plexiform neurofibroma

Digital cases

 HPC:105 : Plexiform neurofibroma NF1
 HPC:401 : Palpebral plexiform neurofibroma in NF1

Definition: Plexiform neurofibroma is the least common of neurofibromas. Plexiform neurofibroma occurs almost exclusively in patients with neurofibromatosis type 1 (NF1) and they are often multiple.

Many of them occur in a superficial location but they can also occur in deep locations.

Macroscopy

Grossly, a typical, small to medium sized tumor appears as a spindle shaped enlargement with the head and tail merging with the surrounding tissue or the nerve trunks that they arise from.

In well preserved surgical specimens that are removed en bloc, plexiform neurofibroma appears as a tangle of enlarged nerve trunks reminiscent of a piece of curled up noodle or “a bag of worms”.

On cut section, the individual enlarged nerve trunks may appear as segregated individual bundles. A myxoid appearance is also characteristic.

Although the tumors tend to confine their involvement in large nerves to the nerve trunks themselves, their diffuse involvement along the length of the nerve makes complete resection difficult if not impossible. In contrast, sporadic neurofibromas are sharply circumscribed.

Microscopy

Microscopically, plexiform neurofibroma shares common features with other neurofibromas. In essence, the tumor has a hypocellular, myxomatous background of variable degree. Fine collagen fibers are also present at a variable density.

The proportion of collagenous to myxomatous component is highly variable. There are widespread spindle cells with ovoid to thin and elongated, bland nuclei with minimal or no pleomorphism and without prominent nucleoli. The neoplastic tissue typically fills and expands the nerve trunk.

In contrast to schwannoma where residual nerve trunk may be identified at the periphery, the nerve trunk is diffusely involved and expanded and no residual nerve trunk can be demonstrated. Mitosis, high cellularity, necrosis and high grade pleomorphism should not be present. Demonstration of these features would raise a strong suspicion for malignant transformation which is not an uncommon event in NF1 patients.

Immunochemistry

Immunohistochemically, the spindle cells are positive for S-100 protein. Entrapped axons can also be demonstrated by immunohistochemistry for neurofilament proteins. In general, the morphologic features of plexiform neurofibroma are highly characteristic and do not require immunohistochemistry for diagnostic purposes.

Synopsis

 closely packed multiple fascicles of neurofibromatous tissue of low to at most moderate cellularity
 myxoid matrix
 fascicles of tissue contain collagen fibers and bland appearing, small, spindle cells with dark nuclei as well as plump cells with open, vesicular nuclei
 cellularity varies from low to moderate
 no areas with high cellularity or significant atypia
 no mitotic figures
 no hemorrhage
 no necrosis

See also

 neurofibromas

• localized neurofibroma

 neurofibromatoses

• neurofibromatosis type 1 (NF1)

References

 Malignant schwannoma of the sciatic nerve originating in a spinal plexiform neurofibroma associated with neurofibromatosis type 1—case report. Topsakal C, Akdemir I, Tiftikci M, Ozercan I, Aydin Y. Neurol Med Chir (Tokyo). 2001 Nov;41(11):551-5. PMID: 11758709