Plexiform neurofibromas are extensive nerve sheath tumors that can become quite large in size. They occur in patients with neurofibromatosis type 1, and present as large soft tissue masses under the skin. Occasionally, these tumors can become malignant. This is heralded by rapid growth and pain. MRI with and without contrast fully defines these tumors. PET scans may be used to help exclude malignancy.