European developments in radiotherapy with beams of large radiobiological effectiveness

Heavy-Ion Microbeams for Biological Science: Development of System and Utilization for Biological Experiments in QST-Takasaki

Target irradiation of biological material with a heavy-ion microbeam is a useful means to analyze the mechanisms underlying the effects of heavy-ion irradiation on cells and individuals. At QST-Takasaki, there are two heavy-ion microbeam systems, one using beam collimation and the other beam focusing. They are installed on the vertical beam lines of the azimuthally-varying-field cyclotron of the TIARA facility for analyzing heavy-ion radiation effects on biological samples. The collimating heavy-ion microbeam system is used in a wide range of biological research not only in regard to cultured cells but also small individuals, such as silkworms, nematode C. elegans, and medaka fish. The focusing microbeam system was designed and developed to perform more precise target irradiation that cannot be achieved through collimation. This review describes recent updates of the collimating heavy ion microbeam system and the research performed using it. In addition, a brief outline of the focusing microbeam system and current development status is described.

Commissioning of a fluoroscopic-based real-time markerless tumor tracking system in a superconducting rotating gantry for carbon-ion pencil beam scanning treatment

PURPOSE

To perform the final quality assurance of our fluoroscopic-based markerless tumor tracking for gated carbon-ion pencil beam scanning (C-PBS) radiotherapy using a rotating gantry system, we evaluated the geometrical accuracy and tumor tracking accuracy using a moving chest phantom with simulated respiration.

METHODS

The positions of the dynamic flat panel detector (DFPD) and x-ray tube are subject to changes due to gantry sag. To compensate for this, we generated a geometrical calibration table (gantry flex map) in 15° gantry angle steps by the bundle adjustment method. We evaluated five metrics: (a) Geometrical calibration was evaluated by calculating chest phantom positional error using 2D/3D registration software for each 5° step of the gantry angle. (b) Moving phantom displacement accuracy was measured (±10 mm in 1-mm steps) with a laser sensor. (c) Tracking accuracy was evaluated with machine learning (ML) and multi-template matching (MTM) algorithms, which used fluoroscopic images and digitally reconstructed radiographic (DRR) images as training data. The chest phantom was continuously moved ±10 mm in a sinusoidal path with a moving cycle of 4 s and respiration was simulated with ±5 mm expansion/contraction with a cycle of 2 s. This was performed with the gantry angle set at 0°, 45°, 120°, and 240°. (d) Four types of interlock function were evaluated: tumor velocity, DFPD image brightness variation, tracking anomaly detection, and tracking positional inconsistency in between the two corresponding rays. (e) Gate on/off latency, gating control system latency, and beam irradiation latency were measured using a laser sensor and an oscilloscope.

RESULTS

By applying the gantry flex map, phantom positional accuracy was improved from 1.03 mm/0.33° to <0.45 mm/0.27° for all gantry angles. The moving phantom displacement error was 0.1 mm. Due to long computation time, the tracking accuracy achieved with ML was <0.49 mm (=95% confidence interval [CI]) for imaging rates of 15 and 7.5 fps; those at 30 fps were decreased to 1.84 mm (95% CI: 1.79 mm-1.92 mm). The tracking positional accuracy with MTM was <0.52 mm (=95% CI) for all gantry angles and imaging frame rates. The tumor velocity interlock signal delay time was 44.7 ms (=1.3 frame). DFPD image brightness interlock latency was 34 ms (=1.0 frame). The tracking positional error was improved from 2.27 ± 2.67 mm to 0.25 ± 0.24 mm by the tracking anomaly detection interlock function. Tracking positional inconsistency interlock signal was output within 5.0 ms. The gate on/off latency was <82.7 ± 7.6 ms. The gating control system latency was <3.1 ± 1.0 ms. The beam irradiation latency was <8.7 ± 1.2 ms.

CONCLUSIONS

Our markerless tracking system is now ready for clinical use. We hope to shorten the computation time needed by the ML algorithm at 30 fps in the future.

