New Ions for Therapy

BIANCA, a biophysical model of cell survival and chromosome damage by protons, C-ions and He-ions at energies and doses used in hadrontherapy

An upgraded version of the BIANCA II biophysical model, which describes more realistically interphase chromosome organization and the link between chromosome aberrations and cell death, was applied to V79 and AG01522 cells exposed to protons, C-ions and He-ions over a wide LET interval (0.6-502 keV µm^-1), as well as proton-irradiated U87 cells. The model assumes that (i) ionizing radiation induces DNA 'cluster lesions' (CLs), where by definition each CL produces two independent chromosome fragments; (ii) fragment (distance-dependent) mis-rejoining, or un-rejoining, produces chromosome aberrations; (iii) some aberrations lead to cell death. The CL yield, which mainly depends on radiation quality but is also modulated by the target cell, is an adjustable parameter. The fragment un-rejoining probability, f, is the second, and last, parameter. The value of f, which is assumed to depend on the cell type but not on radiation quality, was taken from previous studies, and only the CL yield was adjusted in the present work. Good agreement between simulations and experimental data was obtained, suggesting that BIANCA II is suitable for calculating the biological effectiveness of hadrontherapy beams. For both V79 and AG01522 cells, the mean number of CLs per micrometer was found to increase with LET in a linear-quadratic fashion before the over-killing region, where a less rapid increase, with a tendency to saturation, was observed. Although the over-killing region deserves further investigation, the possibility of fitting the CL yields is an important feature for hadrontherapy, because it allows performing predictions also at LET values where experimental data are not available. Finally, an approach was proposed to predict the ion-response of the cell line(s) of interest from the ion-response of a reference cell line and the photon response of both. A pilot study on proton-irradiated AG01522 and U87 cells, taking V79 cells as a reference, showed encouraging results.

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

Proton and carbon ion beams are used in the clinical practice for external radiotherapy treatments achieving, for selected indications, promising and superior clinical results with respect to x-ray based radiotherapy. Other ions, like [Formula: see text] have recently been considered as projectiles in particle therapy centres and might represent a good compromise between the linear energy transfer and the radiobiological effectiveness of [Formula: see text] ion and proton beams, allowing improved tumour control probability and minimising normal tissue complication probability. All the currently used p, [Formula: see text] and [Formula: see text] ion beams allow achieving sharp dose gradients on the boundary of the target volume, however the accurate dose delivery is sensitive to the patient positioning and to anatomical variations with respect to photon therapy. This requires beam range and/or dose release measurement during patient irradiation and therefore the development of dedicated monitoring techniques. All the proposed methods make use of the secondary radiation created by the beam interaction with the patient and, in particular, in the case of [Formula: see text] ion beams are also able to exploit the significant charged radiation component. Measurements performed to characterise the charged secondary radiation created by [Formula: see text] and [Formula: see text] particle therapy beams are reported. Charged secondary yields, energy spectra and emission profiles produced in a poly-methyl methacrylate (PMMA) target by [Formula: see text] and [Formula: see text] beams of different therapeutic energies were measured at 60° and 90° with respect to the primary beam direction. The secondary yield of protons produced along the primary beam path in a PMMA target was obtained. The energy spectra of charged secondaries were obtained from time-of-flight information, whereas the emission profiles were reconstructed exploiting tracking detector information. The obtained measurements are in agreement with results reported in the literature and suggests the feasibility of range monitoring based on charged secondary particle detection: the implications for particle therapy monitoring applications are also discussed.

Particles with similar LET values generate DNA breaks of different complexity and reparability: a high-resolution microscopy analysis of γH2AX/53BP1 foci

Biological effects of high-LET (linear energy transfer) radiation have received increasing attention, particularly in the context of more efficient radiotherapy and space exploration. Efficient cell killing by high-LET radiation depends on the physical ability of accelerated particles to generate complex DNA damage, which is largely mediated by LET. However, the characteristics of DNA damage and repair upon exposure to different particles with similar LET parameters remain unexplored. We employed high-resolution confocal microscopy to examine phosphorylated histone H2AX (γH2AX)/p53-binding protein 1 (53BP1) focus streaks at the microscale level, focusing on the complexity, spatiotemporal behaviour and repair of DNA double-strand breaks generated by boron and neon ions accelerated at similar LET values (∼135 keV μm^-1) and low energies (8 and 47 MeV per n, respectively). Cells were irradiated using sharp-angle geometry and were spatially (3D) fixed to maximize the resolution of these analyses. Both high-LET radiation types generated highly complex γH2AX/53BP1 focus clusters with a larger size, increased irregularity and slower elimination than low-LET γ-rays. Surprisingly, neon ions produced even more complex γH2AX/53BP1 focus clusters than boron ions, consistent with DSB repair kinetics. Although the exposure of cells to γ-rays and boron ions eliminated a vast majority of foci (94% and 74%, respectively) within 24 h, 45% of the foci persisted in cells irradiated with neon. Our calculations suggest that the complexity of DSB damage critically depends on (increases with) the particle track core diameter. Thus, different particles with similar LET and energy may generate different types of DNA damage, which should be considered in future research.

