Fragmentation of 120 and 200 MeV u−14He ions in water and PMMA targets

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.

Feasibility study of a proton CT system based on 4D-tracking and residual energy determination via time-of-flight

Objective.For dose calculations in ion beam therapy, it is vital to accurately determine the relative stopping power (RSP) distribution within the treatment volume. A suitable imaging modality to achieve the required RSP accuracy is proton computed tomography (pCT), which usually uses a tracking system and a separate residual energy (or range) detector to directly measure the RSP distribution. This work investigates the potential of a novel pCT system based on a single detector technology, namely low gain avalanche detectors (LGADs). LGADs are fast 4D-tracking detectors, which can be used to simultaneously measure the particle position and time with precise timing and spatial resolution. In contrast to standard pCT systems, the residual energy is determined via a time-of-flight (TOF) measurement between different 4D-tracking stations.Approach.To show the potential of using 4D-tracking for proton imaging, we studied and optimized the design parameters for a realistic TOF-pCT system using Monte Carlo simulations. We calculated the RSP accuracy and RSP resolution inside the inserts of the CTP404 phantom and compared the results to a simulation of an ideal pCT system.Main results.After introducing a dedicated calibration procedure for the TOF calorimeter, RSP accuracies less than 0.6% could be achieved. We also identified the design parameters with the strongest impact on the RSP resolution and proposed a strategy to further improve the image quality.Significance.This comprehensive study of the most important design aspects for a novel TOF-pCT system could help guide future hardware developments and, once implemented, improve the quality of treatment planning in ion beam therapy.

Relative biological effectiveness of single and split helium ion doses in the rat spinal cord increases strongly with linear energy transfer

BACKGROUND AND PURPOSE

Determination of the relative biological effectiveness (RBE) of helium ions as a function of linear energy transfer (LET) for single and split doses using the rat cervical spinal cord as model system for late-responding normal tissue.

MATERIAL AND METHODS

The rat cervical spinal cord was irradiated at four different positions within a 6 cm spread-out Bragg-peak (SOBP) (LET 2.9, 9.4, 14.4 and 20.7 keV/µm) using increasing levels of single or split doses of helium ions. Dose-response curves were determined and based on TD₅₀-values (dose at 50% effect probability using paresis II as endpoint), RBE-values were derived for the endpoint of radiation-induced myelopathy.

RESULTS

With increasing LET, RBE-values increased from 1.13 ± 0.04 to 1.42 ± 0.05 (single dose) and 1.12 ± 0.03 to 1.50 ± 0.04 (split doses) as TD₅₀-values decreased from 21.7 ± 0.3 Gy to 17.3 ± 0.3 Gy (single dose) and 30.6 ± 0.3 Gy to 22.9 ± 0.3 Gy (split doses), respectively. RBE-models (LEM I and IV, mMKM) deviated differently for single and split doses but described the RBE variation in the high-LET region sufficiently accurate.

CONCLUSION

This study established the LET-dependence of the RBE for late effects in the central nervous system after single and split doses of helium ions. The results extend the existing database for protons and carbon ions and allow systematic testing of RBE-models. While the RBE-values of helium were generally lower than for carbon ions, the increase at the distal edge of the Bragg-peak was larger than for protons, making detailed RBE-modeling necessary.

Physical characterization of3He ion beams for radiotherapy and comparison with4He

There is increasing interest in using helium ions for radiotherapy, complementary to protons and carbon ions. A large number of patients were treated with⁴He ions in the US heavy ion therapy project and novel⁴He ion treatment programs are under preparation, for instance in Germany and Japan.³He ions have been proposed as an alternative to⁴He ions because the acceleration of³He is technically less difficult than⁴He. In particular, beam contaminations have been pointed out as a potential safety issue for⁴He ion beams. This motivated a series of experiments with³He ion beams at Gesellschaft für Schwerionenforschung (GSI), Darmstadt. Measured³He Bragg curves and fragmentation data in water are presented in this work. Those experimental data are compared with FLUKA Monte Carlo simulations. The physical characteristics of³He ion beams are compared to those of⁴He, for which a large set of data became available in recent years from the preparation work at the Heidelberger Ionenstrahl-Therapiezentrum (HIT). The dose distributions (spread out Bragg peaks, lateral profiles) that can be achieved with³He ions are found to be competitive to⁴He dose distributions. The effect of beam contaminations on⁴He depth dose distribution is also addressed. It is concluded that³He ions can be a viable alternative to⁴He, especially for future compact therapy accelerator designs and upgrades of existing ion therapy facilities.

