occurs in 5-10% of larger tumors 5,6. If complete resection is possible, then a cure can be effected, however, due to the infiltrating nature of these tumors, such a resection is usually not possible. 

The term plexiform comes from the infiltrative growth pattern that, histologically, looks like a plexus or a network. Plex- originates from the Latin verb plectere meaning "to plait" or "interweave".

It is typically associated with tumors of neural derivation. Plexiform neurofibromas are considered the prototype of the plexiform pattern 1.

On imaging consider:

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