Monitoring of Hadrontherapy Treatments by Means of Charged Particle Detection

Feasibility study of 4D-online monitoring of density gradients induced by lung cancer treatment using carbon ions

Tumor motion is a major challenge for scanned ion-beam therapy. In the case of lung tumors, strong under- and overdosage can be induced due to the high density gradients between the tumor- and bone tissues compared to lung tissues. This work proposes a non-invasive concept for 4D monitoring of high density gradients in carbon ion beam therapy, by detecting charged fragments. The method implements CMOS particle trackers that are used to reconstruct the fragment vertices, which define the emission points of nuclear interactions between the primary carbon ions and the patient tissues. A 3D treatment plan was optimized to deliver 2 Gy to a static spherical target volume. The goodness of the method was assessed by comparing reconstructed vertices measured in two static cases to the ones in a non-compensated moving case with an amplitude of 20 mm. The measurements, performed at the Marburg Ion-Beam Therapy Center (MIT), showed promising results to assess the conformity of the delivered dose. In particular to measure overshoots induced by high density gradients due to motion with 83.0 ± 1.5% and 92.0 ± 1.5% reliability based on the ground truth provided by the time-resolved motor position and depending on the considered volume and the iso-energy layers.

Experimental validation of a FLUKA Monte Carlo simulation for carbon-ion radiotherapy monitoring via secondary ion tracking

BACKGROUND

In-vivo monitoring methods of carbon ion radiotherapy (CIRT) includes explorations of nuclear reaction products generated by carbon-ion beams interacting with patient tissues. Our research group focuses on in-vivo monitoring of CIRT using silicon pixel detectors. Currently, we are conducting a prospective clinical trial as part of the In-Vivo Monitoring project (InViMo) at the Heidelberg Ion Beam Therapy Center (HIT) in Germany. We are using an innovative, in-house developed, non-contact fragment tracking system with seven mini-trackers based on the Timepix3 technology developed at CERN.

PURPOSE

This article focuses on the implementation of the mini-tracker in Monte Carlo (MC) based on FLUKA simulations to monitor secondary charged nuclear fragments in CIRT. The main objective is to systematically evaluate the simulation accuracy for the InViMo project.

METHODS

The implementation involved integrating the mini-tracker geometry and the scoring mechanism into the FLUKA MC simulation, utilizing the finely tuned HIT beam line. The systematic investigation included varying mini-tracker angles (from15 ∘ $15^\circ$ to45 ∘ $45^\circ$ in5 ∘ $5^\circ$ steps) during the irradiation of a head-sized phantom with therapeutic carbon-ion pencil beams. To evaluate our implemented FLUKA framework, a comparison was made between the experimental data and data obtained from MC simulations. To ensure the fidelity of our comparison, experiments were performed at the HIT using the parameters and setup established in the simulations.

RESULTS

Our research demonstrates high accuracy in reproducing characteristic behaviors and dependencies of the monitoring method in terms of fragment distributions in the mini-tracker, track angles, emission profiles, and fragment numbers. Discrepancies in the number of detected fragments between the experimental data and the data obtained from MC simulations are less than 4% for the angles of interest in the InViMo detection system.

CONCLUSIONS

Our study confirms the potential of our simulation framework to investigate the performance of monitoring inter-fractional anatomical changes in patients undergoing CIRT using secondary nuclear charged fragments escaping from the irradiated patient.

Inhomogeneity detection within a head-sized phantom using tracking of charged nuclear fragments in ion beam therapy

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 2σfor 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 cm3and 1.26 cm3were narrowed down around the detectability threshold of this secondary-fragment-based monitoring method.

An in-vivo treatment monitoring system for ion-beam radiotherapy based on 28 Timepix3 detectors

Ion-beam radiotherapy is an advanced cancer treatment modality offering steep dose gradients and a high biological effectiveness. These gradients make the therapy vulnerable to patient-setup and anatomical changes between treatment fractions, which may go unnoticed. Charged fragments from nuclear interactions of the ion beam with the patient tissue may carry information about the treatment quality. Currently, the fragments escape the patient undetected. Inter-fractional in-vivo treatment monitoring based on these charged nuclear fragments could make ion-beam therapy safer and more efficient. We developed an ion-beam monitoring system based on 28 hybrid silicon pixel detectors (Timepix3) to measure the distribution of fragment origins in three dimensions. The system design choices as well as the ion-beam monitoring performance measurements are presented in this manuscript. A spatial resolution of 4 mm along the beam axis was achieved for the measurement of individual fragment origins. Beam-range shifts of1.5 mm were identified in a clinically realistic treatment scenario with an anthropomorphic head phantom. The monitoring system is currently being used in a prospective clinical trial at the Heidelberg Ion Beam Therapy Centre for head-and-neck as well as central nervous system cancer patients.

