Secondary radiation measurements for particle therapy applications: charged particles produced by 4He and 12C ion beams in a PMMA target at large angle

A simulation study of in-beam visualization system for proton therapy by monitoring scattered protons

Recently, in-beam positron emission tomography (PET) has been actively researched for reducing biological washout effects and dose monitoring during irradiation. However, the positron distribution does not precisely reflect the dose distribution since positron production and ionization are completely different physical processes. Thus, a novel in-beam system was proposed to determine proton dose range by measuring scattered protons with dozens of scintillation detectors surrounding the body surface. While previous studies conducted a preliminary experiment with a simple phantom, we simulated more complex situations in this paper. Especially, we conducted three stepwise simulation studies to demonstrate the feasibility of the proposed method. First, a simple rectangular phantom was reproduced on simulation and irradiated with protons for obtaining current values and Monte Carlo (MC) dose. Next, we trained a deep learning model to estimate 2-dimensional-dose range (2D-DL dose) from measured current values for simulation (A). We simulated plastic scintillators as detectors to measure the scattered protons. Second, a rectangular phantom with an air layer was used, and 3D-DL dose was estimated in simulation (B). Finally, a cylindrical phantom that mimics the human body was used for confirming the estimation quality of the simulation (C). Consequently, the position of the Bragg peak was estimated with an error of 1.0 mm in simulation (A). In addition, the position of the air layer, as well as the verifying peak position with an error of 2.1 mm, was successfully estimated in simulation (B). Although the estimation error of the peak position was 12.6 mm in simulation (C), the quality was successfully further improved to 9.3 mm by incorporating the mass density distribution obtained from the computed tomography (CT). These simulation results demonstrated the potential of the as-proposed verification system. Additionally, the effectiveness of CT utilization for estimating the DL dose was also indicated.

Roadmap: helium ion therapy

Helium ion beam therapy for the treatment of cancer was one of several developed and studied particle treatments in the 1950s, leading to clinical trials beginning in 1975 at the Lawrence Berkeley National Laboratory. The trial shutdown was followed by decades of research and clinical silence on the topic while proton and carbon ion therapy made debuts at research facilities and academic hospitals worldwide. The lack of progression in understanding the principle facets of helium ion beam therapy in terms of physics, biological and clinical findings persists today, mainly attributable to its highly limited availability. Despite this major setback, there is an increasing focus on evaluating and establishing clinical and research programs using helium ion beams, with both therapy and imaging initiatives to supplement the clinical palette of radiotherapy in the treatment of aggressive disease and sensitive clinical cases. Moreover, due its intermediate physical and radio-biological properties between proton and carbon ion beams, helium ions may provide a streamlined economic steppingstone towards an era of widespread use of different particle species in light and heavy ion therapy. With respect to the clinical proton beams, helium ions exhibit superior physical properties such as reduced lateral scattering and range straggling with higher relative biological effectiveness (RBE) and dose-weighted linear energy transfer (LET_d) ranging from ∼4 keVμm^-1to ∼40 keVμm^-1. In the frame of heavy ion therapy using carbon, oxygen or neon ions, where LET_dincreases beyond 100 keVμm^-1, helium ions exhibit similar physical attributes such as a sharp lateral penumbra, however, with reduced radio-biological uncertainties and without potentially spoiling dose distributions due to excess fragmentation of heavier ion beams, particularly for higher penetration depths. This roadmap presents an overview of the current state-of-the-art and future directions of helium ion therapy: understanding physics and improving modeling, understanding biology and improving modeling, imaging techniques using helium ions and refining and establishing clinical approaches and aims from learned experience with protons. These topics are organized and presented into three main sections, outlining current and future tasks in establishing clinical and research programs using helium ion beams-A. Physics B. Biological and C. Clinical Perspectives.

Management of Motion and Anatomical Variations in Charged Particle Therapy: Past, Present, and Into the Future

The major aim of radiation therapy is to provide curative or palliative treatment to cancerous malignancies while minimizing damage to healthy tissues. Charged particle radiotherapy utilizing carbon ions or protons is uniquely suited for this task due to its ability to achieve highly conformal dose distributions around the tumor volume. For these treatment modalities, uncertainties in the localization of patient anatomy due to inter- and intra-fractional motion present a heightened risk of undesired dose delivery. A diverse range of mitigation strategies have been developed and clinically implemented in various disease sites to monitor and correct for patient motion, but much work remains. This review provides an overview of current clinical practices for inter and intra-fractional motion management in charged particle therapy, including motion control, current imaging and motion tracking modalities, as well as treatment planning and delivery techniques. We also cover progress to date on emerging technologies including particle-based radiography imaging, novel treatment delivery methods such as tumor tracking and FLASH, and artificial intelligence and discuss their potential impact towards improving or increasing the challenge of motion mitigation in charged particle therapy.

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

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 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.

