A prototype system for fast clinical treatment with carbon ions using a 3D range modulator

Carbon-ion radiotherapy provides highly conformal dose distributions and enhanced biological effectiveness; however, energy-switching delays in the multi-energy beam scanning system remain a challenge for fast dose delivery to minimize the interplay effect for moving tumors, and ultra-fast dose delivery to trigger the FLASH effect in clinical treatment.

This study presents a novel treatment workflow based on patient-specific 3D range modulators (3DRMs) that enables fast, single-energy carbon-ion treatments while maintaining conformal dose distributions and demonstrating feasibility for clinical application.

We developed and validated a patient-specific 3DRM workflow in the TRiP treatment planning system that converts a conventionally optimized multi-layer virtual range shifter plan into a single-energy delivery. Multiple Coulomb scattering introduced by the modulator material was quantified via the Highland formula and incorporated into beam modeling, spot optimization, and dose calculation. The workflow was demonstrated for a non-small cell lung cancer (NSCLC) case using three patient-specific 3DRMs with modulator-to-target distances (MTD) of 50, 70, and 100 cm and a prescription dose of 3 Gy(RBE) delivered in a single fraction. The impact of MTD to plan quality was assessed using dose-volume histogram (DVH) metrics for the planning target volume (PTV) and relevant organs at risk (OARs). The 3DRMs were manufactured using high-precision 3D printing. The workflow was validated by cross-comparison of phantom-based dose distributions obtained from TRiP calculations, FLUKA Monte Carlo simulations, and experimental measurements.

By integrating the 3DRM into the beamline, all 3DRM-based plans achieved full target coverage using a single energy layer, the irradiation times were reduced to 25-33 s per field, corresponding to 4 synchrotron cycles - significantly faster than conventional beam scanning, which requires approximately 2-5 min. Increasing MTDs resulted in predictable lateral penumbra broadening and increased low- and medium-dose exposure to surrounding lung tissue; however, all plans maintained OAR doses within clinical constraints and good target coverage, with PTV V95 values exceeding 99% and inhomogeneity coefficients (IC) of approximately 0.05. Good agreement (γ_global(2%/2 mm, 10% dose threshold) = 91%-99%) was observed between treatment planning calculations, FLUKA simulations, and experimental measurements in phantom-based validations.

This study demonstrates that patient-specific 3DRMs enable precise and efficient fast carbon-ion treatments. The proposed workflow maintains high dosimetric quality while reducing irradiation time, providing a scalable and clinically feasible pathway toward motion-robust and FLASH-compatible carbon-ion radiotherapy.