• Most common type
• Circumscribed but not encapsulated
• Permeative growth in nerve quickly proceeds to diffuse infiltration of surrounding soft tissue
• Occurs as lumps or bumps on skin
• Painless
• Slowly growing
• Often in dermis and subcutaneous
• ≤2-5 cm in diameter
• Arise from small cutaneous nerves
• Overrun axons may be identified within
• May contain fat
• Starts in teenage years or young adults and rarely starts in childhood
• Increases in size and number over the years
• Clearly defined borders hence, can be removed if necessary
• Number of skin tumors in each patient varies tremendously
• Mostly solitary and sporadic, not associated with NF1
• Malignant transformation very rare

• Second-most common type
• Segmental, fusiform nerve enlargement
• Residual axons traverse through lesion
  • Neurofilament immunohistochemistry and Bielshowsky stain show axons within center of lesion
• Often contains coarse, refractile collagen
• Malignant change infrequent

• Common
• Present underneath the skin
• Can be single tumors or chains
• They are often a form of plexiform neurofibromas

(superficial)

• Superficial
• Uncommon
• Feels soft and squishy
• Most common in head (scalp) and neck region or trunk (often protruding out, like a “love handle”)
• Ill defined margins, can’t tell where tumor stops
• Runs through the full skin thickness (from the surface all the way down to the base of the skin (subcutaneous fascia/fat)
• Doesn’t go deeper than fascia
• Nondestructive, envelops normal structures (e.g., fat cells and adenexal structures)
• Uniform matrix of fine, fibrillary collagen
• Shorter, rounder Schwann cells
• Clusters of pseudo-meissnerian body-like structures may be seen
• Usually seen in early childhood and young adults
• Usually associated with diffuse hyperpigmentation, so looks like a very large squishy café au lait spot
• Solitary lesions and not related to any inherited condition (rarely associated with NF1)
• Rare malignant change

• Common
• Usually isolated tumors in the muscle
• Growths along very small nerves
• Causes pain sometimes
• Can be removed (leaves a scar behind)
• Sometimes can occur as plexiform neurofibromas in form of chains or networks

(deep)

• Deep
• Associated with NF1
• Diffuse involvement along a large nerve and its branches
• Mostly internal/ intraneural
• Can also involve small nerves and superficial skin
• Have more connective tissue/extracellular matrix than cutaneous neurofibromas, that separates the nerve fibers
• Gross enlargement of the nerve with nodular tumor development results in the gross pathologic appearance referred to as “bag of worms”
• Generally believed to be present at birth (congenital)
• Disfiguring
• Affects function due to sheer size as well as neurovascular compromise
• Upto 5% risk of malignant transformation
• Plexiform neurofibroma exhibits following features on T2-weighted MRI:
  • Target sign (low signal intensity centrally with a ring of high signal intensity peripherally)
  • Fascicular sign
  • May appear as a larger and more infiltrating mass with lobulated borders with inhomogeneous enhancement

• Neurofibroma with melanin-bearing pigmented cells, usually only appreciated microscopically
• Not considered a true subtype
• No increased risk of malignant transformation

• It extends through the skin into fascia and muscle
• Lacks clear margins
• Has “little fingers” that invade muscle or other tissue
• May not be easy to see in infants
• May have a large café au lait spot “above” it

• Usually involves nerves coming off the spinal cord or off of larger nerves (such as sciatic nerve)
• Usually enlarges or thickens the nerve
• Looks like little clusters of tumors along the nerve