LET Dependence Research Articles

A preliminary analysis of LET effects in the dosimetry of proton beams using PRESAGE™ and optical CT

PRESAGE™ is a solid dosimeter based on a clear polyurethane matrix doped with radiochromic components (leuco dyes). On exposure to ionizing radiation a colour change is generated in the dosimeter, and hence an optical absorption or optical density change that can be read out by optical CT. The main focus of present investigations has been to investigate the possible LET dependence of PRESAGE™ to the dose deposited at the Bragg maxima using proton beam absorbed dose measurements, and the linearity of response of the dosimeter. Proton irradiations were performed using the proton beam facility at the Douglas Cyclotron, Clatterbridge Centre for Oncology (CCO) using a configuration that approximates the one routinely used in treatment of patients with ocular tumours. The samples were irradiated with both monoenergetic and modulated proton beams. Optical tomography measurements were carried out with our in-house CCD-based optical-CT system. Initial results for monoenergetic beams show that in PRESAGE™ the measured ratio of the Bragg peak dose to entrance dose is approximately 2:1 whereas the true value measured at CCO is approximately 5:1. For range-modulated proton beams, the absorbed dose close to the end of the proton range, i.e. at the Bragg peak, is underestimated by approximately 20% compared to the corresponding diode measurement. Further investigations are necessary to understand and quantify the effect of LET on PRESAGE™, and to measure the uncertainties related to our optical CT.

Response of 6LiF:Mg,Cu,Si and 7LiF:Mg,Cu,Si TLD Pairs to the Neutrons and Photon Mixtures

Replacement of dopant P by Si in LiF:Mg,Cu,P has provided an improved LiF:Mg,Cu,Si TLD material. Responses of indigenously developed Korean NaturalLiF:Mg,Cu,Si. 6LiF:Mg,Cu,Si and 7LiF:Mg,Cu,Si TLDs to gamma rays and neutrons were evaluated and compared with the commercially available LiF based TLDs. The intrinsic gamma ray sensitivity of NaturalLiF:Mg,Cu,Si was found to be the highest among the studied TLDs. The gamma ray sensitivities of 6LiF:Mg,Cu,Si and 7LiF:Mg,Cu,Si were about 27 and 32 times higher than that of Harshaw LiF:Mg,Ti. The relative neutron sensitivities (ratio of TL/mGy to neutron and gamma rays) of 6LiF TLDs doped with Mg,Cu,P and Mg,Cu,Si were about the same but lower than that of Harshaw 6LiF:Mg,Ti (TLD-600) due to the difference in their LET dependences. The high intrinsic TL sensitivity of 6LiF:Mg,Cu,Si outweighs the higher relative neutron sensitivity of TLD-600. In the testing following ANSI criteria, the performance quotient of the pairs of 6LiF:Mg,Cu,Si / 7LiF:Mg,Cu,Si held in Harshaw neutron TLD badge holders for the dosimetry in the mixed field of neutron and gamma rays was found to be less than 0.1 as against the maximum permitted value of 0.4.

Simulation of electron transport during beta irradiation

Optimisation of the laboratory beta irradiation for luminescence dosimetry requires a detailed understanding of the processes involved. For this purpose, we have developed a Monte-Carlo transport based model of the 90Sr/ 90Y source used on Risø TL/OSL readers. Here, we test this model against experimental data for an unshielded source. It is shown to be able to represent well the general form of the dependence of dose rate on depth and substrate. In contrast to data from film dosimetry (a measure of true energy deposition), we found CaF 2 luminescence dosimetry deviated systematically from the predicted values. We suggest that these differences must arise from an LET dependence of the luminescence efficiency on the electron spectrum depositing dose. This modelling begins to provide an analytical basis for the recent observation that the beta to gamma apparent dose rate ratio is phosphor dependent, even for phosphors with essentially identical energy absorption characteristics (e.g. quartz, feldspar, Al 2 O 3 ).

