Real-time energy measurement of clinical carbon ion beams using a cross-correlation time-of-flight method with parallel-plate chambers

Abstract

Background In carbon-ion radiotherapy (CIRT), the beam energy determines both the particle range and the overall dosimetric quality. Range-verification QA devices such as Zebra and Giraffe, which are based on multilayer ionization chambers (MLICs), can verify the range but only under dedicated QA conditions, leaving any energy deviations introduced by nozzle components undetected in real time. In particular, nozzle structures such as ridge filters can broaden or modulate the energy spectrum, causing the effective energy delivered to the patient to differ from the nominal accelerator setting. These limitations highlight the need for a real-time method capable of verifying the beam energy under actual clinical operating conditions.Purpose We proposed a TOF-based beam-energy measurement concept that leverages a cross-correlation analysis of full detector waveforms. Compact and radiation-hard parallel-plate chambers (PPCs) were developed and evaluated, in contrast to prior TOF systems based on semiconductor detectors.Methods PPCs (2.5 cm diameter active area, 0.4 mm gas gap) were operated in CO2. Two detectors were mounted coaxially with detector separations of 22.5 and 46.3 cm. Experiments were performed at Yonsei Heavy-ion Therapy Center (HITC) using four nominal energies (102.6, 140.4, 250.3, 430 MeV/nucleon) and three intensities, covering the clinically interesting ranges. Signals were digitized with a 1 GHz bandwidth oscilloscope. For each spill, paired waveforms were cross-correlated, and peak times were refined by parabolic interpolation to determine TOF. Precision and accuracy were evaluated across energies, intensities, and detector separations.Results The PPCs operated stably for all beam conditions. Under pencil-beam delivery and normalized to 1 s acquisitions, the timing precision of the mean TOF (standard error) remained within 1 ps for both detector separations, scaling with 1/N$1/\sqrt N $ (N: number of TOF samples per acquisition) and not representing the single-particle TOF resolution. Residuals between measured and theoretical TOF remained within 80 ps across energies and distances. After relativistic conversion from TOF to kinetic energy and then to water-equivalent range, all deviations were within a 1 mm range shift, meeting the recommended clinical criteria for range verification.Conclusions We demonstrated that compact CO2-filled PPCs, operated as a TOF pair, can measure carbon-ion beam energy across the clinically relevant range of energies (approximate to 100-430 MeV/u) and intensities used in routine treatment delivery. We achieved sub-picosecond timing precision on the TOF mean (standard error) per 1 s acquisition and submillimeter water-equivalent range accuracy using a robust cross-correlation analysis method. These results open the way to the integration of PPC-based TOF monitoring to tighten beam-delivery tolerances and improve the reliability and safety of carbon-ion radiotherapy.