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PubMed1998

Wall effects observed in tissue-equivalent proportional counters from 1.05 GeV/nucleon iron-56 particles.

Tissue-equivalent proportional counters (TEPCs) have been used to measure energy deposition in simulated volumes of tissue ranging in diameter from 0.1 to 10 microm. There has been some concern that the wall used to define the volume of interest could influence energy deposition within the sensitive volume because it has a density significantly greater than that of the cavity gas. These effects become important for high-velocity heavy ions. Measurements of energy deposition were made for 1 GeV/nucleon iron particles in a TEPC simulating a 1-microm-diameter sphere of tissue. The TEPC was nested within a particle spectrometer that provided identification and flight path of individual particles. Energy deposition was studied as a function of pathlength through the TEPC. Approximately 30% of the energy transfer along trajectories through the center of the detector escapes the sensitive volume. The response of the TEPC, for trajectories through the detector, is always larger than calculations for energy loss in a homogeneous medium. This enhancement is greatest for trajectories near the cavity/wall interface. An integration of the response indicates that charged-particle equilibrium is essentially achieved for a wall thickness of 2.54 mm. However, estimates of the linear energy transfer for the incident particles are influenced by these wall effects.

Gases↗
PubMed1996

[Effects of small fluxes of heavy charged particles on neurons of the rats' cerebral cortex in delayed periods following irradiation].

Dystrophic changes in neurons of the sensorimotor cortex of rat's brain in delayed periods following exposure to small fluxes, i.e. 104 and 105 particles/cm2, of accelerated carbon, fluorine, and oxygen ions with an energy of 300 MeV/nucleon and 137Cs gamma-rays at a dose of 1.0 Gy were quantitatively analyzed. Some structural changes of varying degree including death of a part of neurons were observed; however, most of the cells looked unaltered. Glial elements and small blood vessels also developed a reaction. Dynamics of the morphologic changes was studied. Higher effectiveness of small fluxes of heavy charged particles versus gamma-radiation at a dose of 1.0 Gy and dependence of the depth of structural disorders on linear energy transfer were demonstrated.

Animals↗
PubMed1992

Deoxyribonucleoprotein structure and radiation injury: cellular radiosensitivity is determined by LET infinity -dependent DNA damage in hydrated deoxyribonucleoproteins and the extent of its repair.

For decades, theories of cellular radiosensitivity relied upon the initial patterns of energy deposition to explain radiation lethality. Such theories are unsound: cellular (DNA) repair also underlies cellular radiosensitivity. For the charged particles encountered in deep space, both the types of DNA damage caused in cellular deoxyribonucleoproteins and the efficacies of their repair are dependent on linear energy transfer (LET infinity), and repair efficiency is also influenced by cell and tissue type, i.e., the actual recovery processes involved. Therefore, quality factors derived from radiation quality alone are inadequate parameters for assessing the radiation risks of space flight. Until recently, OH radicals formed in bulk nuclear water were believed to be the major causes of DNA damage that results in cell death, especially for sparsely ionizing radiations. That hypothesis has now been challenged, if not refuted. Lethal genomic DNA damage is determined mainly by energy deposition in deoxyribonucleoproteins, and their hydration shells, and charge (energy) transfer processes within those structures.

Animals↗
PubMed2006

Phenomenological study of the ionisation density-dependence of TLD-100 peak 5a.

Horowitz and collaborators have reported evidence on the structure of TLD-100 peak 5. A satellite peak, called 5a, has been singled out as arising from localised electron-hole recombination in a trap/luminescent centre, its emission mechanism would be geminate recombination and, therefore, its population would depend on incident radiation ionisation density. We report a phenomenological study of peak 4, 5a and 5 strengths for glow curves previously measured at UNAM for gammas, electrons and low-energy ions. The deconvolution procedure has followed strict rules to assure that the glow curve, where the presence of peak 5a is not visually noticeable, is decomposed in a consistent fashion, maintaining fixed widths and relative temperature difference between all the peaks. We find no improvement in the quality of the fit after inclusion of peak 5a. The relative contribution of peak 5a with respect to peak 5 does not seem to correlate with the radiation linear energy transfer.

