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PubMed2004

Chromosome spatial clustering inferred from radiogenic aberrations.

PURPOSE: Analysing chromosome aberrations induced by low linear energy transfer (LET) radiation in order to characterize systematic spatial clustering among the 22 human autosomes in human lymphocytes and to compare their relative participation in interchanges. MATERIALS AND METHODS: A multicolour fluorescence in situ hybridization (mFISH) data set, specifying colour junctions in metaphases of human peripheral blood lymphocytes 72 h after in vitro exposure to low LET radiation, was analysed separately and in combination with previously published results. Monte Carlo computer simulations and mathematical modelling guided data analysis. RESULTS AND CONCLUSIONS: Statistical tests on aberration data confirmed two clusters of chromosomes, [1, 16, 17, 19, 22] and [13, 14, 15, 21, 22], as having their members being on average closer to each other than randomness would predict. The first set has been reported previously to be near the centre of the interphase nucleus and to be formed mainly by gene-rich chromosomes, while the second set comprises the nucleolus chromosomes. The results suggest a possible interplay between chromosome positioning and transcription. A number of other clusters suggested in the literature were not confirmed and considerable randomness of chromosome-chromosome juxtapositions was present. In addition, and consistent with previous results, it was found that chromosome participation in interchanges is approximately proportional to the two-thirds power of the DNA content.

Chromosome Aberrations↗

PubMed2005

Modelled microgravity does not modify the yield of chromosome aberrations induced by high-energy protons in human lymphocytes.

The aim was to evaluate the effect of modelled microgravity on radiation-induced chromosome aberrations (CAs). G0 peripheral blood lymphocytes were exposed to 60 MeV protons or 250 kVp X-rays in the dose range 0-6 Gy, and allowed to repair DNA damage for 24 h under either normal gravity or microgravity modelled by the NASA-designed rotating-wall bioreactor. Cells were then stimulated to proliferate by phytohaemagglutinin (PHA) under normal gravity conditions and prematurely condensed chromosomes were harvested after 48 h. CAs were scored in chromosomes 1 and 2 by fluorescence in-situ hybridization. Proliferation gravisensitivity was examined by cell growth curves and by morphological evaluation of mitogen-induced activation. Cell replication rounds were monitored by bromodeoxyuridine labelling. Modelled microgravity markedly reduced PHA-mediated lymphocyte blastogenesis and cell growth. However, no significant differences between normal gravity and modelled microgravity were found in the dose-response curves for the induction of aberrant cells or total interchromosomal exchange frequency. Rotating-wall bioreactor-based microgravity reproduced space-related alterations of mitogen stimulation in human lymphocytes but did not affect the yield of CAs induced by low-linear energy transfer radiation.

Cell Proliferation↗

PubMed2005

Apoptosis induced by high-LET radiations is not affected by cellular p53 gene status.

To learn more about the biological effects of high-linear energy transfer (LET) radiations, we examined radiation-induced apoptosis in response to high-LET radiations in cells with wild-type, mutated and null p53 gene. Three human lung cancer cell lines were used. These lines had identical genotypes, except for the p53 gene. Cells were exposed to X-rays or high-LET radiations (13 - 200 keV microm(-1)) using different nuclei ion beams. Cellular radiation sensitivities were determined with the use of colony-forming assays. Apoptosis was detected and quantified using Hoechst 33342 staining with fluorescence microscopy. It was found that (1) there was no significant difference in cellular sensitivity to high-LET radiation (>85 keV microm(-1)), although the sensitivity of wild-type p53 cells to X-rays was higher than that of mutated p53 or p53-null cells; (2) X-ray-induced apoptosis at higher frequencies in wild-type p53 cells when compared with mutated p53 and p53-null cells; and (3) Fe beams (200 keV microm(-1)) induced apoptosis in a p53-independent manner. The results indicate that high-LET radiations induces apoptosis in human lung cancer cells in a manner that does not seem to depend on the p53 gene status of the cells.

Apoptosis↗

PubMed2005

Full-color painting reveals an excess of radiation-induced dicentrics involving homologous chromosomes.

