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Neurofibromas are benign (WHO grade 1) peripheral nerve sheath tumors that are usually solitary and sporadic. There is, however, a strong association with neurofibromatosis type 1 (NF1), particularly for the plexiform subtype.
Neurofibromas are generally divided into five morphological forms 1-8:
localized/nodular intraneural neurofibroma
localized/nodular cutaneous neurofibroma
Diffuse cutaneous neurofibromas, plexiform neurofibromas and massive diffuse soft tissue neurofibromas have distinct radiological appearances and are therefore discussed separately. Localized cutaneous neurofibromas are generally not a radiological diagnosis, appearing as incidental findings on scans of patients with neurofibromatosis type 1.
As such, the remainder of this article is a general discussion focussing on localized intraneural neurofibromas which are by far the most common form of neurofibroma, representing 90% of these lesions 2.
Peak presentation of sporadic localized intraneural neurofibromas is between 20 and 30 years of age 5 with no sex predilection. When in the setting of neurofibromatosis type 1 they appear earlier, generally in early childhood 6.
The majority of localized intraneural neurofibromas are solitary and sporadic and not associated with neurofibromatosis type 1. However, when multiple neurofibromas are present (or plexiform neurofibromas) then the diagnosis of neurofibromatosis type 1 is almost assured.
The clinical presentation of localized intraneural neurofibromas is non-specific and the result of either mass effect on surrounding lesions (or palpable lump) or neurogenic dysfunction.
They are considered WHO grade 1 tumors in the 5th edition (2021) WHO classification of CNS tumors 6.
Unlike schwannomas, neurofibromas are not encapsulated and infiltrate between the nerve fascicles, making resection difficult. They primarily affect superficial cutaneous nerves, but occasionally affect larger deep-seated nerves.
Localized intraneural neurofibromas are composed of Schwann cells and fibroblasts, containing a rich network of collagen fibers.
S100 positive in the Schwann cells
CD34 positive in the stroma 6
General imaging features of neurofibromas:
well-defined hypodense mass
minimal or no contrast enhancement
T1 C+ (Gd): heterogeneous enhancement
Treatment and prognosis
For lesions not associated with neurofibromatosis type 1:
localized and diffuse lesions may be treated surgically
however, as neurofibromas infiltrate between nerve fascicles, they are unable to be separated from the parent nerve and complete excision requires the sacrifice of the nerve
deep-seated lesions are therefore often managed conservatively
local recurrence after excision is uncommon and malignant transformation is rare 2
For lesions associated with neurofibromatosis type 1:
due to the multiplicity of lesions, unless debilitating symptoms are present, treatment of patients with neurofibromatosis type 1 is often non-surgical
plexiform neurofibromas are particularly difficult to resect, often leading to incomplete resection
recurrence after resection is frequent
plexiform neurofibromas demonstrate a significant potential for malignant transformation
Although neurofibromas are usually indolent tumors they sometimes have atypical features (atypical neurofibroma) and uncommonly undergo malignant transformation (malignant peripheral nerve sheath tumor). Malignant change of sporadic localized intraneural neurofibromas is most frequently seen in large neurofibromas, and even then it only occurs in 5-10% of tumors 6.
Note, when atypical features are encountered in the setting of neurofibromatosis type 1, then the term atypical neurofibromatous neoplasms of uncertain biologic potential (ANNUBP) should be used 6.