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Neural tumors


Neurofibroma

Key Features

  • Loose arrangement with pale myxoid stroma

  • Haphazard spindle cells with small wavy or S-shaped nuclei

  • Mast cells are numerous

Neurofibromas may occur as solitary lesions. Multiple widespread neurofibromas characterize neurofibromatosis (von Recklinghausen disease) in which they are seen in association with café-au-lait macules, axillary freckling, and pigmented hamartomas of the iris (Lisch nodules).

Fig. 22.1, Neurofibroma

Diffuse neurofibroma

Key Features

  • At scanning magnification, diffuse replacement of dermis and infiltration of fat

  • Higher magnification shows features typical of neurofibroma

Differential Diagnosis

Dermatofibrosarcoma protuberans has a similar growth pattern and can be myxoid, bearing a considerable resemblance to diffuse neurofibroma. Both may be CD34 positive, but S100 staining of the neurofibroma can distinguish the two. Spindle cell lipoma can appear similar, but occurs as an encapsulated nodule.

Fig. 22.2, Diffuse neurofibroma

Plexiform neurofibroma

Key Features

  • Large fascicles of neurofibroma surrounded by perineurium

  • Often embedded within a diffuse neurofibroma

Plexiform neurofibroma clinically resembles a “bag of worms” and is considered pathognomonic of neurofibromatosis. Both diffuse and plexiform neurofibromas probably result from loss of heterozygosity, where segmental loss of the remaining normal allele for the NF1 gene results in a localized complete lack of tumor suppressor protein.

Fig. 22.3, Plexiform neurofibroma

Fig. 22.4, Plexiform neurofibroma

Schwannoma (neurilemmoma)

Key Features

  • Deep dermal or subcutaneous tumors with perineural capsule

  • Arise within a nerve, displacing axons to the periphery, causing pain

  • Antoni A tissue contains parallel rows of nuclei separated by acellular areas (Verocay bodies)

  • Hard schwannomas are composed almost entirely of Antoni A tissue

  • Antoni B tissue represents a degenerative change with edematous stroma, typically just below the capsule

  • Soft schwannomas are composed almost entirely of Antoni B tissue

Schwannomas present as round to oval encapsulated tumors with a subcapsular crescentic zone of edema. Hard schwannomas typically demonstrate many Verocay bodies, whereas soft schwannomas are encapsulated neoplasms composed of loose edematous tissue with widely dilated hyalinized blood vessels.

Psammomatous melanotic schwannoma is a variant with psammoma bodies and melanin. It is associated with Carney complex (myxomas, spotty pigmentation, and endocrinopathy).

Fig. 22.5, (A) Schwannoma. (B) Schwannoma (Verocay bodies)

Fig. 22.6, (A) Soft schwannoma (predominantly Antoni B tissue). (B) Soft schwannoma (S100 immunostain)

Fig. 22.7, Psammomatous melanotic schwannoma

“Ancient” schwannoma

Key Features

  • Benign schwannoma with hyperchromatic pleomorphic nuclei

  • No mitoses are present

These “ancient” changes in schwannomas represent a degenerative phenomenon. The absence of mitotic figures and absence of an expansile growth pattern distinguish benign ancient schwannoma from malignant peripheral nerve sheath tumors (MPNST). Typically, MPNSTs arise from neurofibromas, rather than schwannomas.

Fig. 22.8, Ancient schwannoma

Neuromas

Neuromas are nerve sheath tumors with a roughly 1:1 ratio of axons to Schwann cells.

Traumatic neuroma

Key Features

  • Multiple nerve fascicles embedded within a fibrous scar

  • Clefts between fascicles

Traumatic neuromas occur most commonly on the extremities where mechanical injuries are most frequent. Longitudinal growth of the regenerating nerve trunk results in complex folding. In cross-section, this appears as multiple discrete nerve fascicles.

Fig. 22.9, Traumatic neuroma

Fig. 22.10, Traumatic neuroma

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