The dose conformation and the sparing of neighboring critical healthy structures are improved in carbon-ion beam radiotherapy in comparison to conventional photon radiotherapy. Inter and intrafractional plan adaptation strategies may preclude the quality assurance (QA) of the actually applied treatment plan before the treatment starts. Therefore, independent measurements of the positions of scanned pencil 12C ion beams are of interest in order to monitor the beam application during the treatment and the beam in the isocenter. In this work, secondary ions outgoing from a patient-like phantom are exploited for the assessment of the lateral pencil beam position in a clinic-like 12C treatment fraction. The experiment was performed at the Heidelberg Ion-Beam Therapy Center (HIT) in Germany. A carbon-ion treatment plan was used to treat a 100 cm3 tumor volume in the center of an Alderson head phantom. Two silicon pixel detectors based on the Timepix3 technology developed at CERN were operated in synchronization to detect and to track outgoing secondary ions. We established an analysis of the measured secondary ion track distribution which enabled us to follow the beam scanning movement of the carbon-ion pencil beam by assessing the lateral position of the single beam spots. The precision of the developed method was found to range from 0.84 mm to 2.59 mm. For beam energies greater than 197.58 MeV/n, the mean of absolute distances of the measured lateral pencil beam positions with respect to the pencil beam positions measured by the beam application system (averaged over each energy layer) were smaller than 2 mm. We conclude that the presented method has shown capabilities of monitoring the lateral pencil beam positions by means of secondary ions with precision and sensitivity of clinical interest.

Purpose Ion beam radiotherapy offers enhances dose conformity to the tumor volume while better sparing healthy tissue compared to conventional photon radiotherapy. However, the increased dose gradient also makes it more sensitive to uncertainties. While the most important uncertainty source is the patient itself, the beam delivery is also subject to uncertainties. Most of the proton therapy centers used cyclotrons, which deliver typically a stable beam over time, allowing a continuous extraction of the beam. Carbon‐ion beam radiotherapy (CIRT) in contrast uses synchrotrons and requires a larger and energy‐dependent extrapolation of the nozzle‐measured positions to obtain the lateral beam positions in the isocenter, since the nozzle‐to‐isocenter distance is larger than for cyclotrons. Hence, the control of lateral pencil beam positions at isocenter in CIRT is more sensitive to uncertainties than in proton radiotherapy. Therefore, an independent monitoring of the actual lateral positions close to the isocenter would be very valuable and provide additional information. However, techniques capable to do so are scarce, and they are limited in precision, accuracy and effectivity. Methods The detection of secondary ions (charged nuclear fragments) has previously been exploited for the Bragg peak position of C‐ion beams. In our previous work, we investigated for the first time the feasibility of lateral position monitoring of pencil beams in CIRT. However, the reported precision and accuracy were not sufficient for a potential implementation into clinical practice. In this work, it is shown how the performance of the method is improved to the point of clinical relevance. To minimize the observed uncertainties, a mini‐tracker based on hybrid silicon pixel detectors was repositioned downstream of an anthropomorphic head phantom. However, the secondary‐ion fluence rate in the mini‐tracker rises up to 1.5 × 105 ions/s/cm2, causing strong pile‐up of secondary‐ion signals. To solve this problem, we performed hardware changes, optimized the detector settings, adjusted the setup geometry and developed new algorithms to resolve ambiguities in the track reconstruction. The performance of the method was studied on two treatment plans delivered with a realistic dose of 3 Gy (RBE) and averaged dose rate of 0.27 Gy/s at the Heidelberg Ion‐Beam Therapy Center (HIT) in Germany. The measured lateral positions were compared to reference beam positions obtained either from the beam nozzle or from a multi‐wire proportional chamber positioned at the room isocenter. Results The presented method is capable to simultaneously monitor both lateral pencil beam coordinates over the entire tumor volume during the treatment delivery, using only a 2‐cm2 mini‐tracker. The effectivity (defined as the fraction of analyzed pencil beams) was 100%. The reached precision of (0.6 to 1.5) mm and accuracy of (0.5 to 1.2) mm are in line with the clinically accepted uncertainty for QA measurements of the lateral pencil beam positions. Conclusions It was demonstra...