Answer: Schwannoma

Histology: The tumor is an encapsulated lesion, which grows eccentrically along the periphery of a nerve. The cellular tumor consists of spindle cells arranged in short bundles or interlacing fascicles and foci of nuclear palisading and whirling of cells can be seen. In some areas, the spindle cells form long sweeping fascicles. Mitotic figures are rare. The vessels within the tumor show perivascular hyalinization. The tumor is infiltrated by many foamy histiocytes

Discussion: Classic schwannomas show a mixture of Antoni A and B areas. Antoni A areas are cellular areas composed of compact spindle cells arranged in short bundles or interlacing fascicles. In these cellular areas, there may be nuclear palisading and whirling of cells. Characteristic Verocay bodies may be seen. Antoni B areas are less orderly and less cellular areas composed of loose matrix with foci of microcystic change and inflammatory cells. This schwannoma shows marked infiltration by foamy histiocytes. This lesion lacks atypia and show rare mitoses, arguing against a malignant diagnosis. Neurofibromas may show cellular areas that resemble Antoni A areas of a schwannoma, however, unlike schwannomas, neurofibromas are not encapsulated and small nerves can usually be demonstrated within the lesion. Meningiomas can be confused with schwannomas, as cellular whirling areas can be seen in both lesions. Immunohistochemical stains can be helpful to differentiated between the two entities. Schwannomas are positive for S100 and negative for EMA, while meningiomas show the opposite staining pattern.

Bilateral schwannomas are a characteristic of patients with neurofibromatosis, type II, also known at central neurofibromatosis. Like neurofibromatosis, type I or Von Recklinghausen’s disease, type II neurofibromatosis shows an autosomal dominant inheritance. The gene is on chromosome 22 and encodes for the protein, merlin, which plays a role in cell membrane binding to the intracellular matrix. The cafe-au-lait spots and neurofibromas seen in type I neurofibromatosis can occur in type II neurofibromatosis, but occur fewer in number. Patients with type II neurofibromatosis also tend to get other central nervous system tumors including ependymoma and meningioma.