Helium radiography with a digital tracking calorimeter-a Monte Carlo study for secondary track rejection

Radiation therapy using protons and heavier ions is a fast-growing therapeutic option for cancer patients. A clinical system for particle imaging in particle therapy would enable online patient position verification, estimation of the dose deposition through range monitoring and a reduction of uncertainties in the calculation of the relative stopping power of the patient. Several prototype imaging modalities offer radiography and computed tomography using protons and heavy ions. A Digital Tracking Calorimeter (DTC), currently under development, has been proposed as one such detector. In the DTC 43 longitudinal layers of laterally stacked ALPIDE CMOS monolithic active pixel sensor chips are able to reconstruct a large number of simultaneously recorded proton tracks. In this study, we explored the capability of the DTC for helium imaging which offers favorable spatial resolution over proton imaging. Helium ions exhibit a larger cross section for inelastic nuclear interactions, increasing the number of produced secondaries in the imaged object and in the detector itself. To that end, a filtering process able to remove a large fraction of the secondaries was identified, and the track reconstruction process was adapted for helium ions. By filtering on the energy loss along the tracks, on the incoming angle and on the particle ranges, 97.5% of the secondaries were removed. After passing through 16 cm water, 50.0% of the primary helium ions survived; after the proposed filtering 42.4% of the primaries remained; finally after subsequent image reconstruction 31% of the primaries remained. Helium track reconstruction leads to more track matching errors compared to protons due to the increased available focus strength of the helium beam. In a head phantom radiograph, the Water Equivalent Path Length error envelope was 1.0 mm for helium and 1.1 mm for protons. This accuracy is expected to be sufficient for helium imaging for pre-treatment verification purposes.

Characterization of the Mixed Radiation Field Produced by Carbon and Oxygen Ion Beams of Therapeutic Energy: A Monte Carlo Simulation Study

Purpose:

The main advantages of charged particle radiotherapy compared to conventional X-ray external beam radiotherapy are a better tumor conformality coupled with the capability of treating deep-seated radio-resistant tumors. This work investigates the possibility to use oxygen beams for hadron therapy, as an alternative to carbon ions.

Materials and Methods:

Oxygen ions have the advantage of a higher relative biological effectiveness (RBE) and better conformality to the tumor target. This work describes the mixed radiation field produced by an oxygen beam in water and compares it to the one produced by a therapeutic carbon ion beam. The study has been performed using Geant4 simulations. The dose is calculated for incident carbon ions with energies of 162 MeV/u and 290 MeV/u, and oxygen ions with energies of 192 MeV/u and 245 MeV/u, and hence that the range of the primary oxygen ions projectiles in water was located at the same depth as the carbon ions.

Results:

The results show that the benefits of oxygen ions are more pronounced when using lower energies because of a slightly higher peak-to-entrance ratio, which allows either providing higher dose in tumor target or reducing it in the surrounding healthy tissues. It is observed that, per incident particle, oxygen ions deliver higher doses than carbon ions.

Conclusions:

This result coupled with the higher RBE shows that it may be possible to use a lower fluence of oxygen ions to achieve the same therapeutic dose in the patient as that obtained with carbon ion therapy.

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.

Biophysical modeling and experimental validation of relative biological effectiveness (RBE) for 4He ion beam therapy

BACKGROUND

Helium (4He) ion beam therapy provides favorable biophysical characteristics compared to currently administered particle therapies, i.e., reduced lateral scattering and enhanced biological damage to deep-seated tumors like heavier ions, while simultaneously lessened particle fragmentation in distal healthy tissues as observed with lighter protons. Despite these biophysical advantages, raster-scanning 4He ion therapy remains poorly explored e.g., clinical translational is hampered by the lack of reliable and robust estimation of physical and radiobiological uncertainties. Therefore, prior to the upcoming 4He ion therapy program at the Heidelberg Ion-beam Therapy Center (HIT), we aimed to characterize the biophysical phenomena of 4He ion beams and various aspects of the associated models for clinical integration.