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.

Oxygen beams for therapy: advanced biological treatment planning and experimental verification

Nowadays there is a rising interest towards exploiting new therapeutical beams beyond carbon ions and protons. In particular, [Formula: see text]O ions are being widely discussed due to their increased LET distribution. In this contribution, we report on the first experimental verification of biologically optimized treatment plans, accounting for different biological effects, generated with the TRiP98 planning system with [Formula: see text]O beams, performed at HIT and GSI. This implies the measurements of 3D profiles of absorbed dose as well as several biological measurements. The latter includes the measurements of relative biological effectiveness along the range of linear energy transfer values from  ≈20 up to  ≈750 keV μ [Formula: see text], oxygen enhancement ratio values and the verification of the kill-painting approach, to overcome hypoxia, with a phantom imitating an unevenly oxygenated target. With the present implementation, our treatment planning system is able to perform a comparative analysis of different ions, according to any given condition of the target. For the particular cases of low target oxygenation, [Formula: see text]O ions demonstrate a higher peak-to-entrance dose ratio for the same cell killing in the target region compared to [Formula: see text]C ions. Based on this phenomenon, we performed a short computational analysis to reveal the potential range of treatment plans, where [Formula: see text]O can benefit over lighter modalities. It emerges that for more hypoxic target regions (partial oxygen pressure of  ≈0.15% or lower) and relatively low doses (≈4 Gy or lower) the choice of [Formula: see text]O over [Formula: see text]C or [Formula: see text]He may be justified.

Benchmarking GATE/Geant4 for 16O ion beam therapy

Oxygen ([Formula: see text]) ions are a potential alternative to carbon ions in ion beam therapy. Their enhanced linear energy transfer indicates a higher relative biological effectiveness and a reduced oxygen enhancement ratio. Due to the limited availability of [Formula: see text] ion beams, Monte Carlo (MC) transport codes are important research tools for investigating their potential. The purpose of this study was to validate GATE/Geant4 for [Formula: see text] ion beam therapy using experimental data from literature. Five hadron physics lists and two electromagnetic options were benchmarked against measured depth dose distributions (DDDs) and charge-changing cross sections. The simulated beam ranges deviated by less than 0.5% for all physics configurations and only a few points exceeded the gamma index criterion (2%/1 mm). However, the simulated partial charge-changing cross sections deviated considerably for some hadron physics configurations. Best agreement with the experimental values was obtained with the quantum molecular dynamics model (QMD), and we therefore suggest using this model in Geant4 to accurately describe the fragmentation of [Formula: see text] ion beams into lighter fragments ([Formula: see text]).

Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets

We report on recent experimental results deploying a continuous cryogenic hydrogen jet as a debris-free, renewable laser-driven source of pure proton beams generated at the 150 TW ultrashort pulse laser Draco. Efficient proton acceleration reaching cut-off energies of up to 20 MeV with particle numbers exceeding 109 particles per MeV per steradian is demonstrated, showing for the first time that the acceleration performance is comparable to solid foil targets with thicknesses in the micrometer range. Two different target geometries are presented and their proton beam deliverance characterized: cylindrical (∅ 5 μm) and planar (20 μm × 2 μm). In both cases typical Target Normal Sheath Acceleration emission patterns with exponential proton energy spectra are detected. Significantly higher proton numbers in laser-forward direction are observed when deploying the planar jet as compared to the cylindrical jet case. This is confirmed by two-dimensional Particle-in-Cell (2D3V PIC) simulations, which demonstrate that the planar jet proves favorable as its geometry leads to more optimized acceleration conditions.

Carbon and oxygen minibeam radiation therapy: An experimental dosimetric evaluation

PURPOSE

To perform dosimetric characterization of a minibeam collimator in both carbon and oxygen ion beams to guide optimal setup geometry and irradiation for future radiobiological studies.

METHODS

Carbon and oxygen minibeams were generated using a prototype tungsten multislit collimator presenting line apertures 700 μm wide, which are spaced 3500 μm centre-to-centre distance apart. Several radiation beam spots generated the desired field size of 15 × 15 mm² and production of a 50 mm long spread out Bragg peak (SOBP) centered at 80 mm depth in water. Dose evaluations were performed with two different detectors: a PTW microDiamond® single crystal diamond detector and radiochromic films (EBT3). Peak-to-valley dose ratio (PVDR) values, output factors (OF), penumbras, and full width at half maximum (FWHM) were measured.

