Malignant Peripheral Nerve Sheath Tumor (MPNST) (WHO grades III or IV) is any malignant tumor arising from a peripheral nerve or showing nerve sheath differentiation, with the exception of tumors originating from epineurium or the peripheral nerve vasculature. Approximately 50% of cases are associated with Neurofibromatosis 1 (NF1). This malignant neoplasm is morphologically variable, with a tendency towards divergent mesenchymal differentiation. (Adapted from WHO.)