Effects of nuclear interaction corrections and trichrome fragment spectra modelling on dose and linear energy transfer distributions in carbon ion radiotherapy

Background and Purpose Nuclear interaction correction (NIC) and trichrome fragment spectra modelling improve RBE-weighted dose (D RBE ) and dose-averaged linear energy transfer (LET d ) calculation for carbon ions. The effect of those novel approaches on the clinical dose and LET distributions was investigated. Materials and Methods The effect of the NIC and trichrome algorithm was assessed, creating single beam plans for a virtual water phantom with standard settings and NIC + trichrome corrections. Reference D RBE and LET d distributions were simulated using FLUKA version 2021.2.9. Thirty clinically applied scanned carbon ion treatment plans were recalculated applying NIC, trichrome and NIC + trichrome corrections, using the LEM low dose approximation and compared to clinical plans ( base RS ). Four treatment sites were analysed: six prostate adenocarcinoma, ten head and neck, nine locally advanced pancreatic adenocarcinoma and five sacral chordoma. The FLUKA and clinical plans were compared in terms of D RBE deviations for D 98% , D 50% , D 2% for the clinical target volume (CTV) and D 50% in ring-like dose regions retrieved from isodose curves in base RS plans. Additionally, region-based median LET d deviations and global gamma parameters were evaluated. Results Dose deviations comparing baseRS and evaluation plans were within ± 1 % supported by γ-pass rates over 97 % for all cases. No significant LET d deviations were reported in the CTV, but significant median LET d deviations were up to 80 % for very low dose regions. Conclusion Our results showed improved accuracy of the predicted D RBE and LET d . Considering clinically relevant constraints, no significant modifications of clinical protocols are expected with the introduction of NIC + trichrome.