Computer Simulation↗
PubMed1994

RBE-LET relationships for different types of lethal radiation damage in mammalian cells: comparison with DNA dsb and an interpretation of differences in radiosensitivity.

Relative biological effectiveness (RBE), as a function of linear energy transfer (LET), is evaluated for different types of damage contributing to mammalian cell reproductive death. Survival curves are analysed assuming a linear-quadratic dose dependence of lethal lesions. The linear term represents lethal damage due to single particle tracks, the quadratic term represents lethality due to interaction of lesions from independent tracks. RBE-LET relationships of single-track lethal damage, sublethal damage, potentially lethal damage and DNA double-strand breaks (dsb) are compared. Single-track lethal damage is shown to be composed of two components: damage that remains unrepaired in an interval between irradiation and assay, characterized by a very strong dependence on LET, with RBEs up to 20, and potentially lethal damage, which is weakly dependent on LET with RBEs < 3. Potentially lethal damage and sublethal damage depend similarly on LET as DNA dsb. The identification of these different components of damage leads to an interpretation of differences in radiosensitivity and in RBEs among various types of cells.

Animals↗
PubMed2004

Analysis of DNA damage generated by high-energy particles.

Ionizing radiation generates isolated and localized multiple (i.e., clustered) lesions in DNA, and the latter is believed to be primarily responsible for deleterious biological consequences such as cell killing and mutations. In the present study, we irradiated plasmid and lambda DNA by ionizing radiations with different linear energy transfer (LET) values, and compared the yields of isolated and clustered DNA lesions.

Carbon↗
PubMed1996

Fractionation and therapeutic ratio with neutron therapy.

Dose-response relationships of most tumours and normal tissues after neutron irradiation are characterized by relatively high alpha/beta ratios. Late reacting tissues are not as protected by low doses per fraction as they are after low linear energy transfer (LET) irradiation. Using the linear-quadratic (LQ) model with the time factor (when needed), we have calculated the therapeutic ratio (TR) as the ratio of the effect on a tumour (or an acutely responding tissue) over that on late reacting tissue. Hypothetical cases are given as well as a case derived from LQ parameters obtained experimentally in animals. It is shown that X-ray fractionation schemes that lead to a high TR will lead to a low neutron TR. For epithelial tumours, a neutron gain factor, GF (ratio of TR) is possible if neutron fractionation is compared to hypofractionated X-irradiation. Neutron treatment over conventional overall times of rapidly proliferating tumours carries a low TR and offers no GF. Neutron therapy for tumours with low alpha/beta ratios offer a potential advantage compared to tumours with high alpha/beta ratios.

Animals↗
PubMed1994

Measurements of trapped protons and cosmic rays from recent Shuttle flights.

We have flown two new charged particle detectors in five recent Shuttle flights. In this paper we report on the dose rate, equivalent dose rate, and radiation quality factor for trapped protons and cosmic radiation separately. A comparison of the integral linear energy transfer (LET) spectra with recent transport code calculations show significant disagreement. Using the calculated dose rate from the omni-directional AP8MAX model with IGRF reference magnetic field epoch 1970, and observed dose rate as a function of (averaged over all geographic latitude) and longitude, we have determined the westward drift of the South Atlantic anomaly. We have also studied the east-west effect, and observed a 'second' radiation belt. A comparison of the galactic cosmic radiation lineal energy transfer spectra with model calculations shows disagreement comparable to those of the trapped protons.

Atlantic Ocean↗
PubMed2002

Enhanced values of the RBE and H ratio for cytogenetic effects induced by secondary electrons from an X-irradiated gold gurface.

