high-LET Charged Particles Research Articles

Using the photon isoeffective dose formalism to compare and combine BNCT and CIRT in a head and neck tumour

Boron Neutron Capture Therapy (BNCT) is a radiotherapy technique based on the enrichment of tumour cells with suitable 10-boron concentration and on subsequent neutron irradiation. Low-energy neutron irradiation produces a localized deposition of radiation dose caused by boron neutron capture reactions. Boron is vehiculated into tumour cells via proper borated formulations, able to accumulate in the malignancy more than in normal tissues. The neutron capture releases two high-LET charged particles (i.e., an alpha particle and a lithium ion), losing their energy in a distance comparable to the average dimension of one cell. Thus BNCT is selective at the cell level and characterized by high biological effectiveness. As the radiation field is due to the interaction of neutrons with the components of biological tissues and with boron, the dosimetry requires a formalism to express the absorbed dose into photon-equivalent units. This work analyzes a clinical case of an adenoid cystic carcinoma treated with carbon-ion radiotherapy (CIRT), located close to optic nerve and deep-seated as a practical example of how to apply the formalism of BNCT photon isoeffective dose and how to evaluate the BNCT dose distribution against CIRT. The example allows presenting different dosimetrical and radiobiological quantities and drawing conclusions on the potential of BNCT stemming on the clinical result of the CIRT. The patient received CIRT with a dose constraint on the optic nerve, affecting the peripheral part of the Planning Target Volume (PTV). After the treatment, the tumour recurred in this low-dose region. BNCT was simulated for the primary tumour, with the goal to calculate the dose distribution in isoeffective units and a Tumour Control Probability (TCP) to be compared with the one of the original treatment. BNCT was then evaluated for the recurrence in the underdosed region which was not optimally covered by charged particles due to the proximity of the optic nerve. Finally, a combined treatment consisting in BNCT and carbon ion therapy was considered to show the consistency and the potential of the model. For the primary tumour, the photon isoeffective dose distribution due to BNCT was evaluated and the resulted TCP was higher than that obtained for the CIRT. The formalism produced values that are consistent with those of carbon-ion. For the recurrence, BNCT dosimetry produces a similar TCP than that of primary tumour. A combined treatment was finally simulated, showing a TCP comparable to the BNCT-alone with overall dosimetric advantage in the most peripheral parts of the treatment volume. Isoeffective dose formalism is a robust tool to analyze BNCT dosimetry and to compare it with the photon-equivalent dose calculated for carbon-ion treatment. This study introduces for the first time the possibility to combine the dosimetry obtained by two different treatment modalities, showing the potential of exploiting the cellular targeting of BNCT combined with the precision of charged particles in delivering an homogeneous dose distribution in deep-seated tumours.

Short-term metabolic disruptions in urine of mouse models following exposure to low doses of oxygen ion radiation

Molecular alterations as a result of exposure to low doses of high linear energy transfer (LET) radiation can have deleterious short- and long-term consequences on crew members embarking on long distance space missions. Oxygen ions (16O) are among the high LET charged particles that make up the radiation environment inside a vehicle in deep space. We used mass spectrometry-based metabolomics to characterize urinary metabolic profiles of male C57BL/6J mice exposed to a single dose of 0.1, 0.25 and 1.0 Gy of 16O (600 MeV/n) at 10 and 30 days post-exposure to delineate radiation-induced metabolic alterations. We recognized a significant down regulation of several classes of metabolites including cresols and tryptophan metabolites, ketoacids and their derivatives upon exposure to 0.1 and 0.25 Gy after 10 days. While some of these changes reverted to near normal by 30 days, some metabolites including p-Cresol sulfate, oxalosuccinic acid, and indoxylsulfate remained dysregulated at 30 days, suggesting long term prognosis on metabolism. Pathway analysis revealed a long-term dysregulation in multiple pathways including tryptophan and porphyrin metabolism. These results suggest that low doses of high-LET charged particle irradiation may have long-term implications on metabolic imbalance.