PURPOSE: To determine the ratio of homologous to heterologous dicentric chromosomes induced in human cells by ionizing radiation. This ratio is influenced by, and thus potentially informative about, underlying DNA damage/repair/misrepair processes and also the geometry of individual chromosome domains within the interphase nucleus. MATERIALS AND METHODS: 24-color mFISH (multiplex fluorescent in situ hybridization) was used to determine the ratio of 1-color (homologous) to 2-color (heterologous) dicentrics produced in human lymphocytes or fibroblasts by gamma-rays, alpha particles, or iron ions at various doses. Assuming that randomness independent of homology holds, the expected homologue:heterologue ratio for diploid human male cells is approximately 0.024, as shown by deriving a formula applicable to simple interchanges and then extending the result, via Monte Carlo simulation, to the general situation where complex aberrations are also considered. RESULTS AND CONCLUSIONS: There was a substantial excess of homologous dicentrics, with probability of occurrence by chance less than 0.02 for each of the three radiations and only about 10(-8) for all the data combined. Overall, approximately 18 homologous dicentrics were expected but 47 were found, including 11 involving chromosome 1. Observed excesses were similar for both sparsely and densely ionizing radiations. Geometric proximity of homologues is a possible explanation for the overabundance; in that case more extensive statistics should eventually uncover a linear energy transfer (LET) dependence. An alternative possibility, not ruled out by the present data, is homology-dependent misrepair.

Cell Culture Techniques↗

PubMed2005

Dose rate dependence of the relative biological effectiveness of 103Pd for continuous low dose rate irradiation of BA1112 rhabdomyosarcoma cells in vitro relative to acute exposures.

PURPOSE: To measure the relative biological effectiveness (RBE) of continuous low dose rate irradiation (CLDRI) using 103Pd sources relative to acute high dose rate irradiations (AHDRI) from a 250 kVp x-ray beam and an x-ray beam having an equivalent mono-energetic photon energy equal to the average energy of the 103Pd source for BA1112 rhabdomyosarcoma cells. MATERIALS AND METHODS: A customized 103Pd irradiator was built to provide CLDRI using 103Pd at different dose rates relevant to clinical interstitial brachytherapy to BA1112 rhabdomyosarcoma cells growing in exponential phase in culture. A special x-ray beam that simulates the photon energies emitted by the 103Pd source was also developed to provide acute high dose rate irradiation at those energies. Cell survival curves from different irradiation conditions were measured. The RBE with respect to AHDRI using standard 250 kVp x-rays was determined from the doses required to achieve a cell surviving faction of 0.01. RESULTS: For acute irradiation, the RBE of the x-rays simulating (103)Pd was 1.24 relative to 250 kVp x-rays. A profound dose rate effect was observed at low dose rates in the range of 6.8 - 14.4 cGy/h that are typical of permanent interstitial brachytherapy. At cell-surviving fraction of 0.01, the RBE of CLDRI at 6.8 and 14.4 cGy/h using 103Pd sources was reduced by a factor of 3 and 2, respectively, relative to the acute exposure. This observation is in good agreement with recent in vivo tumor cure studies performed on BA1112 tumor. CONCLUSION: The relative biological effectiveness of the photons emitted by 103Pd depends on both the linear energy transfer (LET) of the low energy photons and the dose rate of the irradiation. The higher LET of 103Pd photons is biologically more effective in killing BA1112 tumor cells compared to conventional 250 kVp x-rays when both are delivered at the same dose rate. But the gain in RBE that results from the higher LET can be quickly negated by the reduced dose rate of the irradiation.

Animals↗

PubMed2005

DNA DSB induced in human cells by charged particles and gamma rays: experimental results and theoretical approaches.