METHODS

Characterization of biological effect for 4He ion beams was performed in both homogenous and patient-like treatment scenarios using innovative models for estimation of relative biological effectiveness (RBE) in silico and their experimental validation using clonogenic cell survival as the gold-standard surrogate. Towards translation of RBE models in patients, the first GPU-based treatment planning system (non-commercial) for raster-scanning 4He ion beams was devised in-house (FRoG).

RESULTS

Our data indicate clinically relevant uncertainty of ±5-10% across different model simulations, highlighting their distinct biological and computational methodologies. The in vitro surrogate for highly radio-resistant tissues presented large RBE variability and uncertainty within the clinical dose range.

CONCLUSIONS

Existing phenomenological and mechanistic/biophysical models were successfully integrated and validated in both Monte Carlo and GPU-accelerated analytical platforms against in vitro experiments, and tested using pristine peaks and clinical fields in highly radio-resistant tissues where models exhibit the greatest RBE uncertainty. Together, these efforts mark an important step towards clinical translation of raster-scanning 4He ion beam therapy to the clinic.

Report of a National Cancer Institute special panel: Characterization of the physical parameters of particle beams for biological research

PURPOSE

To define the physical parameters needed to characterize a particle beam in order to allow intercomparison of different experiments performed using different ions at the same facility and using the same ion at different facilities.

METHODS

At the request of the National Cancer Institute (NCI), a special panel was convened to review the current status of the field and to provide suggested metrics for reporting the physical parameters of particle beams to be used for biological research. A set of physical parameters and measurements that should be performed by facilities and understood and reported by researchers supported by NCI to perform pre-clinical radiobiology and medical physics of heavy ions were generated.

RESULTS

Standard measures such as radiation delivery technique, beam modifiers used, nominal energy, field size, physical dose and dose rate should all be reported. However, more advanced physical measurements, including detailed characterization of beam quality by microdosimetric spectrum and fragmentation spectra, should also be established and reported. Details regarding how such data should be incorporated into Monte Carlo simulations and the proper reporting of simulation details are also discussed.

CONCLUSIONS

In order to allow for a clear relation of physical parameters to biological effects, facilities and researchers should establish and report detailed physical characteristics of the irradiation beams utilized including both standard and advanced measures. Biological researchers are encouraged to actively engage facility staff and physicists in the design and conduct of experiments. Modeling individual experimental setups will allow for the reporting of the uncertainties in the measurement or calculation of physical parameters which should be routinely reported.

The impact of secondary fragments on the image quality of helium ion imaging

Single-event ion imaging enables the direct reconstruction of the relative stopping power (RSP) information required for ion-beam therapy. Helium ions were recently hypothesized to be the optimal species for such technique. The purpose of this work is to investigate the effect of secondary fragments on the image quality of helium CT (HeCT) and to assess the performance of a prototype proton CT (pCT) scanner when operated with helium beams in Monte Carlo simulations and experiment. Experiments were conducted installing the U.S. pCT consortium prototype scanner at the Heidelberg Ion-Beam Therapy Center (HIT). Simulations were performed with the scanner using the TOPAS toolkit. HeCT images were reconstructed for a cylindrical water phantom, the CTP404 (sensitometry), and the CTP528 (line-pair) [Formula: see text] ® modules. To identify and remove individual events caused by fragmentation, the multistage energy detector of the scanner was adapted to function as a [Formula: see text] telescope. The use of the developed filter eliminated the otherwise arising ring artifacts in the HeCT reconstructed images. For the HeCT reconstructed images of a water phantom, the maximum RSP error was improved by almost a factor 8 with respect to unfiltered images in the simulation and a factor 10 in the experiment. Similarly, for the CTP404 module, the mean RSP accuracy improved by a factor 6 in both the simulation and the experiment when the filter was applied (mean relative error 0.40% in simulation, 0.45% in experiment). In the evaluation of the spatial resolution through the CTP528 module, the main effect of the filter was noise reduction. For both simulated and experimental images the spatial resolution was  ∼4 lp cm-1. In conclusion, the novel filter developed for secondary fragments proved to be effective in improving the visual quality and RSP accuracy of the reconstructed images. With the filter, the pCT scanner is capable of accurate HeCT imaging.