Thermomechanical effects caused by heavy ions propagating in tissue

The thermomechanical effects caused by ions propagating in tissue are discussed. Large energy densities in small regions surrounding ion paths cause shock waves propagating in tissue. The strength of the shock waves depends on the linear energy transfer. Molecular dynamics simulations help in determining the necessary strength of shock waves in order for the stresses caused by them to directly produce DNA strand breaks. At much smaller values of linear energy transfer, the shock waves may be instrumental in propagating reactive species formed close to the ion's path to large distances, successfully competing with diffusion.

History of hadron therapy accelerators

In the last 60 years, hadron therapy has made great advances passing from a stage of pure research to a well-established treatment modality for solid tumours. In this paper the history of hadron therapy accelerators is reviewed, starting from the first cyclotrons used in the thirties for neutron therapy and passing to more modern and flexible machines used nowadays. The technical developments have been accompanied by clinical studies that allowed the selection of the tumours which are more sensitive to this type of radiotherapy. This paper aims at giving a review of the origin and the present status of hadron therapy accelerators, describing the technological basis and the continuous development of this application to medicine of instruments developed for fundamental science. At the end the present challenges are reviewed.

Measurement of neutron spectra generated from bombardment of 4 to 24 MeV protons on a thick ⁹Be target and estimation of neutron yields

A systematic study on the measurement of neutron spectra emitted from the interaction of protons of various energies with a thick beryllium target has been carried out. The measurements were carried out in the forward direction (at 0° with respect to the direction of protons) using CR-39 detectors. The doses were estimated using the in-house image analyzing program autoTRAK_n, which works on the principle of luminosity variation in and around the track boundaries. A total of six different proton energies starting from 4 MeV to 24 MeV with an energy gap of 4 MeV were chosen for the study of the neutron yields and the estimation of doses. Nearly, 92% of the recoil tracks developed after chemical etching were circular in nature, but the size distributions of the recoil tracks were not found to be linearly dependent on the projectile energy. The neutron yield and dose values were found to be increasing linearly with increasing projectile energies. The response of CR-39 detector was also investigated at different beam currents at two different proton energies. A linear increase of neutron yield with beam current was observed.

ENLIGHT: The European Network for Light Ion Hadron Therapy

The European Network for Light Ion Hadron Therapy (ENLIGHT) was established in 2002 to coordinate European efforts on hadron therapy (radiotherapy performed with protons and light ions instead of high-energy photons). The ENLIGHT network is formed by the European Hadron Therapy Community, with more than 300 participants from 20 different countries. A major success of ENLIGHT has been uniting traditionally separate communities so that clinicians, physicists, biologists, and engineers with experience and interest in particle therapy work together. ENLIGHT has been a successful initiative in forming a common European platform and bringing together people from diverse disciplines. ENLIGHT demonstrates the advantages of regular and organized exchanges of data, information, and best practices, as well as determining and following strategies for future needs in research and technological development in the hadron therapy field.

Medical Applications of Accelerators

Particle accelerators play an essential role in the field of medical applications. A large variety of systems is in use for diagnostic purposes, such as the production of radioactive tracers for imaging or x-ray radiography. The dominant application, however, is related to the treatment of cancer patients. This article puts emphasis on cancer treatment, presenting the status and developments of the corresponding technical systems, and gives a brief overview of the biophysical properties and medical aspects of these treatments.

Comparison of basic features of proton and helium ion pencil beams in water using GATE

PURPOSE

The aim of this study was to investigate the basic features of helium ions for their possible application in advanced radiotherapy and to benchmark them against protons, the current particle of choice in the low linear energy transfer (LET) range.

