Transformation of Plexiform Neurofibromas to Malignant Peripheral Nerve Sheath Tumors in Neurofibromatosis Type 1

Background:\ \- Many people with neurofibromatosis type 1 (NF1) get tumors of the nervous system. Finding malignant tumors early is important for removing them. Researchers want to find ways of doing this with scans and genetic testing. Objectives: \- To learn more about neurofibromatosis type 1. Eligibility: \- People age 10 and older with NF1 who have a benign tumor or have had a malignant one. Design: * Participants will be screened in another study with medical history, physical exam, and urine and blood tests. They will have a magnetic resonance imaging (MRI) scan. * MRI: Participants will lie on a table that slides into a metal cylinder. They will be in the scanner for 60 90 minutes, lying still for 15 minutes at a time. Participants will get earplugs for the loud sounds. They will get a contrast agent (dye) through a thin plastic tube (catheter) inserted in an arm vein. * As part of their regular care, participants will have: * FDG-PET/CT scan. They will get radioactive glucose (sugar) through a catheter in an arm vein. * \[18F\]-FLT-PET/CT scan. This is like the FDG scan but with a different radioactive chemical. * Biopsy. A piece of tumor tissue is removed with a needle. A piece of tissue from a previous biopsy may also be studied. * Participants may have genetic testing. Blood will be taken. It will be tested along with biopsy samples. Researchers will explain the risks and procedures. They may notify participants if testing shows health problems. * After this study, participants will continue their regular cancer care.