Directional genomic hybridization: inversions as a potential biodosimeter for retrospective radiation exposure

Chromosome aberrations in blood lymphocytes provide a useful measure of past exposure to ionizing radiation. Despite the widespread and successful use of the dicentric assay for retrospective biodosimetry, the approach suffers substantial drawbacks, including the fact that dicentrics in circulating blood have a rather short half-life (roughly 1-2 years by most estimates). So-called symmetrical aberrations such as translocations are far more stable in that regard, but their high background frequency, which increases with age, also makes them less than ideal for biodosimetry. We developed a cytogenetic assay for potential use in retrospective biodosimetry that is based on the detection of chromosomal inversions, another symmetrical aberration whose transmissibility (stability) is also ostensibly high. Many of the well-known difficulties associated with inversion detection were circumvented through the use of directional genomic hybridization, a method of molecular cytogenetics that is less labor intensive and better able to detect small chromosomal inversions than other currently available approaches. Here, we report the dose-dependent induction of inversions following exposure to radiations with vastly different ionization densities [i.e., linear energy transfer (LET)]. Our results show a dramatic dose-dependent difference in the yields of inversions induced by low-LET gamma rays, as compared to more damaging high-LET charged particles similar to those encountered in deep space.

The more important heavy charged particle radiotherapy of the future is more likely to be with heavy ions rather than protons

The more important heavy charged particle radiotherapy of the future is more likely to be with heavy ions rather than protons

SU-D-137-05: Impact of Dosimetric Uncertainties On the Computed Relative Biological Effectiveness (RBE) of High Linear Energy Transfer (LET) Particles

Purpose: Quantify uncertainties in cellular and sub‐cellular dosimetry required for the analysis of biological experiments in which cell cultures are irradiated by charged particles. Assess the impact of dosimetric uncertainties on the computed RBE of high LET charged particles. Methods: Estimates of the frequency‐mean specific energy, lineal energy and absorbed dose per unit fluence (D/Φ) are computed using analytical, deterministic (MCDS) and Monte Carlo (MCNPX and TOPAS/Geant4) methods for representative cell culture geometries. Uncertainties in dosimetric quantities associated with thickness of the culture dish (0.85 mg/cm2 of Melinex) and the location of the nucleus (0–8 urn from the Melinex surface) within a 2 mm layer of culture medium (water) are quantified for a variety of high LET particles (e−, 1H+, 2H+, 3He2 + 4He2 +, and 12C6 +). Results: Uncertainties in D/Φ and related quantities may be as large as 75% for a 4 MeV 4He2 + (range about 27 μm). Substantial differences are seen in dosimetric quantities for all of the considered charged particles with a CSDA range less than about 150 μm. For particles with a CSDA range larger than about 30 μm, dosimetric quantities computed using the deterministic MCDS algorithm are typically within 3% of the estimates obtained with Monte Carlo. The default TOPAS/Geant4 physics parameters sometimes produce estimates of fluence and absorbed dose that differ from the MCDS and MCNPX estimates by as much as 15%. Conclusion: The size and location of the cell nucleus during mitotic cell cycle has a significant impact on the dose delivered to cellular and subcellular targets for short range (high LET) particles. Path and energy straggling have a small impact on deterministic dose quantities for particles with a CSDA range larger than about 30 μm (Z < 6). Dosimetric uncertainties may have a significant impact on the inferred RBE of high LET particles.

Yield of Single- and Double-Strand Breaks and Nucleobase Lesions in Fully Hydrated Plasmid DNA Films Irradiated with High-LET Charged Particles

We measured the yield and spectrum of strand breaks and nucleobase lesions produced in fully hydrated plasmid DNA films to determine the linear energy transfer (LET) dependence of DNA damage induced by ion-beam irradiation in relation to the change in the atomic number of ions. The yield of isolated damage was revealed as a decrease in prompt SSBs with increasing LET of He(2+), C(5+,6+) and Ne(8+,10+) ions. On the other hand, the yields of prompt DSBs increased with increasing ion LET. SSBs were additionally induced in ion-irradiated DNA film by treatment with two kinds of base excision repair proteins (glycosylases), Nth and Fpg, indicating that base lesions are produced in the hydrated DNA film. This result shows that nucleobase lesions are produced via both chemical reactions with diffusible water radicals, such as OH radicals, and direct energy deposition onto DNA and the hydrated water layer. Nth-sensitive sites deduced to be pyrimidine lesions, such as 5,6-dihydrothymine (DHT), showed a relatively larger yield than Fpg-sensitive sites deduced to be purine lesions, such as 7,8-dihydro-8-oxo-2'deoxyguanine (8-oxoGua), for all ion exposures tested. The yield of SSBs or DSBs observed by enzyme treatment decreased noticeably with increasing LET for all tested ions. These results indicated that higher-LET ions preferentially produce a complex type of damage that might compromise the activities of the glycosylases used in this study. These findings are biologically important since, under cell mimicking conditions, persistent DNA damage occurs in part due to direct energy deposition on the DNA or hydrated water shell that is specifically induced by dense ionization in the track.