A novel experimental approach to characterize neutron fields at high- and low-energy particle accelerators

The characterization of particle accelerator induced neutron fields is challenging but fundamental for research and industrial activities, including radiation protection, neutron metrology, developments of neutron detectors for nuclear and high-energy physics, decommissioning of nuclear facilities, and studies of neutron damage on materials and electronic components. This work reports on the study of a novel approach to the experimental characterization of neutron spectra at two complex accelerator environments, namely the CERF, a high-energy mixed reference field at CERN in Geneva, and the Bern medical cyclotron laboratory, a facility used for multi-disciplinary research activities, and for commercial radioisotope production for nuclear medicine. Measurements were performed through an innovative active neutron spectrometer called DIAMON, a device developed to provide in real time neutron energy spectra without the need of guess distributions. The intercomparison of DIAMON measurements with reference data, Monte Carlo simulations, and with the well-established neutron monitor Berthold LB 6411, has been found to be highly satisfactory in all conditions. It was demonstrated that DIAMON is an almost unique device able to characterize neutron fields induced by hadrons at 120 GeV/c as well as by protons at 18 MeV colliding with different materials. The accurate measurement of neutron spectra at medical cyclotrons during routine radionuclide production for nuclear medicine applications is of paramount importance for the facility decommissioning. The findings of this work are the basis for establishing a methodology for producing controlled proton-induced neutron beams with medical cyclotrons.

Compton imaging for medical applications

Compton imaging exploits inelastic scattering, known as Compton scattering, using a Compton camera consisting of a scatterer detector in the front layer and an absorber detector in the back layer. This method was developed for astronomy, and in recent years, research and development for environmental and medical applications has been actively conducted. Compton imaging can discriminate gamma rays over a wide energy range from several hundred keV to several MeV. Therefore, it is expected to be applied to the simultaneous imaging of multiple nuclides in nuclear medicine and prompt gamma ray imaging for range verification in particle therapy. In addition, multiple gamma coincidence imaging is expected to be realized, which allows the source position to be determined from a single coincidence event using nuclides that emit multiple gamma rays simultaneously, such as nuclides that emit a single gamma ray simultaneously with positron decay. This review introduces various efforts toward the practical application of Compton imaging in the medical field, including in vivo studies, and discusses its prospects.

Ultrasound-assisted carbon ion dosimetry and range measurement using injectable polymer-shelled phase-change nanodroplets: in vitro study

Methods allowing for in situ dosimetry and range verification are essential in radiotherapy to reduce the safety margins required to account for uncertainties introduced in the entire treatment workflow. This study suggests a non-invasive dosimetry concept for carbon ion radiotherapy based on phase-change ultrasound contrast agents. Injectable nanodroplets made of a metastable perfluorobutane (PFB) liquid core, stabilized with a crosslinked poly(vinylalcohol) shell, are vaporized at physiological temperature when exposed to carbon ion radiation (C-ions), converting them into echogenic microbubbles. Nanodroplets, embedded in tissue-mimicking phantoms, are exposed at 37 °C to a 312 MeV/u clinical C-ions beam at different doses between 0.1 and 4 Gy. The evaluation of the contrast enhancement from ultrasound imaging of the phantoms, pre- and post-irradiation, reveals a significant radiation-triggered nanodroplets vaporization occurring at the C-ions Bragg peak with sub-millimeter shift reproducibility and dose dependency. The specific response of the nanodroplets to C-ions is further confirmed by varying the phantom position, the beam range, and by performing spread-out Bragg peak irradiation. The nanodroplets' response to C-ions is influenced by their concentration and is dose rate independent. These early findings show the ground-breaking potential of polymer-shelled PFB nanodroplets to enable in vivo carbon ion dosimetry and range verification.

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.

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

PURPOSE

Non-invasive 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 cm² , 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 (6 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 9 standard deviations, and up to 17 standard deviations respectively. The cavity could be detected at all depths and its position measured within 6.5 mm ± 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 non-invasive monitoring of carbon-ion treatments. This article is protected by copyright. All rights reserved.

