Neurofibromatosis type 1 (NF1) is a frequent and inherited disease with a predisposition for malignant peripheral nerve sheath tumor (MPNST) development. MPNST are soft tissue sarcomas that arise from peripheral nerves, being one of the most aggressive malignancies in humans with extremely poor prognosis. MPNST frequently arise from a previously undetected plexiform neurofibroma (PNF). The malignant transformation of an internal PNF to an MPNST is difficult to assess and requires advanced imaging techniques like magnetic resonance imaging or positron emission tomography. Despite the high quality of current diagnostics, the changing tumor biology inside a plexiform neurofibroma cannot currently be visualized accurately. We report 4 cases of NF1 patients with PNF who showed imaging findings suspicious for malignant degeneration, but proved to have MPNST in only one case. Three tumors might represent an intermediate type between PNF and MPNST. Ablative surgery and complete histological work-up of specimens is the only way to clarify tumor status, thereby enabling provision of adequate local treatment.
