Quality assurance method for monitoring of lateral pencil beam positions in scanned carbon‐ion radiotherapy using tracking of secondary ions

Abstract

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...

Detecting perturbations of a radiation field inside a head‐sized phantom exposed to therapeutic carbon‐ion beams through charged‐fragment tracking

“…Pixels having a ToA difference greater than or equal to 150 ns were considered to come from different secondary‐ion hits and thus to correspond to different clusters. This clustering time window was chosen to account for the collection time of the charge generated by a particle impinging on the sensor as detailed in Félix‐Bautista et al 14 Subsequently, within a group of pixels with a similar ToA, pixels were clustered using a minimal spatial distance with a nearest‐neighbor algorithm.…”

“…The two modules of the mini-tracker were connected via a synchronization cable, as described in Félix-Bautista et al 13,14 where this mini-tracker was used for the first time.…”

“…Recent results showed that with this method,the lateral pencil beam positions of the scanned carbon-ion beam can be monitored during a full realistic carbon-ion treatment fraction, delivered to an anthropomorphic head phantom. 13,14 In contrast to semiconductor tracking technology, the INSIDE collaboration based their tracking technology on scintillating fibers. 15 They reported the first clinical results for CIRT monitoring using prompt secondary ions 16 where inter-fractional changes in anatomy in one of three patients could be identified in the measured secondary-ion emission profiles and traced back to the emptying of a paranasal sinus cavity.…”

“…However, a new generation of the Timepix technology, the Timepix3 detector, now enables a practically dead‐time‐free data acquisition and is an attractive candidate to be studied in clinical conditions. Recent results showed that with this method, the lateral pencil beam positions of the scanned carbon‐ion beam can be monitored during a full realistic carbon‐ion treatment fraction, delivered to an anthropomorphic head phantom 13,14 . In contrast to semiconductor tracking technology, the INSIDE collaboration based their tracking technology on scintillating fibers 15 .…”