The low-energy secondary electrons emerging from the entrance surface of an X-irradiated gold foil increase the dose to cells in contact with or at micrometer distances from this surface (Radiat. Res. 150, 92-100, 1998). We examined the effect of the spectrum of these low-energy electrons on the RBE for cytogenetic effects and showed that this RBE was increased. A monolayer of surface-attached human T lymphocytes was exposed to 60 kV X rays in the absence or presence of a gold foil positioned immediately behind the cell layer or separated from it by a Mylar foil 0.9 or 2 microm thick. The enhancement of dose in the cell nuclei caused by the photoelectrons and Auger electrons emerging from the entrance surface of the gold foil was measured by TSEE dosimetry. Dose enhancement factors of 55.7, 46.6 and 37.5 were obtained with 0, 0.9 and 2 microm of Mylar inserted between the gold surface and the cell layer. This large enhancement results from the photoelectric effect in the gold foil, as shown by the accompanying Monte Carlo calculations of the secondary electron spectra at the gold surface. Auger electrons from the gold foil generally were not able to penetrate into the cell nuclei except for that fraction of the cells that had a very thin (< 0.7 microm) layer of cytoplasm and membranes between gold surface and cell nucleus. The dose-yield curves for dicentric chromosomes plus centric rings and for acentric fragments obtained after exposures without or with the gold foil were linear-quadratic. The coefficient alpha, the slope of the linear yield component, was increased in the presence of the gold foil and showed RBE values ranging from 1.7 to 2.2 compared to exposures in absence of the gold foil. The ratio of the yield of interstitial deletions and dicentrics (H ratio) was significantly increased from about 0.17 in the absence of the gold foil to about 0.22 in the presence of the gold foil. The increases in the RBE and the H ratio are interpreted in microdosimetric terms: The preferred occurrence of electron track ends in the vicinity of the gold surface causes an increase in the dose-mean restricted linear energy transfer in cell nuclei exposed to the photoelectrons and Auger electrons.

Chromosome Aberrations↗
PubMed2004

A new approach to the dosimetry of mixed radiation using a recombination chamber.

A new method of handling data derived from saturation curves of a recombination chamber is proposed. The method involves formation and extrapolation of a graph y(x), where y and x are simple, well-defined functions of the ratio of the ionisation current of the recombination chamber irradiated in the radiation field investigated to that in the field of a reference gamma radiation, using the same set of voltages, applied consecutively to the chamber. It makes it possible to determine separately the low linear energy transfer (LET) and high-LET components of an absorbed dose and also other important dosimetric quantities characterising a mixed radiation field.

Algorithms↗
PubMed1998

Sacrococcygeal chordomas: potential role of high LET therapy.

Chordomas are rare malignant neoplasms representing less than 3% of all primary bone tumors. They usually have a benign histological appearance and a slow growth rate. Their locoregional progression nearly always causes suffering and eventually death. Efforts to increase local control have included aggressive surgery, radiation therapy or combined approaches but locally free survival rates remain relatively low and have not exceeded 30% at 5 years. Sacrococcygeal chordomas, which represent approximately 50% of the localizations, have been investigated less frequently than those at the base of the skull or cervical region with primary or post-operative radiation therapy. The disappointing results with photon therapy in a multimodality approach and the good results reported by Schoenthaler with charged particles in the Lawrence Berkeley laboratory led us to propose fast neutron therapy in the management of inoperable or recurrent sacrococcygeal chordomas. Preliminary results obtained from only 12 patients indicate that high linear energy transfer (LET) therapy seems to be a good alternative for radical treatment of chordomas in the case of microscopic or macroscopic residual tumor.

Chordoma↗
PubMed2003

Study of induced thermoluminescence in CVD diamond film by low-energy X-rays.