水の放射線分解 - LET、直接効果と間接効果

水の放射線分解 - LET、直接効果と間接効果

DNA fragments induction in human fibroblasts by radiations of different qualities

Experimental data on DNA double strand break (DSB) induction in human fibroblasts (AG1522), following irradiation with several radiation qualities, namely gamma rays, 0.84 MeV protons, 58.9 MeV u(-1) carbon ions, iron ions of 115 MeV u(-1), 414 MeV u(-1), 1 GeV u(-1), and 5 GeV u(-1), are presented. DSB yields were measured by calibrated Pulsed Field Gel Electrophoresis in the DNA fragment size range 0.023-5.7 Mbp. The DSB yields show little LET dependence, in spite of the large variation of the latter among the beams, and are slightly higher than that obtained using gamma rays. The highest yield was found for the 5 GeV u(-1) iron beam, that gave a value 30% higher than the 1 GeV u(-1) iron beam. A phenomenological method is used to parametrise deviation from randomness in fragment size spectra.

LET and Ion Species Dependence for Cell Killing in Normal Human Skin Fibroblasts

We studied the LET and ion species dependence of the RBE for cell killing to clarify the differences in the biological effects caused by the differences in the track structure that result from the different energy depositions for different ions. Normal human skin fibroblasts were irradiated with heavy-ion beams such as carbon, neon, silicon and iron ions that were generated by the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Science (NIRS) in Japan. Cell killing was measured as reproductive cell death using a colony formation assay. The RBE-LET curves were different for carbon ions and for the other ions. The curve for carbon ions increased steeply up to around 98 keV/microm. The RBE of carbon ions at 98 keV/microm was 4.07. In contrast, the curves for neon, silicon and iron ions had maximum peaks around 180 keV/microm, and the RBEs at the peak position ranged from 3.03 to 3.39. When the RBEs were plotted as a function of Z*2/beta2 (where Z* is the effective charge and beta is the relative velocity of the ion) instead of LET, the discrepancies between the RBE-LET curves for the different ion beams were reduced, but branching of the RBE-Z*2/beta2 curves still remained. When the inactivation cross section was plotted as a function of either LET or Z*2/beta2, it increased with increasing LET. However, the inactivation cross section was always smaller than the geometrical cross section. These results suggest that the differences in the energy deposition track structures of the different ion sources have an effect on cell killing.

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.

Dose and LET dependences of cellular oxygen consumption rate

Dose and LET dependences of cellular oxygen consumption rate

Exploration of `Over Kill Effect' of High-LET Ar- and Fe-ions by Evaluating the Fraction of Non-hit Cell and Interphase Death

The reason why RBE for cell killing fell to less than unity (1.0) with very high-LET heavy-ions ((40)Ar: 1,640 keV/microm; (56)Fe: 780, 1,200, 2,000 keV/microm) was explored by evaluating the fraction of non-hit cell (time-lapse observation) and cells undergoing interphase death (calculation based on our previous data). CHO cells were exposed to 4 Gy (30% survival dose) of Ar (1,640 keV/microm) or Fe-ions (2,000 keV/microm). About 20% of all cells were judged to be non-hit, and about 10% cells survived radiation damage. About 70% cells died after dividing at least once (reproductive death) or without dividing (interphase death). RBE for reproductive (RBE[R]) and interphase (RBE[I]) death showed a similar LET dependence with maximum around 200 keV/microm. In this LET region, at 30% survival level, about 10% non-survivors underwent interphase death. The corresponding value for very high-LET Fe-ions (2,000 keV/microm) was not particularly high (approximately 15%), whereas that for X-rays was less than 3%. However, reproductive death (67%) predominated over interphase death (33%) even in regard to rather severely damaged cells (1% survival level) after exposure to Fe-ions (2,000 keV/microm). These indicate that interphase death is a type of cell death characteristic for the cells exposed to high-LET radiation and is not caused by "cellular over kill effect". Both NHF37 (non-hit fraction at 37% survival) and inactivation cross-section for reproductive death (sigma[R]) began to increase when LET exceeded 100 keV/microm. The exclusion of non-hit fraction in the calculation of surviving fraction partially prevented the fall of RBE[R] when LET exceeded 200 keV/microm. On the other hand, the mean number of lethal damage per unit dose (NLD/Gy) showed the same LET-dependent pattern as RBE[R]. These suggest that the increase in non-hit fraction and sigma[R] with an increasing LET is caused by enhanced clustering of ionization and DNA damage which lowers the energy efficiency for producing damage and RBE.