MATERIAL AND METHODS

Geant4 Application for Emission Tomography (GATE) simulations were performed with beams of 1x10(7) monodirectional particles traversing a water phantom. Initial energies ranged from 50 to 250 MeV per nucleon (MeV/A). The following parameters were evaluated: particle range at the distal 80% of maximum energy deposition (E(max)), width of the Bragg peak (BP) at 60% of E(max), and dose fall-off width between 80% and 20% of E(max) for longitudinal spectra. In addition the fragmentation tail was quantified in terms of length, percental energy deposition, and contributing particles. For each energy lateral profiles were registered along the beam axis and the FWHM at four different depths was extracted. Besides the comparison of parameters between the two particle types, results were also compared to data in the literature.

RESULTS

As expected, the position of the BP as a function of initial kinetic energy showed similar values for protons and helium ions, with deviations smaller than 1.3%. The quantitative results of the Monte Carlo (MC) study showed less range straggling effects and smaller lateral deflections for helium ions compared to protons for the investigated energy range. On average, an about 56% reduction of the width of the BP and a 48% reduction of the dose fall-off was observed for helium ions compared to protons. Both the width of the BP and the dose fall-off width as a function of particle range or energy showed an almost linear increase with increasing energy. The tail length increased from 55.9 mm to 592.7 mm and the deposited energy increased from 0.5% to 7.3% for energies between 90 and 250 MeV/A. Lateral profiles of helium ions were about 52% narrower than those of protons.

CONCLUSIONS

Due to their mass and charge helium ions distinguish themselves from protons in reduced range straggling effects, smaller lateral deflections, and a fragmentation tail. The MC based comprehensive data set for 21 clinically relevant energies can be used to create look-up tables for semi-analytical pencil beam model for helium ions.

Calculation of complex DNA damage induced by ions

This paper is devoted to the analysis of the complex damage of DNA irradiated by ions. The assessment of complex damage is important because cells in which it occurs are less likely to survive because the DNA repair mechanisms may not be sufficiently effective. We study the flux of secondary electrons through the surface of nucleosomes and calculate the radial dose and the distribution of clustered damage around the ion's path. The calculated radial dose distribution is compared to simulations. The radial distribution of the complex damage is found to be different from that of the dose. A comparison with experiments may solve the question of what is more lethal for the cell, damage complexity or absorbed energy. We suggest a way to calculate the probability of cell death based on the complexity of the damage. This work is done within the framework of the phenomenon-based multiscale approach to radiation damage by ions.

Dosimetry for ion beam radiotherapy

Recently, ion beam radiotherapy (including protons as well as heavier ions) gained considerable interest. Although ion beam radiotherapy requires dose prescription in terms of iso-effective dose (referring to an iso-effective photon dose), absorbed dose is still required as an operative quantity to control beam delivery, to characterize the beam dosimetrically and to verify dose delivery. This paper reviews current methods and standards to determine absorbed dose to water in ion beam radiotherapy, including (i) the detectors used to measure absorbed dose, (ii) dosimetry under reference conditions and (iii) dosimetry under non-reference conditions. Due to the LET dependence of the response of films and solid-state detectors, dosimetric measurements are mostly based on ion chambers. While a primary standard for ion beam radiotherapy still remains to be established, ion chamber dosimetry under reference conditions is based on similar protocols as for photons and electrons although the involved uncertainty is larger than for photon beams. For non-reference conditions, dose measurements in tissue-equivalent materials may also be necessary. Regarding the atomic numbers of the composites of tissue-equivalent phantoms, special requirements have to be fulfilled for ion beams. Methods for calibrating the beam monitor depend on whether passive or active beam delivery techniques are used. QA measurements are comparable to conventional radiotherapy; however, dose verification is usually single field rather than treatment plan based. Dose verification for active beam delivery techniques requires the use of multi-channel dosimetry systems to check the compliance of measured and calculated dose for a representative sample of measurement points. Although methods for ion beam dosimetry have been established, there is still room for developments. This includes improvement of the dosimetric accuracy as well as development of more efficient measurement techniques.