PURPOSE: To quantify the role played by radiation track structure and background fragments in modulating DNA fragmentation in human cells exposed to gamma-rays and light ions. MATERIALS AND METHODS: Human fibroblasts were exposed in vitro to different doses (in the range from 40 - 200 Gy) of (60)Co gamma-rays and 0.84 MeV protons (Linear Energy Transfer, LET, in tissue 28.5 keV/microm). The resulting DNA fragments were scored under two electrophoretic conditions, in order to optimize separation in the size ranges 0.023 - 1.0 Mbp and 1.0 - 5.7 Mbp. In parallel, DNA fragmentation was simulated both with a phenomenological approach based on the "generalized broken-stick" model, and with a mechanistic approach based on the PARTRAC (acronym of PARticle TRACk) Monte Carlo code (1.32 MeV photons were used for the simulation of (60)Co gamma-rays). RESULTS: For both gamma-rays and protons, the experimental dose response in the range 0.023 - 5.7 Mbp could be approximated as a straight line, the slope of which provided a yield of (5.3 +/- 0.4) x 10(-9) Gy(-1) bp(-1) for gamma-rays and (7.1 +/- 0.6) x 10(-9) Gy(-1) bp(-1) for protons, leading to a Relative Biological Effectiveness (RBE) of 1.3 +/- 0.2. From both theoretical analyses it appeared that, while gamma-ray data were consistent with double-strand breaks (DSB) random induction, protons at low doses showed significant deviation from randomness, implying enhanced production of small fragments in the low molecular weight part of the experimental range. The theoretical analysis of fragment production was then extended to ranges where data were not available, i.e. to fragments larger than 5.7 Mbp and smaller than 23 kbp. The main outcome was that small fragments (<23 kbp) are produced almost exclusively via non-random processes, since their number is considerably higher than that produced by a random insertion of DSB. Furthermore, for protons the number of these small fragments is a significant fraction (about 20%) of the total number of fragments; these fragments remain undetected in these experiments. Calculations for 3.3 MeV alpha particle irradiation (for which no experimental data were available) were performed to further investigate the role of fragments smaller than 23 kbp; in this case, besides the non-random character of their production, their number resulted to be at least as much as half of the total number of fragments. CONCLUSION: Comparison between experimental data and two different theoretical approaches provided further support to the hypothesis of an important role of track structure in modulating DNA damage. According to the theoretical approaches, non-randomness of fragment production was found for proton irradiation for the smaller fragments in the experimental size range and, in a significantly larger extent, for fragments of size less than 23 kbp, both for protons and alpha particles.

Cells, Cultured↗

PubMed2006

Minor contribution of direct ionization to DNA base damage inducedby heavy ions.

PURPOSE: The deleterious processes triggered by heavy ions on DNA were studied through the determination of the yield of a series of oxidized bases. Emphasis was placed on the estimation of the respective contribution of direct ionization and indirect effects, mostly by comparison with low linear energy transfer (LET) gamma-rays. MATERIAL AND METHODS: DNA samples and human monocytes were exposed either to gamma-rays emitted by a (60)Co source or to (12)C(6+) or (36)Ar(18+) ions. The levels of thymidine and 2'-deoxyguanosine oxidation products were determined by liquid chromatography coupled to tandem mass spectrometry subsequently to DNA digestion into nucleosides. RESULTS: The yields of thymidine lesions were similar to those of 8-oxo-7,8-dihydro-2'-deoxyguanosine within isolated DNA exposed either to gamma-rays or argon ions. Addition of spermine and Tris aimed at minimizing the indirect effect modified this ratio to the same extent with both types of radiation. In cells, the level of radiation-induced base damage was found to be correlated with the radiolytic yield of degrees OH that depends on the LET of the particle. In addition, radiation-induced thymidine and 2'-deoxyguanosine lesions were produced in similar amounts. In contrast, oxidation of 2'-deoxyguanosine was the main process when ionization was triggered in cellular DNA by ultraviolet laser-induced biphotonic processes. CONCLUSIONS: Predominant oxidation of 2'-deoxyguanosine is expected to be the hallmark of direct DNA ionization. The observation that thymidine and 2'-deoxyguanosine are equally damaged rules out a major contribution of the direct ionization in radiation-induced base damage to both isolated and cellular DNA by heavy ions. Dependence of the yield of lesions on the LET provides further support for this conclusion.

DNA↗

PubMed2006

Combined electron emission effects on the biologically damaging efficiency of 125I.

PURPOSE: One important obstacle in modeling the biological damage of 125I lies in the collective effects of the emitted electrons. The huge number of possible different ways of atomic rearrangement causes a strong variability in the number and energies of the emitted electrons. The aim of this work is to tabulate, separately for each atomic rearrangement pathway, the absorbed doses and the single-strand breaks (SSB) probabilities for different size nanospheres of liquid water. METHODS: A simple deterministic (non-stochastic) atomic rearrangement model of only 265 pathways is appropriate for modeling the electron-capture and internal conversion processes in 125I. The feasibility of the atomic rearrangement model is verified by comparing the SSB yield for ultrasoft X-ray sources with Lobachevsky's experimental data. RESULTS: A linear equation is sufficient for describing the SSB probability as a function of the dose absorbed in a small volume surrounding the labelled nucleotide 125I-dC. CONCLUSIONS: The application of a deterministic atomic rearrangement model to Auger electron emission spectra gives a new perspective to the study of low linear energy transfer (LET) radiation effects on DNA. The damages induced by point electron-capture (EC) sources and by macroscopic X-ray sources are compared.

DNA↗

PubMed2006

Analysis of gamma-rays induced chromosome aberrations: a fingerprint evaluation with a combination of pan-centromeric and pan-telomeric probes.