DNA DSB induced by iron ions in human fibroblasts: LET dependence and shielding efficiency

This paper reports on DNA DSB induction in human fibroblasts by iron ions of different energies, namely 5, 1 GeV/u, 414 and 115 MeV/u, in absence or presence of different shields (PMMA, Al and Pb). Measure of DNA DSB was performed by calibrated Pulsed Field Gel Electrophoresis using the fragment counting method. The RBE–LET relationships for unshielded and shielded beams were obtained both in terms of dose average LET and of track average LET. Weak dependence on these parameters was observed for DSB induction. The shielding efficiency, evaluated by the ratio between the cross sections for unshielded and shielded beams, depends not only on the shield type and thickness, but also on the beam energy. Protection is only observed at high iron ions energy, especially at 5 GeV/u, where PMMA shield gives higher protection compared to Al or Pb shields of the same thickness expressed in g/cm 2.

Calculated DNA double-strand break and fragmentation yields after irradiation with He ions

Tracks of He ions with LET values ranging from 7.6 up to 210 keV/μm were calculated by PARTRAC and radiation damage to DNA induced in human fibroblast cells was analysed using a higher-order DNA target model. Taking into account the resolution of experimental measurements of DNA fragmentation, the computed yields of DSBs relate to the experimentally observed LET dependence. Whereas the yields of total DSBs and of short DNA fragments continue to rise with increasing LET, the yields of intermediate DNA fragments (about 1–1000 kbp) have a maximum around an LET value of 100 keV/μm. A similar LET dependence in measured data for cell inactivation supports the hypothesis that multiple DSBs leading to fragments in this size region have an important role for this end point.

Laparoscopic oophorectomy combined with breast surgery for breast cancer patients

We started a research program using heavy ion beams at the GSI Darmstadt accelerator to investigate radiation effects of heavy ions in microelectronic devices. The understanding of these radiation effects is of particular importance to estimate the radiation rise for microelectronic parts exposed to the cosmic radiation during space missions. In order to expose static RAM devices to heavy ion beams a test system was installed. Additionally we use CCD image sensors to measure the amount of charge deposited inside the sensitive volume of single picture elements. Measurements will be performed with ions of different charge and energy up to 1 GeV/N to investigate the LET dependence of the effects and possible influence of the track structure. The experimental setup is described in this paper and first results are presented.

LET dependence for somatic mutation induction by heavy ion beam with low-dose rate

LET dependence for somatic mutation induction by heavy ion beam with low-dose rate

LET dependency in radioadaptive response in human cultured cells

LET dependency in radioadaptive response in human cultured cells

LET and Ion-species Dependence for Cell Killing and Mutation Induction in Normal Human Fibroblasts.

We have been studying LET and ion species dependence of RBE values in cell killing and mutation induction. Normal human skin fibroblasts were irradiated with heavy-ion beams such as carbon (290 Mev/u and 135 Mev/u), neon (230 Mev/u and 400 Mev/u), silicon (490 Mev/u) and iron (500 Mev/u) ion beams, generated by Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences (NIRS). Cell killing effect was detected as reproductive cell death using a colony formation assay. Mutation induction in hprt locus was detected to measure 6-thioguanine resistant colonies. The RBE-LET curves of cell killing and mutation induction were different each ion beam. So, we plotted RBE for cell killing and mutation induction as function of Z*2/beta2 instead of LET. RBE-Z*2/beta2 curves of cell killing indicated that the discrepancy of RBE-LET curves was reconciled each ion species. But RBE-Z*2/beta2 curves of mutation induction didn't corresponded between carbon- and silicon-ion beams. These results suggested that different biological endpoints may be suitable for different physical parameter, which represent the track structure of energy deposition of ion beams.

A correction method for diamond detector signal dependence with proton energy.