What can we learn from the neutron clinical experience for improving ion-beam techniques and high-LET patient selection?

Abstract Historically, improvements in radiotherapy have been mainly due to improvements in physical selectivity: beam penetration, collimation, dosimetry, treatment planning; and advances in imaging. Neutrons were the first high-LET (linear energy transfer) radiation to be used clinically and showed improvement in the differential response of radiation resistant tumors and normal tissues. The benefits of fast neutrons (and other forms of high LET radiations) are due to their biological effects: a reduction of the OER, a reduction in the differential cell radiosensitivity related to their position in the mitotic cycle, and a reduction in cellular repair capacity (thus less importance of fractionation). The poor physical selectivity of the early neutron therapy beams introduced a systematic bias in comparison with the photon treatments and created a negative perception for neutron therapy. However, significant improvements in the neutron therapy equipment resulted in a physical selectivity similar to modern MV photon therapy. The tumor types or sites where the best therapeutic results were obtained included inoperable or recurrent salivary gland tumors locally extended prostatic adenocarcinomas, and slowly growing well-differentiated sarcomas. The benefit of neutrons for some other well-defined groups of patients was demonstrated in randomized trials. It was estimated that about 20 % of all radiotherapy patients could benefit from fast neutrons (if neutrons are delivered under satisfactory physical conditions). An important issue for fast neutron therapy is the selection of the types of patients who could most benefit from high-LET radiations. The same issue is raised today with other high-LET radiations (e.g., 12C ions). It is reasonable to assume that the same types of patients would benefit from 12C irradiation. Of course the better physical selectivity of ion beams enhances the treatment possibilities but this is true for both the high-LET and low-LET radiations (i.e., moving from neutrons to 12C ions and from photons to protons, respectively). An important area of research involves developing criteria to identify specific patients suitable for high-LET radiation. One promising technique is to measure the RBE of the cancer cell population in vitro mainly in head and neck tumors. Modern molecular imaging allows the identification of hypoxic or proliferative regions in the tumor. Special MRI examinations are also able to identify hypoxic regions. A promising predictive test recently initiated, is the study of non-repairable double strand breaks but the utility of the technique needs to be confirmed. The extensive experience with fast neutron therapy can greatly assist the transition to high-LET charged-particle therapy.

Induction of Micronuclei in Germinating Onion Seed Root Tip Cells Irradiated with High Energy Heavy Ions

Effects of high LET charged particles on a perfect in-vivo system are an essential theme for the study of the biological effects of radiation. Germinating onion seeds are independent complete organisms and the radiation induced micronuclei in the root chip cells can be examined quantitatively and theoretically. We irradiated with three types of high energy accelerated heavy ions germinating onion seeds using a synchrotron and observed micronuclei in the root tip cells. Micronuclei induction showed characteristic dose responses of an upward convex bell shape and a steep rise near zero doses for all types of the ions. The bell curve dose responses, however, could be explained by a simple mathematical model. A parameter in the model which indicates micronuclei induction frequency and another parameter which indicates induction frequency of lethal damages (or damages delaying cell divisions) per heavy ion track were both proportional to square of the LET. Because we suspected by-stander effect concerning the dose responses rising steeply near zero doses and tapering off for higher doses, we tested acute irradiation to remove time of information transmittance between cells using a single spill (about 0.3 s) of the synchrotron beam. No difference was detected between normal multiple spill irradiations and single spill.