Respiratory motion management in particle therapy

Clinical outcomes of charged particle therapy are very promising. Currently, several dedicated centers that use scanning beam technology are either close to clinical use or under construction. Since scanned beam treatments of targets that move with respiration most likely result in marked local over- and underdosage due to interplay of target motion and dynamic beam application, dedicated motion mitigation techniques have to be employed. To date, the motion mitigation techniques, rescanning, beam gating, and beam tracking, have been proposed and tested in experimental studies. Rescanning relies on repeated irradiations of the target with the number of particles reduced accordingly per scan to statistically average local misdosage. Specific developments to prohibit temporal correlation between beam scanning and target motion will be required to guarantee adequate averaging. For beam gating, residual target motion within gating windows has to be mitigated in order to avoid local misdosage. Possibly the most promising strategy is to increase the overlap of adjacent particle pencil beams laterally as well as longitudinally to effectively reduce the sensitivity against small residual target motion. The most conformal and potentially most precise motion mitigation technique is beam tracking. Individual particle pencil beams have to be adapted laterally as well as longitudinally according to the target motion. Within the next several years, it can be anticipated that rescanning as well as beam gating will be ready for clinical use. For rescanning, treatment planning margins that incorporate the full extent of target motion as well as motion induced density variations in the beam paths will result in reduced target conformity of the applied dose distributions. Due to the limited precision of motion monitoring devices, it seems likely that beam gating will be used initially to mitigate interplay effects only but not to considerably decrease treatment planning margins. Then, in the next step, beam gating, based on more accurate motion monitoring systems, provides the possibility to restore target conformity as well as steep dose gradients due to reduced treatment planning margins. Accurate motion monitoring systems will be required for beam tracking. Even though beam tracking has already been successfully tested experimentally, full clinical implementation requires direct feedback of the actual target position in quasireal time to the treatment control system and can be anticipated to be several more years ahead.

Complex exchanges are responsible for the increased effectiveness of C-ions compared to X-rays at the first post-irradiation mitosis

The purpose of the present study was to investigate as to what extent differences in the linear energy transfer (LET) are reflected at the chromosomal level. For this study human lymphocytes were exposed to 9.5 MeV/u C-ions (1 or 2 Gy, LET=175 keV/microm) or X-rays (1-6 Gy), harvested at 48, 72 or 96 h post-irradiation and aberrations were scored in first cycle metaphases using 24 color fluorescence in situ hybridization (mFISH). Additionally, in selected samples aberrations were measured in prematurely condensed G2-phase cells. Analysis of the time-course of aberrations in first cycle metaphases showed a stable yield of simple and complex exchanges after X-ray irradiation. In contrast, after C-ion exposure the yields profoundly increased with harvesting time complicating the estimation of the frequency of aberrations produced by high LET particles within the entire cell population. This is especially true for the yield of complex exchanges. Complex aberrations dominate the aberration spectrum produced by C-ions. Their fraction was about 50% for the two measured doses. In contrast, isodoses of X-rays induced smaller proportions of complex aberrations (i.e. 5% and 15%, respectively). For both radiation qualities the fraction of complexes did not change with harvesting time. As expected from the different dose deposition of high and low LET radiation, complex exchanges produced by high LET C-ions involved more breaks and more chromosomes than those induced by isodoses of X-rays. Noteworthy, C-ions but not X-rays induced a small number of complex chromatid-isochromatid exchanges that are not expected for cells exposed in the G0-phase. The results obtained so far for cells arrested in G2-phase confirm these patterns. Altogether our data show that the increased effectiveness of C-ions for the induction of aberrations in first cycle cells is determined by complex exchanges, whereas for simple exchanges the relative biological effectiveness is about one.

Review on heavy ion radiotherapy facilities and related ion sources (invited)

Heavy ion radiotherapy awakens worldwide interest recently. The clinical results obtained by the Heavy Ion Medical Accelerator in Chiba at the National Institute of Radiological Sciences in Japan have clearly demonstrated the advantages of carbon ion radiotherapy. Presently, there are four facilities for heavy ion radiotherapy in operation, and several new facilities are under construction or being planned. The most common requests for ion sources are a long lifetime and good stability and reproducibility. Sufficient intensity has been achieved by electron cyclotron resonance ion sources at the present facilities.

