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Objective. The highly conformal carbon-ion radiotherapy is associated with an increased sensitivity of the dose distributions to internal changes in the patient during the treatment course. Hence, monitoring methodologies capable of detecting such changes are of vital importance. We established experimental setup conditions to address the sensitivity of a monitoring approach based on secondary-fragment tracking for detecting clinically motivated air cavity dimensions in a homogeneous head-sized PMMA phantom in 40 mm depth. Approach. The air cavities were positioned within the entrance channel of a treatment field of 50 mm diameter at three lateral positions. The measured secondary-fragment emission profiles were compared to a reference measurement without cavities. The experiments were conducted at the Heidelberg Ion-Beam Therapy Center in Germany at typical doses and dose rates. Main results. Significances above a detectability threshold of 2 σ for the larger cavities (20 mm diameter and 4 mm thickness, and 20 mm diameter and 2 mm thickness) across the entire treatment field. The smallest cavity of 10 mm diameter and 2 mm thickness, which is on the lower limit of clinical interest, could not be detected at any position. We also demonstrated that it is feasible to reconstruct the lateral position of the cavity on average within 2.8 mm, once the cavity is detected. This is sufficient for the clinicians to estimate medical effects of such a cavity and to decide about the need for a control imaging CT. Significance. This investigation defines well-controlled reference conditions for the evaluation of the performance of any kind of treatment monitoring method and its capability to detect internal changes within head-sized objects. Four air cavities with volumes between 0.31 cm 3 and 1.26 cm 3 were narrowed down around the detectability threshold of this secondary-fragment-based monitoring method., (© 2024 Institute of Physics and Engineering in Medicine. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

Objective. There is an increasing interest in calculating and measuring linear energy transfer (LET) spectra in particle therapy in order to assess their impact in biological terms. As such, the accuracy of the particle fluence energy spectra becomes paramount. This study focuses on quantifying energy depositions of distinct proton, helium, carbon, and oxygen ion beams using a silicon pixel detector developed at CERN to determine LET spectra in silicon. Approach. While detection systems have been investigated in this pursuit, the scarcity of detectors capable of providing per-ion data with high spatial and temporal resolution remains an issue. This gap is where silicon pixel detector technology steps in, enabling online tracking of single-ion energy deposition. The used detector consisted of a 300 µ m thick silicon sensor operated in partial depletion. Main results. During post-processing, artifacts in the acquired signals were identified and methods for their corrections were developed. Subsequently, a correlation between measured and Monte Carlo-based simulated energy deposition distributions was performed, relying on a two-step recalibration approach based on linear and saturating exponential models. Despite the observed saturation effects, deviations were confined below 7% across the entire investigated range of track-averaged LET values in silicon from 0.77 keV µ m -1 to 93.16 keV µ m -1 . Significance. Simulated and measured mean energy depositions were found to be aligned within 7%, after applying artifact corrections. This extends the range of accessible LET spectra in silicon to clinically relevant values and validates the accuracy and reliability of the measurements. These findings pave the way towards LET-based dosimetry through an approach to translate these measurements to LET spectra in water. This will be addressed in a future study, extending functionality of treatment planning systems into clinical routine, with the potential of providing ion-beam therapy of utmost precision to cancer patients., (Creative Commons Attribution license.)

Purpose: Noninvasive methods to monitor carbon-ion beams in patients are desired to fully exploit the advantages of carbon-ion radiotherapy. Prompt secondary ions produced in nuclear fragmentations of carbon ions are of particular interest for monitoring purposes as they can escape the patient and thus be detected and tracked to measure the radiation field in the irradiated object. This study aims to evaluate the performance of secondary-ion tracking to detect, visualize, and localize an internal air cavity used to mimic inter-fractional changes in the patient anatomy at different depths along the beam axis., Methods: In this work, a homogeneous head phantom was irradiated with a realistic carbon-ion treatment plan with a typical prescribed fraction dose of 3 Gy(RBE). Secondary ions were detected by a mini-tracker with an active area of 2 cm 2 , based on the Timepix3 semiconductor pixel detector technology. The mini-tracker was placed 120 mm behind the center of the target at an angle of 30 degrees with respect to the beam axis. To assess the performance of the developed method, a 2-mm thick air cavity was inserted in the head phantom at several depths: in front of as well as at the entrance, in the middle, and at the distal end of the target volume. Different reconstruction methods of secondary-ion emission profile were studied using the FLUKA Monte Carlo simulation package. The perturbations in the emission profiles caused by the air cavity were analyzed to detect the presence of the air cavity and localize its position., Results: The perturbations in the radiation field mimicked by the 2-mm thick cavity were found to be significant. A detection significance of at least three standard deviations in terms of spatial distribution of the measured tracks was found for all investigated cavity depths, while the highest significance (six standard deviations) was obtained when the cavity was located upstream of the tumor. For a tracker with an eight-fold sensitive area, the detection significance rose to at least nine standard deviations and up to 17 standard deviations, respectively. The cavity could be detected at all depths and its position measured within 6.5 ± 1.4 mm, which is sufficient for the targeted clinical performance of 10 mm., Conclusion: The presented systematic study concerning the detection and localization of small inter-fractional structure changes in a realistic clinical setting demonstrates that secondary ions carry a large amount of information on the internal structure of the irradiated object and are thus attractive to be further studied for noninvasive monitoring of carbon-ion treatments., (© 2022 The Authors. Medical Physics published by Wiley Periodicals LLC on behalf of American Association of Physicists in Medicine.)

The dose conformity of carbon-ion beam radiotherapy, which allows the reduction of the dose deposition in healthy tissue and the escalation of the dose to the tumor, is associated with a high sensitivity to anatomical changes during and between treatment irradiations. Thus, the monitoring of inter-fractional anatomical changes is crucial to ensure the dose conformity, to potentially reduce the size of the safety margins around the tumor and ultimately to reduce the irradiation of healthy tissue. To do so, monitoring methods of carbon-ion radiotherapy in depth using secondary-ion tracking are being investigated. In this work, the detection and localization of a small air cavity of 2 mm thickness were investigated at different detection angles of the mini-tracker relative to the beam axis. The experiments were conducted with a PMMA head phantom at the Heidelberg Ion-Beam Therapy Center (HIT) in Germany. In a clinic-like irradiation of a single field of 3 Gy (RBE), secondary-ion emission profiles were measured by a 2 cm 2 mini-tracker composed of two silicon pixel detectors. Two positions of the cavity in the head phantom were studied: in front and in the middle of the tumor volume. The significance of the cavity detection was found to be increased at smaller detection angles, while the accuracy of the cavity localization was improved at larger detection angles. Detection angles of 20° - 30° were found to be a good compromise for accessing both, the detectability and the position of the air cavity along the depth in the head of a patient., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Ghesquière-Diérickx, Schlechter, Félix-Bautista, Gehrke, Echner, Kelleter and Martišíková.)

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 × 10 5 ions/s/cm 2 , 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-cm 2 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 demonstrated that the performance of the method for a non-invasive lateral position monitoring of pencil beams is sufficient for a potential clinical implementation. The next step is to evaluate the method clinically in a group of patients in a future observational clinical study., (© 2021 American Association of Physicists in Medicine.)