Dose response of CaF2:Tm to charged particles of different LETa)

Thermoluminescent dosimeters are well established for performing calibrations in radiotherapy and for monitoring dose to personnel exposed to low linear energy transfer (LET) ionizing radiation. Patients undergoing light ion therapy and astronauts engaged in space flight are, however, exposed to radiation fields consisting of a mix of low- and high-LET charged particles. In this study, glow curves from chips were examined after exposure to various electron and ion beams. The annealing and readout procedures for these chips were optimized for these beams. After a 10 min prereadout annealing at , the optimized glow curve samples the light output between 95 and with a heating rate of . The ratio of the integral of the glow curve under peaks 4–6 to the integral under peak 3 was approximately 0.9 for electrons, 1.0 for entrance protons, 1.6 for peak protons, and 2.2 for entrance carbon, silicon, and iron ions. The integral light output per unit dose in water for the iron exposures was about half as much as for the electron exposures. The peak-area-ratio can be used to determine a dose response factor for different LET radiations.

High yields of isochromatid breaks and successive formation of chromosome exchanges may lead to reproductive cell death following high-LET irradiation

Abstract To clarify the relationship between cell death and chromosomal aberrations following exposure to heavy-charged ion particles beams, exponentially growing Human Salivary Gland Tumor cells (HSG cells) were irradiated with various kinds of high energy heavy ions; 13 keV/μm carbon ions as a low-LET charged particle radiation source, 120 keV/μm carbon ions and 440 keV/μm iron ions as high-LET charged particle radiation sources. X-rays (200 kVp) were used as a reference. Reproductive cell death was evaluated by clonogenic assays, and the chromatid aberrations in G2/M phase and their repairing kinetics were analyzed by the calyculin A induced premature chromosome condensation (PCC) method. High-LET heavy-ion beams introduced much more severe and un-repairable chromatid breaks and isochromatid breaks in HSG cells than low-LET irradiation. In addition, the continuous increase of exchange aberrations after irradiation occurred in the high-LET irradiated cells. The cell death, initial production of isochromatid breaks and subsequent formation of chromosome exchange seemed to be depend similarly on LET with a maximum RBE peak around 100–200 keV/μm of LET value. Conversely, un-rejoined isochromatid breaks or chromatid breaks/gaps seemed to be less effective in reproductive cell death. These results suggest that the continuous yield of chromosome exchange aberrations induced by high-LET ionizing particles is a possible reason for the high RBE for cell death following high-LET irradiation, alongside other chromosomal aberrations additively or synergistically.

Responses of a diamond detector to high-LET charged particles

The responses of a commercial diamond detector (type 60003, PTW-Freiburg) to several heavy ions were examined. The responses to heavy-ion beams reached stable levels with relatively small pre-irradiation doses compared to photon-beam irradiations. The responses also reached stable levels with a smaller pre-irradiation dose when the dose rate of the He beams was increased. A total accumulated dose of about 5 Gy was required for the pre-irradiation dose of heavy-ion beams. No angular dependence of the detector responses was observed within a deviation of 5%. The dose-rate dependence of the detector responses to heavy-ion beams was far smaller than that to gamma rays. The decrease in the response was within 0.9%, with a variation from 0.88 to 18.2 Gy min−1 in the carbon beam. We examined the LET dependence of the diamond detector responses using various kinds of heavy-ion beams. The responses had particle dependence in addition to LET dependence. The responses decreased more with higher LET particles and decreased less with large-Z particles. We proposed a gradual-saturation model based on the track structure under several simple assumptions to explain the LET and particle dependences of the response.

DNA fragmentation by charged particle tracks

High-LET (linear energy transfer) charged particles induce DNA double-strand breaks (DSB) in a non-random fashion in mammalian cells. The clustering of DSB, probably determined by track structure as well as chromatin conformation, results in an excess of small- and intermediate-sized DNA fragments. DNA fragmentation in normal human fibroblasts (GM5758) was analyzed by pulsed-field gel electrophoresis after irradiation with photons ( 60Co) or 125 keV/μm nitrogen ions. Compared to conventional DSB analysis, i.e. assays only measuring the fraction of DNA smaller than a single threshold, the relative biological effectiveness (RBE) for DSB induction increased with 100%. Further, the size distribution of DNA fragments showed a significant dependence on radiation quality, with an excess of fragments up to 1 Mbp. Irradiation of naked genomic DNA without histone proteins increased the DSB yields 25 and 13 times for photons and nitrogen ions, respectively. The results suggest possible roles of both track structure and chromatin organization in the distribution of DNA double-strand breaks along the chromosome.

