Dosimetric and LETd comparison of horizontal port and gantry carbon-ion radiotherapy for early-stage non–small cell lung cancer: an exploratory study

Purpose: We aimed to evaluate the feasibility of using a horizontal port (HP) system for carbon ion radiotherapy (CIRT) in non-small cell lung cancer (NSCLC) by comparing the dosimetric characteristics and dose-averaged linear energy transfer (LETd) with those of a gantry-based plan. Materials and Methods: Five patients with NSCLC treated with CIRT were included in this study. Both gantry-based and HP plans were generated using a single-field uniform dose optimization in RayStation 2024A with a pencil beam algorithm. Gantry plan beam angles followed institutional clinical protocols, whereas HP plans used beam angles of 90° or 270° with a couch roll of ±15° and yaw of ±30°. Plan evaluation was conducted using dose-volume histograms, LETd-volume histograms, and beam range analysis. Results: Dose distributions for the clinical target volume (CTV) and ipsilateral lung were similar between plans. The CTV V95% was 100% in both plans. For the ipsilateral lung, mean V5 Gy (relative biological effectiveness [RBE]) and V20 Gy (RBE) were slightly lower in the gantry plan (12.40% and 5.77%, respectively, compared to 13.54% and 6.15%, respectively). Mean LETd98% within the CTV was higher in the gantry plan (65.27 keV/μm compared to 57.07 keV/μm). Beam energy and beam range were consistently lower and shorter, respectively, in the gantry plan. Conclusion: The preliminary dosimetric performance of the HP plan was generally comparable to that of the gantry plan. However, these findings should be interpreted with caution. The gantry system may provide greater reliability and clinical efficiency owing to the simpler workflow and reduced sensitivity to setup variability and organ motion.