Perineural spread of tumour is a form of local invasion in which primary tumours cells spread along the tissues of the nerve sheath.  It is a well-recognised phenomenon in head and neck cancers.

Terminology

An important distinction has to be made between perineural invasion (PNI) and perineural spread (PNS). The former is a histological finding of tumour cell infiltration or associated with small nerves that cannot be radiologically imaged, while the latter is macroscopic tumour involvement along a nerve extending away from the primary tumour; this can be radiologically apparent. A third term, neurotropism, simply means that a tumour has an affinity for growth along nerves.

Pathology

Perineural tumour spread is more frequently associated with ^1,2,5:

• mucosal/cutaneous squamous cell carcinoma
  • oral cavity/laryngeal (2-30%) > cutaneous (3-8%)
  • most common overall ⁵
• salivary gland carcinoma
  • adenoid cystic carcinoma (highest incidence per individual tumour ⁵)
  • mucoepidermoid carcinoma
• mucosal/cutaneous basal cell carcinoma (2-5% demonstrate perineural tumour spread) ⁴
• melanoma 
  • 0.8 - 2.6% demonstrate perineural spread⁸ 
    • 65% are desmoplastic subtype 
• lymphoma
• sarcoma
• meningioma (rare) ⁶

Radiographic features

MRI

Signs on MRI suggesting perineural tumour spread are:

• nerve thickening
• widening of the neural foramen
• loss of the fat surrounding the nerve
• abnormal perineural contrast enhancement

MRI can depict perineural tumour spread with a sensitivity of 95%, that falls to 63% regarding the entire spread's map. The presence of perineural tumour spread can be determined but for deducing its extent, cautious analysis and patterns of enhancement may be needed ⁹. 

Differential diagnosis

• radiation-induced neuritis