A comparison of mutations induced by accelerated iron particles versus those induced by low earth orbit space radiation in the FEM-3 gene of Caenorhabditis elegans

The fem-3 gene of Caenorhabditis elegans was employed to determine the mutation frequency as well as the nature of mutations induced by low earth orbit space radiation ambient to Space Shuttle flight STS-76. Recovered mutations were compared to those induced by accelerated iron ions generated by the AGS synchrotron accelerator at Brookhaven National Laboratory. For logistical reasons, dauer larvae were prepared at TCU, transported to either Kennedy Space Center or Brookhaven National Laboratory, flown in space or irradiated, returned to TCU and screened for mutants. A total of 25 fem-3 mutants were recovered after the shuttle flight and yielded a mutation frequency of 2.1×10 −5, roughly 3.3-fold higher than the spontaneous rate of 6.3×10 −6. Four of the mutations were homozygous inviable, suggesting that they were large deletions encompassing fem-3 as well as neighboring, essential genes. Southern blot analyses revealed that one of the 25 contained a polymorphism in fem-3, further evidence that space radiation can induce deletions. While no polymorphisms were detected among the iron ion-induced mutations, three of the 15 mutants were homozygous inviable, which is in keeping with previous observations that high LET iron particles generate deficiencies. These data provide evidence, albeit indirect, that an important mutagenic component of ambient space radiation is high LET charged particles such as iron ions.

TLD efficiency of 7LiF for doses deposited by high-LET particles

The efficiency of 7LiF TLDs (TLD-700) in registering dose from high-LET (⩾10 keV/μm) charged particles (relative to 137Cs γ rays) has been measured for a number of accelerated heavy ions at various particle accelerator facilities. These measured efficiency values have been compared with similar results obtained from the open literature and a dose efficiency function has been fitted to the combined data set. While it was found that the dose efficiency is not only a function of LET, but also of the charge of the incident particle, the fitted function can be used to correct the undermeasured value of dose from exposures made in mixed radiation fields where LET information is available. This LET-dependent dose efficiency function is used in our laboratory in determining total absorbed dose and dose equivalent from combined TLD and CR-39 plastic nuclear track detector measurements.

Charged-particle mutagenesis II. Mutagenic effects of high energy charged particles in normal human fibroblasts

The biological effects of high LET charged particles are a subject of great concern with regard to the prediction of radiation risk in space. In this report, mutagenic effects of high LET charged particles are quantitatively measured using primary cultures of human skin fibroblasts, and the spectrum of induced mutations are analyzed. The LET of the charged particles ranged from 25 KeV/μm to 975 KeV/gmm with particle energy (on the cells) between 94 – 603 MeV/u. The X-chromosome linked hypoxanthine guanine phosphoribosyl transferase ( hprt) locus was used as the target gene. Exposure to these high LET charged particles resulted in exponential survival curves; whereas, mutation induction was fitted by a linear model. The Relative Biological Effect (RBE) for cell-killing ranged from 3.73 to 1.25, while that for mutant induction ranged from 5.74 to 0.48. Maximum RBE values were obtained at the LET of 150 keV/μm. The inactivation cross-section (αi) and the action-section for mutant induction (αm) ranged from 2.2 to 92.0 μm 2 and 0.09 to 5.56 × 10 −3 μm 2, respectively. The maximum values were obtained by 56Fe with an LET of 200 keV/μm. The mutagenicity (αm/αi) ranged from 2.05 to 7.99 × 10 −5 with the maximum value at 150 keV/μm. Furthermore, molecular analysis of mutants induced by charged particles indicates that higher LET beams are more likely to cause larger deletions in the hprt locus.

Clinical gain from improved beam delivery systems.

The feasibility of dynamic conformal heavy charged particle radiotherapy has been investigated at UCLBL, and shows high promise of: 1. an improved therapeutic ratio and 2. reduction in the number of treatment portals required for efficient treatment delivery. Assessment of dose to tumor and critical structures for several anatomical sites have been carried out using a normal tissue complication algorithm developed at LBL. For high-LET charged particle treatment delivery, dynamic conformal therapy using a raster scanned beam with variable modulation and multileaf collimator appears to be the optimal technique for treatment delivery.

