This study is for slow growing tumors called plexiform neurofibromas (PNF) which are a relatively common problem in people with neurofibromatosis type 1 (NF1). These tumors are benign but as they grow, they can become disfiguring as well as disabling or even life threatening. They often cause pain, difficulty using arms or legs because of spinal cord compression, and/or nerve damage. At present, the only available therapy for plexiform neurofibromas is to try to surgically remove as much of the tumor as is possible. Because these tumors grow into the surrounding areas, total surgical resection is often impossible. Most tumors will re-grow after surgery if the entire tumor cannot be removed. To date, other treatments including chemotherapy and radiotherapy have not been able to shrink these tumors.