Monitoring Carbon Ion Beams Transverse Position Detecting Charged Secondary Fragments: Results From Patient Treatment Performed at CNAO

Particle therapy in which deep seated tumours are treated using ¹²C ions (Carbon Ions RadioTherapy or CIRT) exploits the high conformity in the dose release, the high relative biological effectiveness and low oxygen enhancement ratio of such projectiles. The advantages of CIRT are driving a rapid increase in the number of centres that are trying to implement such technique. To fully profit from the ballistic precision achievable in delivering the dose to the target volume an online range verification system would be needed, but currently missing. The ¹²C ions beams range could only be monitored by looking at the secondary radiation emitted by the primary beam interaction with the patient tissues and no technical solution capable of the needed precision has been adopted in the clinical centres yet. The detection of charged secondary fragments, mainly protons, emitted by the patient is a promising approach, and is currently being explored in clinical trials at CNAO. Charged particles are easy to detect and can be back-tracked to the emission point with high efficiency in an almost background-free environment. These fragments are the product of projectiles fragmentation, and are hence mainly produced along the beam path inside the patient. This experimental signature can be used to monitor the beam position in the plane orthogonal to its flight direction, providing an online feedback to the beam transverse position monitor chambers used in the clinical centres. This information could be used to cross-check, validate and calibrate, whenever needed, the information provided by the ion chambers already implemented in most clinical centres as beam control detectors. In this paper we study the feasibility of such strategy in the clinical routine, analysing the data collected during the clinical trial performed at the CNAO facility on patients treated using ¹²C ions and monitored using the Dose Profiler (DP) detector developed within the INSIDE project. On the basis of the data collected monitoring three patients, the technique potential and limitations will be discussed.

Feasibility of quasi-prompt PET-based range verification in proton therapy

Compared to photon therapy, proton therapy allows a better conformation of the dose to the tumor volume with reduced radiation dose to co-irradiated tissues. In vivo verification techniques including positron emission tomography (PET) have been proposed as quality assurance tools to mitigate proton range uncertainties. Detection of differences between planned and actual dose delivery on a short timescale provides a fast trigger for corrective actions. Conventional PET-based imaging of ¹⁵O (T_1/2 = 2 min) and ¹¹C (T_1/2 = 20 min) distributions precludes such immediate feedback. We here present a demonstration of near real-time range verification by means of PET imaging of ¹²N (T_1/2 = 11 ms). PMMA and graphite targets were irradiated with a 150 MeV proton pencil beam consisting of a series of pulses of 10 ms beam-on and 90 ms beam-off. Two modules of a modified Siemens Biograph mCT PET scanner (21 × 21 cm² each), installed 25 cm apart, were used to image the beam-induced PET activity during the beam-off periods. The modifications enable the detectors to be switched off during the beam-on periods. ¹²N images were reconstructed using planar tomography. Using a 1D projection of the 2D reconstructed ¹²N image, the activity range was obtained from a fit of the activity profile with a sigmoid function. Range shifts due to modified target configurations were assessed for multiples of the clinically relevant 10⁸ protons per pulse (approximately equal to the highest intensity spots in the pencil beam scanning delivery of a dose of 1 Gy over a cubic 1 l volume). The standard deviation of the activity range, determined from 30 datasets obtained from three irradiations on PMMA and graphite targets, was found to be 2.5 and 2.6 mm (1σ) with 10⁸ protons per pulse and 0.9 and 0.8 mm (1σ) with 10⁹ protons per pulse. Analytical extrapolation of the results from this study shows that using a scanner with a solid angle coverage of 57%, with optimized detector switching and spot delivery times much smaller than the ¹²N half-life, an activity range measurement precision of 2.0 mm (1σ) and 1.3 mm (1σ) within 50 ms into an irradiation with 4 × 10⁷ and 10⁸ protons per pencil beam spot can be potentially realized. Aggregated imaging of neighboring spots or, if possible, increasing the number of protons for a few probe beam spots will enable the realization of higher precision range measurement.

Inter-fractional monitoring of [Formula: see text]C ions treatments: results from a clinical trial at the CNAO facility

The high dose conformity and healthy tissue sparing achievable in Particle Therapy when using C ions calls for safety factors in treatment planning, to prevent the tumor under-dosage related to the possible occurrence of inter-fractional morphological changes during a treatment. This limitation could be overcome by a range monitor, still missing in clinical routine, capable of providing on-line feedback. The Dose Profiler (DP) is a detector developed within the INnovative Solution for In-beam Dosimetry in hadronthErapy (INSIDE) collaboration for the monitoring of carbon ion treatments at the CNAO facility (Centro Nazionale di Adroterapia Oncologica) exploiting the detection of charged secondary fragments that escape from the patient. The DP capability to detect inter-fractional changes is demonstrated by comparing the obtained fragment emission maps in different fractions of the treatments enrolled in the first ever clinical trial of such a monitoring system, performed at CNAO. The case of a CNAO patient that underwent a significant morphological change is presented in detail, focusing on the implications that can be drawn for the achievable inter-fractional monitoring DP sensitivity in real clinical conditions. The results have been cross-checked against a simulation study.

