Tumor, Malignant Peripheral Nerve Sheath
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These are rare, poorly differentiated neoplasms of the peripheral nerves and/or their intraneural connective tissue. They may occur sporadically (estimated 1/100,000), in patients with neurofibromatosis, and after radiation therapy. Malignant peripheral nerve sheath tumors (MPNST) may resemble sarcomas and can be very aggressive, including high rates of local recurrence as well as metastases. Approximately 3-5% of patients with neurofibromatosis develop an MPNST within a pre-existing plexiform neurofibroma during their lifetime.
Patients usually present with a rapidly growing tumor mass causing pain, radiculopathy, and neurological deficit. MPNSTs are characteristically firm, immobile, tender, and usually have a positive Hoffman-Tinel's sign. Neurological deficit is not uncommon. This is in contrast to benign peripheral nerve sheath tumors that frequently are soft, mobile, and more rarely cause overt neurological deficits. Patients with neurofibromatosis often report a chronic , stable plexiform tumor that recently has become larger and more painful.
An MRI with and without IV contrast defines the the tumor and nearby, involved neurovascular structures. MPNST often are indistinguishable from benign neurofibromas and plexiform tumors on MRI. Therefore, to confirm the diagnosis a biopsy is often required. For large plexiform tumors, multiple biopsies, or one large biopsy is often performed to reduce sampling error. PET scanning shows promise in diagnosing malignancy especially in patients with neurofibromotosis.
En bloc surgical resection with clear margins followed by radiation and chemotherapy are the treatments of choice. Patients often have a biopsy, or gross-total, nerve-sparing tumor removal that reveals an MPNST. In these situations, re-exploration and en bloc re-resection of the involved neurovascular structures is usually recommended. Performing an en bloc resection based solely on a frozen section result, is not recommended because of the occasional misdiagnosis with this technique. Placement of nerve grafts to repair the resected nerve is controversial and guidelines regarding this are not available.
The prognosis is poor with MPNSTs, with many patients not surviving longer than 5 years. Patients who are younger and who are able to undergo resection with clear margins have a better life expectancy.