Response of human hematopoietic stem and progenitor cells to energetic carbon ions

PURPOSE

To characterise the radiation response of human hematopoietic stem and progenitor cells (HSPC) with respect to X and carbon ion irradiation.

MATERIALS AND METHODS

HSPC from peripheral blood of healthy donors treated with granulocyte-colony stimulating factor (G-CSF) were enriched for the transmembrane glycoprotein CD34 (cluster of differentiation) and irradiated with X rays or carbon ions (29 keV/microm monoenergetic beam and 60-85 keV/microm spread-out Bragg peak), mimicking radiotherapy conditions. Apoptotic cell death, cell cycle progression and the frequency of chromosomal aberrations were determined.

RESULTS

After radiation exposure no inhibition in the progression of the cell cycle was detected. However, an enhanced frequency of apoptotic cells and an increase in aberrant cells were observed, both effects being more pronounced for carbon ions than X rays, resulting in a relative biological effectiveness (RBE) of 1.4-1.7. The fraction of complex-type aberrations was higher following carbon ion exposure.

CONCLUSIONS

RBE values of carbon ions are low, as expected for radiosensitive cells. The observed frequencies of apoptotic cells and chromosome aberrations in HSPC are similar to those reported for human peripheral blood lymphocytes suggesting that at least with respect to apoptosis and chromosomal aberrations mature lymphocytes reflect the respective radiation responses of their proliferating progenitors.

Modeling radiation-induced cell cycle delays

Ionizing radiation is known to delay the cell cycle progression. In particular after particle exposure significant delays have been observed and it has been shown that the extent of delay affects the expression of damage, such as chromosome aberrations. Thus, to predict how cells respond to ionizing radiation and to derive reliable estimates of radiation risks, information about radiation-induced cell cycle perturbations is required. In the present study we describe and apply a method for retrieval of information about the time-course of all cell cycle phases from experimental data on the mitotic index only. We study the progression of mammalian cells through the cell cycle after exposure. The analysis reveals a prolonged block of damaged cells in the G2 phase. Furthermore, by performing an error analysis on simulated data valuable information for the design of experimental studies has been obtained. The analysis showed that the number of cells analyzed in an experimental sample should be at least 100 to obtain a relative error <20%.

Temperature and pressure spikes in ion-beam cancer therapy

The inelastic thermal spike model is applied to liquid water in relation to high-energy 12C6+ beams (hundreds of MeV/u) used for cancer therapy. The goal of this project is to calculate the heat transfer in the vicinity of the incident-ion track. Thermal spike calculations indicate a very large temperature increase in the vicinity of ion tracks near the Bragg peak during the time interval from 10(-15) to 10(-9) s after the ion's passage and an increase in pressure, as large as tens of MPa, can be induced during that time. These effects suggest a possibility of thermomechanical pathways to disruption of irradiated DNA. An extension of the model for hydrogen, beryllium, argon, krypton, xenon, and uranium ions around the Bragg peak is presented as well.

Ion beam radiobiology and cancer: time to update ourselves

High-energy protons and carbon ions exhibit an inverse dose profile allowing for increased energy deposition with penetration depth. Additionally, heavier ions like carbon beams have the advantage of a markedly increased biological effectiveness characterized by enhanced ionization density in the individual tracks of the heavy particles, where DNA damage becomes clustered and therefore more difficult to repair, but is restricted to the end of their range. These superior biophysical and biological profiles of particle beams over conventional radiotherapy permit more precise dose localization and make them highly attractive for treating anatomically complex and radioresistant malignant tumors but without increasing the severe side effects in the normal tissue. More than half a century since Wilson proposed their use in cancer therapy, the effects of particle beams have been extensively investigated and the biological complexity of particle beam irradiation begins to unfold itself. The goal of this review is to provide an as comprehensive and up-to-date summary as possible of the different radiobiological aspects of particle beams for effective application in cancer treatment.

