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.

* INCLUSION CRITERIA: 1\. Age: * No upper age limit for patient enrollment. * FLT PET: will only be performed in patients greater than or equal to 10 years old * Research biopsies in consenting patients with MPNST: will only be performed in patients greater than or equal to 18 years old 2\. Diagnosis: * Patients who are diagnosed with NF1 using the NIH Consensus Conference criteria or have a confirmed NF1 mutation with analysis performed in a CLIA certified laboratory. NF1 mutation testing to confirm eligibility will not be performed on this protocol, but as part the POB separate screening study. * For the clinical diagnosis of NF1 all study subjects must have at two or more diagnostic criteria for NF1 listed below (NIH Consensus Conference): 1. Six or more caf(SqrRoot)(Copyright)-au-lait spots (greater than or equal to 0.5 cm in prepubertal subjects or greater than or equal to 1.5 cm in postpubertal subjects) 2. Greater than or equal to 2 neurofibromas or 1 plexiform neurofibroma 3. Freckling in the axilla or groin 4. Optic glioma 5. Two or more Lisch nodules 6. A distinctive bony lesion (dysplasia of the sphenoid bone or dysplasia or thinning of long bone cortex) 7. A first-degree relative with NF1 3\. NFI tumor manifestations Subjects must have: 1. Diagnosis of NF1 with a lesion concerning for MPNST -Criteria include pain, growth of a known plexiform neurofibroma, abnormality on functional imaging study (FDG-PET) or change in clinical exam. OR 2. Diagnosis of NF1 with a histologically confirmed MPNST. 4\. Subjects must be eligible for and willing to participate and sign consent for NCI protocol 08-C-0079: Natural History Study and Longitudinal Assessment of Children, Adolescents, and Adults with Neurofibromatosis Type 1, for the clinical evaluation necessary for this study. 5\. Prior and current therapy: For NF1 related benign tumor manifestations there is no standard effective medical treatment, and surgery is the only standard treatment. Chemotherapy and radiation therapy are additional treatment options for malignant NF1 related tumors. For the purpose of this study subjects who have not previously received medical or surgical treatment, patients who have previously received medical or surgical treatment, and subjects who are currently receiving medical treatment and or radiation for a NF1 related manifestation will be eligible. Patients must be recovered from acute toxicities of prior therapy in order to be able to safely undergo biopsies proposed on the trial. Prior and current treatment for NF1 related manifestations will be recorded on protocol 08-C-0079. Prior radiation therapy and chemotherapy in patients with MPNST must not have been administered within 4 weeks prior to enrollment. 6\. Performance Status: ECOG less than or equal to 3. Subjects who are wheelchair bound because of paralysis will be considered ambulatory when they are up in their wheelchair. Subjects have to be able to travel to the NIH for evaluations. 7\. Informed Consent: All patients or their legal guardians (if the patients is\