RESULTS

Measured lateral dose profiles exhibited spatial fractionation of dose at depth in a water phantom in the expected form of peaks and valleys for both carbon and oxygen radiation fields. The diamond detector and radiochromic film provided measurements of PVDR in good agreement. PVDR values at shallow depth were about 60 and decreased to about 10 at 80 mm depth in water. OF in the center of the SOBP was about 0.4; this value is larger than the corresponding one in proton minibeam radiation therapy measured using a comparable collimator due to a reduced lateral scattering for carbon and oxygen minibeams.

CONCLUSIONS

Carbon and oxygen minibeams may be produced by a mechanical collimator. PVDR values and output factors measured in this first study of these minibeam radiation types indicate there is potential for their therapeutic use. Optimization of minibeam collimator design and the number and size of focal spots for irradiation are advocated to improve PDVR values and dose distributions for each specific applied use.

Lateral variations of radiobiological properties of therapeutic fields of 1H, 4He, 12C and 16O ions studied with Geant4 and microdosimetric kinetic model

As known, in cancer therapy with ion beams the relative biological effectiveness (RBE) of ions changes in the course of their propagation in tissues. Such changes are caused not only by increasing the linear energy transfer (LET) of beam particles with the penetration depth towards the Bragg peak, but also by nuclear reactions induced by beam nuclei leading to the production of various secondary particles. Although the changes of RBE along the beam axis have been studied quite well, much less attention has been paid to the evolution of RBE in the transverse direction, perpendicular to the beam axis. In order to fill this gap, we simulated radiation fields of ¹H, ⁴He, ¹²C and ¹⁶O nuclei of 20 mm in diameter by means of a Geant4-based Monte Carlo model for heavy-ion therapy connected with the modified microdosimetric kinetic model to describe the response of normal ([Formula: see text] Gy) and early-responding ([Formula: see text] Gy) tissues. Depth and radial distributions of saturation-corrected dose-mean lineal energy, RBE and RBE-weighted dose are investigated for passive beam shaping and active beam scanning. The field of ⁴He has a small lateral spread as compared with ¹H field, and it is characterised by a modest lateral variation of RBE suggesting the use of fixed RBE values across the field transverse cross section at each depth. Reduced uncertainties of RBE on the boundary of a ⁴He treatment field can be advantageous in a specific case of an organ at risk located in lateral proximity to the target volume. It is found that the lateral distributions of RBE calculated for ¹²C and ¹⁶O fields demonstrate fast variations in the radial direction due to changes of dose and composition of secondary fragments in the field penumbra. Nevertheless, the radiation fields of all four projectiles at radii larger than 20 mm can be characterized by a common RBE value defined by tissue radiosensitivity. These findings can help, in particular, in accessing the transverse homogeneity of radiation fields of ions used in studies in vitro.

The Immunoregulatory Potential of Particle Radiation in Cancer Therapy

Cancer treatment, today, consists of surgery, chemotherapy, radiation, and most recently immunotherapy. Combination immunotherapy-radiotherapy (CIR) has experienced a surge in public attention due to numerous clinical publications outlining the reduction or elimination of metastatic disease, following treatment with specifically ipilimumab and radiotherapy. The mechanism behind CIR, however, remains unclear, though it is hypothesized that radiation transforms the tumor into an in situ vaccine which immunotherapy modulates into a larger immune response. To date, the majority of attention has focused on rotating out immunotherapeutics with conventional radiation; however, the unique biological and physical benefits of particle irradiation may prove superior in generation of systemic effect. Here, we review recent advances in CIR, with a particular focus on the usage of charged particles to induce or enhance response to cancerous disease.

Secondary radiation measurements for particle therapy applications: prompt photons produced by 4He, 12C and 16O ion beams in a PMMA target

Charged particle beams are used in particle therapy (PT) to treat oncological patients due to their selective dose deposition in tissues with respect to the photons and electrons used in conventional radiotherapy. Heavy (Z  >  1) PT beams can additionally be exploited for their high biological effectiveness in killing cancer cells. Nowadays, protons and carbon ions are used in PT clinical routines. Recently, interest in the potential application of helium and oxygen beams has been growing. With respect to protons, such beams are characterized by their reduced multiple scattering inside the body, increased linear energy transfer, relative biological effectiveness and oxygen enhancement ratio. The precision of PT demands online dose monitoring techniques, crucial to improving the quality assurance of any treatment: possible patient mis-positioning and biological tissue changes with respect to the planning CT scan could negatively affect the outcome of the therapy. The beam range confined in the irradiated target can be monitored thanks to the neutral or charged secondary radiation emitted by the interactions of hadron beams with matter. Among these secondary products, prompt photons are produced by nuclear de-excitation processes, and at present, different dose monitoring and beam range verification techniques based on prompt-γ detection are being proposed. It is hence of importance to perform γ yield measurement in therapeutic-like conditions. In this paper we report on the yields of prompt photons produced by the interaction of helium, carbon and oxygen ion beams with a poly-methyl methacrylate (PMMA) beam stopping target. The measurements were performed at the Heidelberg Ion-Beam Therapy Center (HIT) with beams of different energies. An LYSO scintillator, placed at [Formula: see text] and [Formula: see text] with respect to the beam direction, was used as the photon detector. The obtained γ yields for the carbon ion beams are compared with results from the literature, while no other results from helium and oxygen beams have been published yet. A discussion on the expected resolution of a slit camera detector is presented, demonstrating the feasibility of a prompt-γ-based monitoring technique for PT treatments using helium, carbon and oxygen ion beams.