Differential effects of irradiation with carbon ions and x-rays on macrophage function

Macrophages are potent elicitors of inflammatory reactions that can play both positive and negative roles in radiotherapy. While several studies have investigated the effects of X-rays or gamma-rays on macrophages, virtually no work has been done on the responses of these cells to irradiation with carbon ions. Investigations into the effects of carbon ion irradiation are of particular interest in light of the fact that this type of radiation is being used increasingly for cancer therapy. In the present investigation we compared the effects of 250 kV X-rays with those of 9.8 MeV/u carbon ions on RAW 264.7 macrophages over a wide range of radiation doses. Macrophage functions including vitality, phagocytic activity, production of the proinflammatory cytokines IL-1beta and TNFalpha and production of nitric oxide (NO) were measured. In comparison to lymphocytes and fibroblasts, macrophages showed only a small decrease in vitality after irradiation with either X-rays or carbon ions. Proinflammatory cytokines and NO were induced in macrophages by LPS but not by irradiation alone. X-rays or carbon ions had little modulating effect on LPS-induced TNFalpha production. However, LPS-induced NO increased in a dose dependent manner up to 6-fold after carbon ion irradiation, while X-ray irradiation did not have this effect. Carbon ion irradiation mediated a concomitant decrease in IL-1beta production. Carbon ions also had a greater effect than X-rays in enhancing the phagocytic activity of macrophages. These results underscore the greater potential of carbon ion irradiation with regard to radiobiological effectiveness.

Effectiveness of monoenergetic and spread-out bragg peak carbon-ions for inactivation of various normal and tumour human cell lines

This work aimed at measuring cell-killing effectiveness of monoenergetic and Spread-Out Bragg Peak (SOBP) carbon-ion beams in normal and tumour cells with different radiation sensitivity. Clonogenic survival was assayed in normal and tumour human cell lines exhibiting different radiosensitivity to X- or gamma-rays following exposure to monoenergetic carbon-ion beams (incident LET 13-303 keV/microm) and at various positions along the ionization curve of a therapeutic carbon-ion beam, corresponding to three dose-averaged LET (LET(d)) values (40, 50 and 75 keV/microm). Chinese hamster V79 cells were also used. Carbon-ion effectiveness for cell inactivation generally increased with LET for monoenergetic beams, with the largest gain in cell-killing obtained in the cells most radioresistant to X- or gamma-rays. Such an increased effectiveness in cells less responsive to low LET radiation was found also for SOBP irradiation, but the latter was less effective compared with monoenergetic ion beams of the same LET. Our data show the superior effectiveness for cell-killing exhibited by carbon-ion beams compared to lower LET radiation, particularly in tumour cells radioresistant to X- or gamma-rays, hence the advantage of using such beams in radiotherapy. The observed lower effectiveness of SOBP irradiation compared to monoenergetic carbon beam irradiation argues against the radiobiological equivalence between dose-averaged LET in a point in the SOBP and the corresponding monoenergetic beams.

Target motion tracking with a scanned particle beam

Treatment of moving targets with scanned particle beams results in local over- and under-dosage due to interplay of beam and target motion. To mitigate the impact of respiratory motion, a motion tracking system has been developed and integrated in the therapy control system at Gesellschaft für Schwerionenforschung. The system adapts pencil beam positions as well as the beam energy according to target motion to irradiate the planned position. Motion compensation performance of the tracking system was assessed by measurements with radiographic films and a 3D array of 24 ionization chambers. Measurements were performed for stationary detectors and moving detectors using the tracking system. Film measurements showed comparable homogeneity inside the target area. Relative differences of 3D dose distributions within the target volume were 1 +/- 2% with a maximum of 4%. Dose gradients and dose to surrounding areas were in good agreement. The motion tracking system successfully preserved dose distributions delivered to moving targets and maintained target conformity.