Malignant peripheral nerve sheath tumour, light micrograph. Malignant peripheral nerve sheath tumour (MPNST) is a high-grade sarcoma that shows differentiation along one of the nerve sheath elements such as Schwann cell, perineural cell, or fibroblast. They make up 5% to 10% of all soft tissue sarcomas. About 50% arise de novo; the remainder occur in the setting of neurofibromatosis 1 (NF1). They usually occur in adults along one of the major nerve trunks such as sciatic nerve, brachial plexus, and sacral plexus. MPNSTs are large, fleshy tumours and commonly show areas of necrosis and haemorrhage. MPNSTs have a tendency to local recurrences (about 40% of cases) as well as distant metastases (40% to 60% of cases) after resection. This image shows a high-grade fibrosarcomatous-appearing tumour on the lower left. The uninvolved nerve is seen on the upper right. The two areas are separated by well-defined epineurium (pink band running from upper left to lower right). Magnification: x200.