Small dosimeters as solid state detectors can be useful for the dosimetric characterization and periodic quality control of radiotherapy proton beams. The calibration of solid state detectors for proton beams is not a solved problem especially for ophthalmologic proton beams, where these detectors present a LET-dependent signal. In this work a PTW diamond detector has been selected because of its good signal reproducibility (0.3%) and stable response with accumulated dose. A method that takes into account the LET dependence of the diamond detector signal, at 62 MeV proton beam, is here proposed. In particular an empirical correction factor, kDD(Eo) (Rres), has been determined as a function of the residual range quality index, to correct the diamond detector signal for a proton beam of incident effective energy E0= 62 MeV. A dedicated software allows us to use the diamond detector as an on-line reference dosimeter, where an ionization chamber may be difficult to use, or for periodic quality control procedures. The article also reports a comparison between the signal dependence on proton energy of silicon, diamond, and radiochromic film detectors.

A new perspective of carcinogenesis from protracted high-let radiation arises from the two-stage clonal expansion model

When applied to the Colorado Plateau miner population, the two-stage clonal expansion (TSCE) model of radiation carcinogenesis predicts that radiation-induced promotion dominates radiation-induced initiation. Thus, according to the model, at least for alpha-particle radiation from inhaled radon daughters, lung cancer induction over long periods of protracted irradiation appears to be dominated by radiation-induced modification of the proliferation kinetics of already-initiated cells rather than by direct radiation-induced initiation (i.e., mutation) of normal cells. We explore the possible consequences of this result for radiation exposures to space travelers on long missions. Still unknown is the LET dependence of this effect. Speculations of the cause of this phenomenon include the suggestion that modification of cell kinetics is caused by a “bystander” effect, i.e., the traversal of normal cells by alpha particles, followed by the signaling of these cells to nearby initiated cells which then modify their proliferation kinetics.

TLD-100 glow-curve deconvolution for the evaluation of the thermal stress and radiation damage effects

In this work, the dose response of TLD-100 dosimeters has been studied in a 62 MeV clinical proton beams. The signal versus dose curve has been compared with the one measured in a 60Co beam. Different experiments have been performed in order to observe the thermal stress and the radiation damage effects on the detector sensitivity. A LET dependence of the TL response has been observed. In order to get a physical interpretation of these effects, a computerised glow-curve deconvolution has been employed. The results of all the performed experiments and deconvolutions are extensively reported, and the TLD-100 possible fields of application in the clinical proton dosimetry are discussed.

Heavy ion radiolysis of organic materials

Radiation chemical processes induced by the passage of heavy charged particles in organic materials are discussed. An overview of the physics of energy loss processes on the subsequent chemical reactions is given. The effects of the dominant physical parameter, the linear energy transfer, (LET), on the radical and molecular products from organic compounds are examined. It is shown how the complex interaction of excited states and radical intermediates can sometimes make it difficult to predict the variation of final product yields with high LET radiation. Molecular hydrogen yields are examined in detail because of their fundamental and practical aspects. Hydrogen formation is especially interesting because it is produced by a variety of reactions with different relative LET dependences. The yield of H 2 is found to be very similar for liquid organic molecules and solid polymers containing the same molecule as the side group connected to the main carbon–carbon backbone. Biradical products and cross-linking yields are also found to be comparable suggesting very similar chemistry between the liquid and solid compounds.

LET dependence of scintillation yields in liquid xenon

By the end of the 1980s, a number of experiments were conducted on the LET dependence of scintillation yields in liquid argon in an effort to confirm the maximum response for relativistic heavy ions over the LET region from 10 2 to 10 3 MeV cm 2/ g , identified as “flat-top response”. Based on the experimental results, a flat-top response would be expected in liquid xenon as well, since liquid xenon possesses an electronic structure identical to that of liquid argon. In order to confirm such expectation, a series of scintillation yield measurements were made in liquid xenon using heavy ion beams of C (400 MeV/ n), Si (600 MeV/ n), Ar (650 MeV/ n) and Fe (500 MeV/ n) . As a result, the experiments have successfully confirmed the above-mentioned expectation.