In 1994, Woodruff et al. included 2 extracranial examples of cranial nerve schwannoma with malignant change in their review of literature related to this form of MPNST.
Their criteria for a diagnosis of MPNST ex-schwannoma included :
(a) the presence of a benign schwannoma,
(b) the finding within the schwannoma of a histologically malignant component,
(c) lack of a tumor elsewhere that might have metastasized to the schwannoma.
In their review of such tumors, the malignant component in the majority of the cases was epithelioid in appearance, and the remainder showed small neuroepithelial cell features.
Of the previously published tumors, 1 involved the fifth cranial nerve and had been originally reported as an epithelioid schwannoma.
More rarely, malignant change in schwannomas may also take the form of an angiosarcoma. The angiosarcomatous element in these examples is also frequently epithelioid.
Thus, immunohistochemical stains (eg, CD34, CD31) are important in confirming the diagnosis.
In 2001, McMenamin and Fletcher reported the clinicopathologic features of 17 schwannomas having undergone malignant change. The study included 1 example arising adjacent to a vagus nerve and another from the submandibular region, but none originating from a cranial nerve or in patients with either form of neurofibromatosis.
Their series confirmed the tendency of such malignant nerve sheath tumors to exhibit epithelioid cytology and the rare occurrence of angiosarcoma in schwannomas.
MPNST ex-schwannomas can show variation in cell morphology ranging from small round cells to epithelioid and spindle cells. The latter superficially resembled angiosarcoma, but immunohistochemistry (CD31, CD34 negative) can exclud that diagnosis.