Nanoparticles for radiooncology: Mission, vision, challenges

Cancer is one of the leading non-communicable diseases with highest mortality rates worldwide. About half of all cancer patients receive radiation treatment in the course of their disease. However, treatment outcome and curative potential of radiotherapy is often impeded by genetically and/or environmentally driven mechanisms of tumor radioresistance and normal tissue radiotoxicity. While nanomedicine-based tools for imaging, dosimetry and treatment are potential keys to the improvement of therapeutic efficacy and reducing side effects, radiotherapy is an established technique to eradicate the tumor cells. In order to progress the introduction of nanoparticles in radiooncology, due to the highly interdisciplinary nature, expertise in chemistry, radiobiology and translational research is needed. In this report recent insights and promising policies to design nanotechnology-based therapeutics for tumor radiosensitization will be discussed. An attempt is made to cover the entire field from preclinical development to clinical studies. Hence, this report illustrates (1) the radio- and tumor-biological rationales for combining nanostructures with radiotherapy, (2) tumor-site targeting strategies and mechanisms of cellular uptake, (3) biological response hypotheses for new nanomaterials of interest, and (4) challenges to translate the research findings into clinical trials.

Nuclear physics in particle therapy: a review

Charged particle therapy has been largely driven and influenced by nuclear physics. The increase in energy deposition density along the ion path in the body allows reducing the dose to normal tissues during radiotherapy compared to photons. Clinical results of particle therapy support the physical rationale for this treatment, but the method remains controversial because of the high cost and of the lack of comparative clinical trials proving the benefit compared to x-rays. Research in applied nuclear physics, including nuclear interactions, dosimetry, image guidance, range verification, novel accelerators and beam delivery technologies, can significantly improve the clinical outcome in particle therapy. Measurements of fragmentation cross-sections, including those for the production of positron-emitting fragments, and attenuation curves are needed for tuning Monte Carlo codes, whose use in clinical environments is rapidly increasing thanks to fast calculation methods. Existing cross sections and codes are indeed not very accurate in the energy and target regions of interest for particle therapy. These measurements are especially urgent for new ions to be used in therapy, such as helium. Furthermore, nuclear physics hardware developments are frequently finding applications in ion therapy due to similar requirements concerning sensors and real-time data processing. In this review we will briefly describe the physics bases, and concentrate on the open issues.

Monitoring of Hadrontherapy Treatments by Means of Charged Particle Detection

The interaction of the incoming beam radiation with the patient body in hadrontherapy treatments produces secondary charged and neutral particles, whose detection can be used for monitoring purposes and to perform an on-line check of beam particle range. In the context of ion-therapy with active scanning, charged particles are potentially attractive since they can be easily tracked with a high efficiency, in presence of a relatively low background contamination. In order to verify the possibility of exploiting this approach for in-beam monitoring in ion-therapy, and to guide the design of specific detectors, both simulations and experimental tests are being performed with ion beams impinging on simple homogeneous tissue-like targets (PMMA). From these studies, a resolution of the order of few millimeters on the single track has been proven to be sufficient to exploit charged particle tracking for monitoring purposes, preserving the precision achievable on longitudinal shape. The results obtained so far show that the measurement of charged particles can be successfully implemented in a technology capable of monitoring both the dose profile and the position of the Bragg peak inside the target and finally lead to the design of a novel profile detector. Crucial aspects to be considered are the detector positioning, to be optimized in order to maximize the available statistics, and the capability of accounting for the multiple scattering interactions undergone by the charged fragments along their exit path from the patient body. The experimental results collected up to now are also valuable for the validation of Monte Carlo simulation software tools and their implementation in Treatment Planning Software packages.

The Emerging Role of Carbon-Ion Radiotherapy

Carbon-ion radiotherapy (CIRT) has progressed rapidly in technological delivery, indications, and efficacy. Owing to a focused dose distribution in addition to high linear energy transfer and subsequently high relative biological effect, CIRT is uniquely able to target otherwise untreatable hypoxic and radioresistant disease while opening the door for substantially hypofractionated treatment of normal and radiosensitive disease. CIRT has increasingly garnered international attention and is nearing the tipping point for international adoption.