Purposeto investigate outcome and toxicity of patients affected by malignant peripheral nerve sheath tumors (MPNSTs) treated with high-dose carbon ion radiotherapy (CIRT). Patients and methodswe retrospectively analyzed the outcome of 23 patients with MPNST treated between July 2013 and December 2020. Out of these, 13 patients (56.5%) had incompletely resected tumors, 8 patients (34.7%) experienced recurrence after surgery, and 2 patients (8.7%) had unresectable tumors. Before CIRT treatment, 4 patients underwent a second surgery after the first local recurrence (LR), and 1 patient underwent a third surgery for the second local relapse of disease. Six (26%) of patients received neoadjuvant chemotherapy. The most frequent tumor site was brachial plexus (N=9; 39.1%). In five patients (21.7%) neurofibromatosis type 1 (NF1) disorder was found, while 4 patients (17, 4%) had radiation-induced MPNST. The median CIRT prescribed total dose was 69.8 Gy [RBE] (range, 54-76.8) delivered in a median of 16 fractions (range, 15-22). Eleven patients (47.82%) were treated according to a sequential boost protocol with a median prescribed dose to CTV-LR of 45 Gy[RBE] (range, 41.4 – 54). ResultsAfter a median follow-up time of 23 months (range 3-100 months), the OS rates at 1-year and 2-year were respectively: 82.38%, 61.51%. The 1-year and 2-year LRFS rates were respectively: 65.07%, 48.80%; the 1-year and 2-year PFS rates were respectively: 56.37%, 40.99%. No patients showed acute or late Grade 4 toxicity or any treatment-related deaths. Ten patients (43.48%) reported acute toxicities of Grade ≥2: dermatitis in 6 patients, mucositis in 2 patients, peripheral neuropathy in 4 patients. Eight patients (34.78%) reported late toxicities of Grade ≥2, mainly due to loco-regional neuropathy. Conclusionshigh dose CIRT shows favorable local effect with acceptable toxicities in patients with gross residual and LR after surgery, or unresectable malignant peripheral nerve sheath tumors. Advanced treatment modalities such as particle therapy should be considered for MPNSTs.