For diamond film the one-hit model that is used to interpret low-energy X-ray thermoluminescence (TL) will require some modifications. After the films were irradiated with a superficial X-ray machine with different peak voltages, a two-compartment model with three parameters, the target size, the microscopic saturation factor and the high-LET saturation factor, was used to more precisely describe the TL response to X-ray with energies down to 10 kV. The microdosimetric distribution was calculated using single-event Monte Carlo code developed by authors together with EEDL cross-section data library. Some mechanistic insight into the physical aspect of radiation interaction with solid detectors can be obtained. The sensitive size in diamond was found to be about 15 nm. The saturation of one group of sublevels combined with the activation of another group of sublevels caused the relative efficiency to have a local minimum near 20 keV. The relative efficiency becomes higher below 10 keV, which is similar to the increasing relative biological effectiveness when the linear energy transfer passing through a biological system increases. The similarity made this material to be a molecular-scale dosimeter in the future.

Cobalt Radioisotopes↗
PubMed2000

Induction of minisatellite mutations in the mouse germline by low-dose chronic exposure to gamma-radiation and fission neutrons.

Germline mutation induction at mouse minisatellite loci by paternal low-dose (0.125-1 Gy) exposure to chronic (1.66 x 10(-4) Gy min(-1)) low-linear energy transfer (low-LET) gamma-irradiation and high-LET fission neutrons (0.003 Gy min(-1)) was studied at pre-meiotic stages of spermatogenesis. Both types of radiation produced linear dose-response curves for mutation of the paternal allele. In contrast to previous results using higher doses, the pattern of induction of minisatellite mutation after chronic gamma-irradiation was similar to acute (0.5 Gy min(-1)) exposure to X-rays, indicating that the elevated mutation rate was independent of the ability of the cell to repair damage induced immediately or over a period of up to 100 h. Chronic exposure to fission neutrons was more effective than acute or chronic low-LET exposure (relative biological effectiveness, RBE=3.36). The data also provide strong support for the previous conclusion that increases in minisatellite mutation rate are not caused by radiation-induced DNA damage at minisatellite loci themselves, but rather from damage induced by ionising radiation elsewhere in the genome/cell.

Animals↗
PubMed2007

Doses and LET spectra in the beam of 12C with energy 500 MeV amu-1.

This paper describes the results of experimental studies performed at the Joint Institute for Nuclear Research (JINR) in a (12)C ion beam with the primary nominal energy 500 MeV amu(-1). Data measured by means of a diamond detector and a spectrometer of linear energy transfer (LET) based on chemically etched track detectors are presented, analysed and discussed. LET spectra are also calculated by program SRIM.

Animals↗
PubMed2004

In vivo mammary tumourigenesis in the Sprague-Dawley rat and microdosimetric correlates.

Standard methods for risk assessments resulting from human exposures to mixed radiation fields in Space consisting of different particle types and energies rely upon quality factors. These are generally defined as a function of linear energy transfer (LET) and are assumed to be proportional to the risk. In this approach, it is further assumed that the risks for single exposures from each of the radiation types add linearly. Although risks of cancer from acute exposures to photon radiations have been measured in humans, quality factors for protons and ions of heavier atomic mass are generally inferred from animal and/or cellular data. Because only a small amount of data exists for such particles, this group has been examining tumourigenesis initiated by energetic protons and iron ions. In this study, 741 female Sprague-Dawley rats were irradiated or sham irradiated at approximately 60 days of age with 250 MeV protons, 1 GeV/nucleon iron ions or both protons and iron ions. The results suggest that the risk of mammary tumours in the rats sequentially irradiated with 1 GeV/nucleon 56Fe ions and 250 MeV protons is less than additive. These data in conjunction with earlier results further suggest that risk assessments in terms of only mean LETs of the primary cosmic rays may be insufficient to accurately evaluate the relative risks of each type of particle in a radiation field of mixed radiation qualities.

Animals↗
PubMed1992

Quality factor and dose equivalent investigations aboard the Soviet space station MIR.