A novel method for assessment of fragmentation and beam-material interactions in helium ion radiotherapy with a miniaturized setup

Radiotherapy with protons and carbon ions enables to deliver dose distributions of high conformation to the target. Treatment with helium ions has been suggested due to their physical and biological advantages. A reliable benchmarking of the employed physics models with experimental data is required for treatment planning. However, experimental data for helium interactions is limited, in part due to the complexity and large size of conventional experimental setups. We present a novel method for the investigation of helium interactions with matter using miniaturized instrumentation based on highly integrated pixel detectors. The versatile setup consisted of a monitoring detector in front of the PMMA phantom of varying thickness and a detector stack for investigation of outgoing particles. The ion type downstream from the phantom was determined by high-resolution pattern recognition analysis of the single particle signals in the pixelated detectors. The fractions of helium and hydrogen ions behind the used targets were determined. As expected for the stable helium nucleus, only a minor decrease of the primary ion fluence along the target depth was found. E.g. the detected fraction of hydrogen ions on axis of a 220MeV/u ⁴He beam was below 6% behind 24.5cm of PMMA. Monte-Carlo simulations using Geant4 reproduce the experimental data on helium attenuation and yield of helium fragments qualitatively, but significant deviations were found for some combinations of target thickness and beam energy. The presented method is promising to contribute to the reduction of the uncertainty of treatment planning for helium ion radiotherapy.

On the lateral dose profile of 4He beams in water

PURPOSE

We investigate the possibility to improve the accuracy of the lateral dose profile for ⁴He beams with a novel approach, by extending an already validated model for proton beams to heavier ions.

METHODS

The full Molière theory for the Coulomb multiple scattering is applied to the case of ⁴He beams, with a complete separation of the electromagnetic and of the nuclear contributions in the calculation of the total dose. The latter is described with only three free parameters.

RESULTS

The accuracy of the results compared with Monte Carlo predictions already validated with experimental data is comparable with other studies at low energy, but improves by a factor 2 at high energy. In addition the found solution is more stable with respect to (multi-) Gaussian and other parameterizations. This result makes this method of interest for applications to Treatment Planning Systems (TPS) in ion beam therapy.

CONCLUSIONS

We propose a model, named MONETα (MOdel of ioN dosE for Therapy for α), for the calculation of the lateral dose of ⁴He beams in water that allows fast and accurate dose calculations by requiring a small data base of parameters as input.

A Novel Method for Fragmentation Studies in Particle Therapy: Principles of Ion Identification

Purpose

In carbon ion beam radiation therapy, fragmentation processes within the patient lead to changes in the composition of the particle field with increasing depth. Consequences are alterations of the resulting dose distribution and its biological effectiveness. To enable accurate treatment planning, the characteristics of the ion spectra resulting from fragmentation processes need to be known for various ion energies and target materials. In this work, we present a novel method for ion type identification using a small and highly flexible setup based on a single detector and designed to simplify measurements and overcome current shortages in available fragmentation data.

Materials and Methods

The presented approach is based on the pixelated, semiconductor detector Timepix. The large number of pixels with small pitch, all individually calibrated for energy deposition, enables detection and visualization of single particle tracks. For discrimination among different ion species, the pattern recognition analysis of the detector signal is used. Fragmentation spectra resulting from a primary carbon ion beam at various depths of tissue-equivalent material were studied to identify different ion species in mixed particle fields. The performance of the method was evaluated quantitatively using reference data from an established technique.

Results

All ion species resulting from carbon ion fragmentation in tissue-equivalent material could be separated. For measurements behind a 158-mm-thick water tank, the relative fractions of H, He, Be, and B ions detected agreed with corresponding reference data within the limits of uncertainty. For the relatively rare lithium ions, the agreement was within 2.3 Δ_ref (uncertainty of reference).

Conclusion

For designated configurations, the presented ion type identification method enables studies of therapeutic carbon ion beams with a simple, small, and configurable detection setup. The technique is promising to enable online fragmentation studies over a wide range of beam and target parameters in the future.