Malignant peripheral nerve sheath tumors (MPNST) usually occur in adults (age range 20-50 yrs; median 35 yrs.) but rarely may arise in children as well. Most involve major nerve trunks such as sciatic nerve, brachial plexus, and sacral plexus and this is reflected in locations including proximal upper and lower extremities and the trunk. MPNST cause fusiform or eccentric swelling of the originating nerve trunk. The tumor may spread both proximally and distally for considerable distances along the nerve. This cross section through a nerve trunk shows involvement by the tumor (dark purple areas).