Since Dec 1988, date of the French-Soviet joint space mission "ARAGATZ", the CIRCE device (Compteur Intégrateur de Rayonnement Complexe dans l'Espace) had recorded dose equivalent and quality factor inside the MIR station (380-410 km, 51.5 degrees). After the initial gas filling two years ago, the low pressure tissue equivalent proportional counter is still in good working conditions. Some results of three periods, viz Dec 1988, Mar-Apr 1989 and Jan-Feb 1990 are presented. The average dose equivalent rates measured are respectively 0.6, 0.8 and 0.6 mSv/day with a quality factor equal to 1.9. Some detailed measurements show the increasing of the dose equivalent rates through the SAA and near polar horns. The real time determination of the quality factors allows to point out high LET (Linear Energy Transfer) events with quality factors in the range 10-20.

Atlantic Ocean↗
PubMed1999

A comparison of gamma and neutron irradiation on Raji cells: effects on DNA damage, repair, cell cycle distribution and lethality.

The Comet assay (microgel electrophoresis) was used to study DNA damage in Raji cells, a B-lymphoblastoid cell line, after treatment with different doses of neutrons (0.5 to 16 Gy) or gamma rays (1.4 to 44.8 Gy). A better growth recovery was observed in cells after gamma-ray treatments compared with neutron treatments. The relative biological effectiveness (RBE) of neutron in cell killing was determined to be 2.5. Initially, the number of damaged cells per unit dose was approximately the same after neutron and gamma-ray irradiation. One hour after treatment, however, the number of normal cells per unit dose was much lower for neutrons than for gamma rays, suggesting a more efficient initial repair for gamma rays. Twenty-four hours after treatment, the numbers of damaged cells per unit dose of neutrons or gamma rays were again at comparable level. Cell cycle kinetic studies showed a strong G2/M arrest at equivalent unit dose (neutrons up to 8 Gy; gamma rays up to 5.6 Gy), suggesting a period in cell cycle for DNA repair. However, only cells treated with low doses (up to 2 Gy) seemed to be capable of returning into normal cell cycle within 4 days. For the highest dose of neutrons, decline in the number of normal cells seen at already 3 days after treatment was deeper compared with equivalent unit doses of gamma rays. Our present results support different mechanisms of action by these two irradiations and suggest the generation of locally multiply damaged sites (LMDS) for high linear energy transfer (LET) radiation which are known to be repaired at lower efficiency.

Cell Cycle↗
PubMed1995

Monte Carlo study of electron spectra and dose from backscattered radiation in the vicinity of media interfaces for monoenergetic photons of 50-1250 keV.

In the present paper electron fluence spectra and backscatter dose factors for monoenergetic photons (50, 100, 250 and 1250 keV) are presented. The influence of the atomic number of the backscatter materials glass, bone, steel and titanium is studied. For all radiations increases in dose were found in the immediate vicinity of the interface in a region extending over a distance determined by the photon energy. In a thin layer extending several tens of micrometers from the interface, a large increase in dose was found for 50 and 100 keV photons. The largest backscatter dose factor (3.5) was found for 50 keV photons and a steel interface. This large effect was found to be due largely to backscattered photoelectrons. K-shell fluorescent photons from titanium and steel were also included but were found to have almost no effect on backscatter dose factors or electron spectra. With both K-shell fluorescence and sampling of the photoelectron angular distribution switched off, much smaller backscatter factors were obtained. Beyond the thin region near the interface where the dose is increased, significant dose depressions were found for photons of these energies in a region extending several centimeters from the interface. The effect was maximum for 50 keV photons, for which dose depressions of about 35% for steel and titanium and 15% for glass and bone were obtained. For 250 and 1250 keV photons a more modest dose enhancement was found close to the interface (40-50%) but extending over a larger region (e.g. 5 mm for 1250 keV photons). Small differences in radiation quality nearer to the interface were found as expressed by track-averaged and dose-averaged restricted linear energy transfer, LET100,T and LET100,D, with a maximum effect for the 100 keV photons. For this radiation quality a decrease of 50% in LET100,T was found close to the interface.

Dose-Response Relationship, Radiation↗