Neurofibroma

Neurofibromas are benign peripheral nerve sheath tumours usually solitary and sporadic, however there is an association with neurofibromatosis type 1 (NF1). Three types have been described:

• localised neurofibroma (~90%)
• diffuse neurofibroma
• plexiform neurofibroma (pathognomonic of NF1)

These tumours generally present as a well well-defined hypodense mass with minimal minimal or no contrast enhancement on CT. On MRI they usually are T1 T1 hyointense and and T2 hyperintese with heterogeneous contrast enhancement. 

Epidemiology

Peak presentation is between between 20 and 30 years of age ⁵ with no sex predilection.

Associations

The majority of neurofibromas are solitary and sporadic, however there is an association with NF1 (abnormality of chromosome 17). Multiple neurofibromas are virtually diagnostic of NF1.

Pathology

Neurofibromas are benign neoplasms composed schwann cells and fibroblasts, containing a rich network of collagen fibres (WHO grade I). Unlike schwannomas, they are not encapsulated and infiltrate between the nerve fascicles.

Neurofibromas might might undergo malignant transformation into into a malignant malignantperipheral nerve sheath tumor (MPNST). 

Three types have been described:

• localised neurofibroma
  • the most common form of neurofibroma, representing 90% of these lesions²
  • the majority are solitary lesions and not associated with neurofibromatosis type 1 (NF1) ²
  • primarily affect superficial cutaneous nerves, however occasionally affect larger deep-seated nerves
  • slow growing
  • usually <5 cm in size at presentation³
• diffuse neurofibroma
  • mainly affect children and young adults
  • most frequently located within the subcutaneous tissues of the head and neck
  • 90% are solitary lesions and not associated with NF1²
  • cause plaque-like elevation of the skin with thickening of the entire subcutis³
• plexiform neurofibroma
  • diffuse involvement of a long nerve segment and its branches, often extending beyond the epineurium into the surrounding tissue
  • pathognomonic ofneurofibromatosis type 1 (NF1)
  • usually occur in early childhood
  • significant potential for malignant transformation⁴

Radiographic features

General imaging features of neurofibromas:

CT

• well-defined hypodense mass
• minimal or no contrast enhancement

MRI

• T1: hypointense
• T2: hyperintense
  • a hyperintense hyperintense rim and central area of low signal resulting in a target sign may be seen; this is thought to be due to a dense central area of collagenous stroma
  • although this sign is highly suggestive of neurofibroma, it is occasionally also seen in schwannomas and malignant peripheral nerve sheath tumours
• T1 C+ (Gd): heterogenous enhancement

Plexiform neurofibromas appear on CT and MRI as large multilobulated and conglomerated masses extending along nerves and nerve branches.

Treatment and prognosis

• lesions not associated with NF1
  • localised 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 sacrifice of the nerve
  • deep-seated lesions are therefore often managed conservatively
  • local recurrence after excision is uncommon and malignant transformation is rare ²
• lesions associated with NF1
  • due to multiplicity of lesions, unless debilitating symptoms are present, treatment of patients with NF1 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

See also

• spinal neurofibroma