The production of positron emitters with millisecond half-life during helium beam radiotherapy

Therapy with helium ions is currently receiving significantly increasing interest because helium ions have a sharper penumbra than protons and undergo less fragmentation than carbon ions and thus require less complicated dose calculations. For any ion of interest in hadron therapy, the accuracy of dose delivery is limited by range uncertainties. This has led to efforts by several groups to develop in vivo verification techniques, including positron emission tomography (PET), for monitoring of the dose delivery. Beam-on PET monitoring during proton therapy through the detection of short-lived positron emitters such as ¹²N (T _1/2  =  11 ms), an emerging PET technique, provides an attractive option given the achievable range accuracy, minimal susceptibility to biological washout and provision of near prompt feedback. Extension of this approach to helium ions requires information on the production yield of relevant short-lived positron emitters. This study presents the first measurements of the production of short-lived positron emitters in water, graphite, calcium and phosphorus targets irradiated with 59 MeV/u ³He and 50 MeV/u ⁴He beams. For these targets, the most produced short-lived nuclides are ¹³O/¹²N (T _1/2  =  8.6/11 ms) on water, ¹³O/¹²N on graphite, ⁴³Ti/⁴¹Sc/⁴²Sc (T _1/2  =  509-680 ms) on calcium, ²⁸P (T _1/2  =  268 ms) on phosphorus. A translation of the results from elemental targets to PMMA and representative tissues such as adipose tissue, muscle, compact and cortical bone, shows the dominance of ¹³O/¹²N in at least the first 20 s of an irradiation with ⁴He and somewhat longer with ³He. As the production of ¹³O/¹²N in a ³He irradiation is 3-4 times higher than in a ⁴He irradiation, from a statistical point of view, range verification using ¹³O/¹²N PET imaging will be about 2 times more precise for a ³He irradiation compared to a ⁴He irradiation.

Secondary radiation measurements for particle therapy applications: Charged secondaries produced by 16O ion beams in a PMMA target at large angles

Particle therapy is a therapy technique that exploits protons or light ions to irradiate tumor targets with high accuracy. Protons and ¹²C ions are already used for irradiation in clinical routine, while new ions like ⁴He and ¹⁶O are currently being considered. Despite the indisputable physical and biological advantages of such ion beams, the planning of charged particle therapy treatments is challenged by range uncertainties, i.e. the uncertainty on the position of the maximal dose release (Bragg Peak - BP), during the treatment. To ensure correct 'in-treatment' dose deposition, range monitoring techniques, currently missing in light ion treatment techniques, are eagerly needed. The results presented in this manuscript indicate that charged secondary particles, mainly protons, produced by an ¹⁶O beam during target irradiation can be considered as candidates for ¹⁶O beam range monitoring. Hereafter, we report on the first yield measurements of protons, deuterons and tritons produced in the interaction of an ¹⁶O beam impinging on a PMMA target, as a function of detected energy and particle production position. Charged particles were detected at 90° and 60° with respect to incoming beam direction, and homogeneous and heterogeneous PMMA targets were used to probe the sensitivity of the technique to target inhomogeneities. The reported secondary particle yields provide essential information needed to assess the accuracy and resolution achievable in clinical conditions by range monitoring techniques based on secondary charged radiation.

Review and performance of the Dose Profiler, a particle therapy treatments online monitor

Particle therapy (PT) can exploit heavy ions (such as He, C or O) to enhance the treatment efficacy, profiting from the increased Relative Biological Effectiveness and Oxygen Enhancement Ratio of these projectiles with respect to proton beams. To maximise the gain in tumor control probability a precise online monitoring of the dose release is needed, avoiding unnecessary large safety margins surroundings the tumor volume accounting for possible patient mispositioning or morphological changes with respect to the initial CT scan. The Dose Profiler (DP) detector, presented in this manuscript, is a scintillating fibres tracker of charged secondary particles (mainly protons) that will be operating during the treatment, allowing for an online range monitoring. Such monitoring technique is particularly promising in the context of heavy ions PT, in which the precision achievable by other techniques based on secondary photons detection is limited by the environmental background during the beam delivery. Developed and built at the SBAI department of "La Sapienza", within the INSIDE collaboration and as part of a Centro Fermi flagship project, the DP is a tracker detector specifically designed and planned for clinical applications inside a PT treatment room. The DP operation in clinical like conditions has been tested with the proton and carbon ions beams of Trento proton-therapy center and of the CNAO facility. In this contribution the detector performances are presented, in the context of the carbon ions monitoring clinical trial that is about to start at the CNAO centre.