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. Malignant peripheral nerve sheath tumour (MPNST) is a highly cellular neoplasm resembling fibrosarcoma and is composed of monomorphic spindle cells arranged in interlacing fascicles, palisades, and whorls. The cells have comma-shaped, wavy or buckled hyperchromatic nuclei and indistinct pale or eosinophilic cytoplasm. Some cases have plump oval or rounded nuclei (as shown here).