Investigation of Suitable Detection Angles for Carbon-Ion Radiotherapy Monitoring in Depth by Means of Secondary-Ion Tracking

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 cm2 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.

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.

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.

Measurement of PET isotope production cross sections for protons and carbon ions on carbon and oxygen targets for applications in particle therapy range verification

Measured cross sections for the production of the PET isotopes [Formula: see text], [Formula: see text] and [Formula: see text] from carbon and oxygen targets induced by protons (40-220 [Formula: see text]) and carbon ions (65-430 [Formula: see text]) are presented. These data were obtained via activation measurements of irradiated graphite and beryllium oxide targets using a set of three scintillators coupled by a coincidence logic. The measured cross sections are relevant for the PET particle range verification method where accurate predictions of the [Formula: see text] emitter distribution produced by therapeutic beams in the patient tissue are required. The presented dataset is useful for validation and optimization of the nuclear reaction models within Monte Carlo transport codes. For protons the agreement of a radiation transport calculation using the measured cross sections with a thick target PET measurement is demonstrated.

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.

An overview of recent developments in FLUKA PET tools

The new developments of the FLUKA Positron-Emission-Tomography (PET) tools are detailed. FLUKA is a fully integrated Monte Carlo (MC) particle transport code, used for an extended range of applications, including Medical Physics. Recently, it provided the medical community with dedicated simulation tools for clinical applications, including the PET simulation package. PET is a well-established imaging technique in nuclear medicine, and a promising method for clinical in vivo treatment verification in hadrontherapy. The application of clinically established PET scanners to new irradiation environments such as hadrontherapy requires further experimental and theoretical research to which MC simulations could be applied. The FLUKA PET tools, besides featuring PET scanner models in its library, allow the configuration of new PET prototypes via the FLUKA Graphical User Interface (GUI) Flair. Both the beam time structure and scan time can be specified by the user, reproducing PET acquisitions in time, in a particle therapy scenario. Furthermore, different scoring routines allow the analysis of single and coincident events, and identification of parent isotopes generating annihilation events. Two reconstruction codes are currently supported: the Filtered Back-Projection (FBP) and Maximum-Likelihood Expectation Maximization (MLEM), the latter embedded in the tools. Compatibility with other reconstruction frameworks is also possible. The FLUKA PET tools package has been successfully tested for different detectors and scenarios, including conventional functional PET applications and in beam PET, either using radioactive sources, or simulating hadron beam irradiations. The results obtained so far confirm the FLUKA PET tools suitability to perform PET simulations in R&D environment.

Three dimensional reconstruction of therapeutic carbon ion beams in phantoms using single secondary ion tracks

Carbon ion beam radiotherapy enables a very localised dose deposition. However, even small changes in the patient geometry or positioning errors can significantly distort the dose distribution. A live, non-invasive monitoring system of the beam delivery within the patient is therefore highly desirable, and could improve patient treatment. We present a novel three-dimensional method for imaging the beam in the irradiated object, exploiting the measured tracks of single secondary ions emerging under irradiation. The secondary particle tracks are detected with a TimePix stack-a set of parallel pixelated semiconductor detectors. We developed a three-dimensional reconstruction algorithm based on maximum likelihood expectation maximization. We demonstrate the applicability of the new method in the irradiation of a cylindrical PMMA phantom of human head size with a carbon ion pencil beam of [Formula: see text] MeV u^-1. The beam image in the phantom is reconstructed from a set of nine discrete detector positions between [Formula: see text] and [Formula: see text] from the beam axis. Furthermore, we demonstrate the potential to visualize inhomogeneities by irradiating a PMMA phantom with an air gap as well as bone and adipose tissue surrogate inserts. We successfully reconstructed a three-dimensional image of the treatment beam in the phantom from single secondary ion tracks. The beam image corresponds well to the beam direction and energy. In addition, cylindrical inhomogeneities with a diameter of [Formula: see text] cm and density differences down to [Formula: see text] g cm^-3 to the surrounding material are clearly visualized. This novel three-dimensional method to image a therapeutic carbon ion beam in the irradiated object does not interfere with the treatment and requires knowledge only of single secondary ion tracks. Even with detectors with only a small angular coverage, the three-dimensional reconstruction of the fragmentation points presented in this work was found to be feasible.