Multi-angle beam range measurement framework for carbon-ion radiotherapy using a commercial multi-layer ionization chamber

Accurate beam range verification is critical in carbon-ion radiotherapy, where the sharp Bragg peak and complex material-dependent nuclear interactions pose greater measurement challenges compared to proton therapy; however, conventional water phantom methods for frequent and comprehensive checks required for routine quality assurance (QA) are extremely time-consuming. This study proposes and validates a framework using a commercial multi-layer ionization chamber (MLIC) to enable efficient, accurate, and comprehensive range verification. The framework integrates three key innovations: (1) a beam-model-based fitting method that uses reference integral depth-dose curves to reduce range determination uncertainty for carbon ions, (2) a time-optimized, trigger-free acquisition process, and (3) a custom gantry-compatible mount for stable multi-angle measurements. The framework's measurement uncertainty was evaluated via range-shift experiments, and its clinical feasibility was tested through year-long stability and multi-angle consistency measurements. The proposed method demonstrated a low range-determination uncertainty, with an expanded uncertainty of 0.24 mm (k = 3), and showed excellent agreement with standard water phantom measurements (mean deviation of 0.03 ± 0.15 mm). The framework reduced the measurement time for all 600 energy levels from >50 h to approximately 15 min, representing >100-fold improvement. Clinical validation confirmed high stability, with long-term and multi-angle deviations of 0.17 mm and 0.05 mm (k = 1), respectively. The system also successfully identified machine-related beam range inconsistencies that were not detectable using conventional QA protocols. The proposed MLIC-based framework provides a reliable, accurate, and highly efficient solution for routine beam range QA in carbon-ion therapy. Its successful clinical application demonstrates its feasibility for frequent and comprehensive verification across all energy levels and gantry angles.