Charged-Particle Mutagenesis: I. Cytotoxic and Mutagenic Effects of High-LET Charged Iron Particles on Human Skin Fibroblasts

Cytotoxic and mutagenic effects of high-LET charged iron (56Fe) particles were measured quantitatively using primary cultures of human skin fibroblasts. Argon and lanthanum particles and gamma rays were used in comparative studies. The span of LETs selected was from 150 keV/microns (330 MeV/u) to 920 keV/microns (600 MeV/u). Mutations were scored at the hypoxanthine guanine phosphoribosyl transferase (HPRT) locus using 6-thio-guanine (6-TG) for selection. Exposure to these high-LET charged particles resulted in exponential survival curves. Mutation induction, however, was fitted by the linear model. The relative biological effectiveness (RBE) for cell killing ranged from 3.7 to 1.3, while that for mutation induction ranged from 5.7 to 0.5. Both the RBE for cell killing and the RBE for mutagenesis decreased with increasing LET over the range of 1.50 to 920 keV/microns. The inactivation cross section (sigma i) and the action cross section for mutation induction (sigma m) ranged from 32.9 to 92.0 microns2 and 1.45 to 5.56 X 10(-3) microns2; the maximum values were obtained by 56Fe with an LET of 200 keV/microns. The mutagenicity (sigma m/sigma i) ranged from 2.05 to 7.99 X 10(-5) with an inverse relationship to LET.

Single-particle irradiation of mammalian cells at the radiological research accelerator facility: induction of chromosomal changes

Ionizing radiations have been shown to be able to induce the death of mammalian cells and initiate mutagenic or carcinogenic change. While all three end points are related through chromosomal changes, the latter in particular is of profound concern to human populations. We have undertaken a series of studies wherein mammalian cells were irradiated with low fluences of charged particles (protons, deuterons, helium ions) of defined LET from 10 to about 200 keV per micrometer. Frequencies of induced chromosomal changes were related to fluence at each LET, such that the induction of chromosomal changes per cell per charged particle could be estimated. However, for human exposures from densely ionizing radiation, such as the alpha particles from radon daughters, effects are dominated by the traversal of cells by single particles. Conventional experiments inevitably result in cells being exposed to a distribution (Poisson) of particle traversals. As the effect is unlikely to be a linear function of the number of traversals, a preferred approach would be to irradiate cells with exactly one (or any known number) of particles. To this end we are developing a dedicated beam line (microbeam) on a 4.2 MV Van de Graaff accelerator such that individual particles will vertically traverse individual living mammalian cells positioned by a microscope-based imaging system under computer control. “Conventional” low-fluence and “single-particle” studies will be compared, allowing critical evaluations of the potential of individual high LET charged particles to initiate change. This will have particular relevance both to consideration of the human health risks of radon daughter alpha particles and of basic mechanisms of chromosome aberration formation.

Radiobiological effects of high-let particles: DNA strand breaks

Fundamental studies in radiation biology with high-LET charged particles involve a systematic study of the physical, chemical, molecular and cellular processes. Water molecules and DNA present inside a cell constitute the important targets for energy deposition which eventually lead to either cell death or mutation or transformation. High-LET charged particles are very efficient in causing these types of damages. One of the primary lesions for causing injury to a cell is the production of DNA strand breaks. A good understanding of these breaks is essential before ultimate biological effects of heavy particles can be predicted. Based on known molecular mechanisms of the formation of strand breaks, a theoretical model is presented along with a comparison between the predictions of the model and experimental data. The studies have shown that LET is not a convenient physical parameter to relate the extent of strand breaks and one needs to know the microscopic distribution of energy (track structure). A discussion has also been presented to provide a background on various radiobiological characteristics of high-LET charged particles from the point of view of their uniqueness in damaging cancer cells.

Applications of the BEVALAC in biophysical research

The BEVALAC is a compound accelerator capable of producing energetic charged particle beams. These high-energy ion beams have been used in multidisciplinary research related to fundamental radiobiology and cancer therapy. Selected examples of research areas under investigation are presented. Available data on the phenomena of nuclear fragmentation and microscopic distribution of energy deposition are given as examples of research in physics with high-LET charged particles. Dose-response data are provided for cell survival and cell mutation end points which demonstrate the greater efficiency of BEVALAC ions compared to X-rays. The usefulness of nuclear secondaries (radioactive beams) in diagnostic studies is described from the point of view of Bragg peak therapy. The results of the last fourteen years of high-LET research have generated interest in the construction of hospital-based medical accelerators.