Predicted probabilities of brain injury after carbon ion radiotherapy for head and neck and skull base tumors in long-term survivors

Background and purpose: We aimed to determine the risk factors for radiation-induced brain injury (RIBI1) after carbon ion radiotherapy (CIRT) to predict their probabilities in long-term survivors. Materials and methods: We evaluated 104 patients with head, neck, and skull base tumors who underwent CIRT in a regimen of 32 fractions and were followed up for at least 24 months. RIBI was assessed using the Common Terminology Criteria for Adverse Events. Results: The median follow-up period was 45.5 months; 19 (18.3 %) patients developed grade >= 2 RIBI. The maximal absolute dose covering 5 mL of the brain (D5ml) was the only significant risk factor for grade >2 RIBI in the multivariate logistic regression analysis (p = 0.001). The tolerance doses of D5ml for the 5% and 50% probabilities of developing grade >= 2 RIBI were estimated to be 55.4 Gy (relative biological effectiveness [RBE]) and 68.4 Gy (RBE) by a logistic model, respectively. Conclusion: D5ml was most significantly associated with grade >= 2 RIBI and may enable the prediction of its probability. (C) 2021 Elsevier B.V. All rights reserved.