PURPOSE: To evaluate the types of induced chromosome aberrations after the exposure of peripheral blood to gamma-rays by the simultaneous detection of all centromeres and telomeres; and to analyse the suitability of different radiation fingerprints for the assessment of radiation quality in cases of recent exposures. MATERIAL AND METHODS: Peripheral blood samples were irradiated at 2, 4 and 6 Gy of gamma-rays. Cytogenetic analysis was carried out by fluorescence in situ hybridization (FISH) technique with pan-centromeric and peptide nucleic acid (PNA)-telomeric DNA probes. Cells were analysed using a Cytovision FISH workstation, chromosome aberrations and the length of the acentric fragments were recorded. RESULTS: The total number of the incomplete chromosome elements was 276. The ratio between incomplete elements and multicentrics was 0.38. The number of acentrics was 1096, 71% were complete acentrics, 15% incomplete acentrics, and 14% interstitial fragments. The relative length of complete, incomplete and interstitial acentrics fragments were 2.70 +/- 0.04, 1.91 +/- 0.07, and 1.42 +/- 0.04 respectively. The mean value of the F-ratio was 11.5 higher than the one, 5.5, previously obtained for alpha-particles. For the G-ratio there was no difference between gamma-rays and alpha-particles, 2.8 and 2.8 respectively. The mean value of the H-ratio for gamma-rays, 0.25, was lower than for alpha-particles 0.40. CONCLUSION: The results support that the percentage of incomplete chromosome aberrations depends on radiation type; low-linear energy transfer (LET) radiation would produces less incomplete aberrations than high-LET radiation. The F- and H-ratios seem to be good indicators of radiation quality, although a real estimation of the H-ratio is only possible using pan-telomeric probes.

Adult↗

PubMed1975

The oxygen-enhancement ratio for reproductive death induced by 3H or 125I damage in mammalian cells.

The oxygen-enhancement ratio (o.e.r.) for 3H- and 125I-induced cell death at 4 degrees C was determined in cultured Chinese hamster cells. The o.e.r. for cell death induced by 3H-thymidine was 3-2, essentially the same value as that previously reported for X-ray induced cell death. For cell death induced by 125I-iododeoxyuridine (125IdUrd), the o.e.r. was less than 1-4. The lower o.e.r. for 125I-induced death was not due to the presence of the base analogue itself, since cells that had incorporated unlabelled IdUrd and were X-irradiated had an o.e.r. of 2-8 and cells that were inactivated by 3H-IdUrd damage at 4 degrees C had an o.e.r. even greater than 3. These results suggest that 125I-decay damage, like high-linear-energy-transfer radiation damage, is only slightly increased by the presence of oxygen.

Cell Survival↗

PubMed1979

Modification of high LET radiation-induced damage and its repair in yeast by hypoxia.

The lethal response of a diploid yeast strain BZ34 to densely ionizing radiations from the reaction 10B(n, alpha)7 Li was studied. The values for relative biological effectiveness (r.b.e.) and oxygen enhancement ratio (o.e.r.) for this radiation compare favourably with the data obtained with charged particles on the same strain of yeast. Recovery from potentially lethal damage was also studied by post-irradiation holding under non-nutrient conditions. In order to understand the role of oxygen in the recovery process, the investigation covered the following treatment regimens: (a) aerobic irradiation and aerobic holding (A-A), (b) aerobic irradiation and hypoxic holding (A-H), (c) hypoxic irradiation and hypoxic holding (H-H) and (d) hypoxic irradiation and aerobic holding (H-A). It has been found that the presence of oxygen is essential for recovery from the damage induced by both gamma rays and high linear energy transfer (LET) radiations. The extent of recovery was larger for gamma-induced damage than for damage induced by high LET radiation (alpha + 7Li) for the A-A condition. In the H-H condition, while only a slight recovery was seen for gamma-induced damage, it was totally absent for high LET damage. For the modality A-H, it was found that there is not recovery from the sparsely ionising gamma radiation-induced damage. The implications of these results for the treatment of malignant tumours by radiotherapy are briefly discussed.

Alpha Particles↗

PubMed1981

Effects of heavy ions on rabbit tissues: loss of electroretinogram and DNA repair in retinal photoreceptor cells.

Optic and proximate tissues of New Zealand white rabbits were exposed to the Bragg plateau regions of 530 MeV/amu Ar ions and 365 MeV/amu Ne ions and also to 60Co gamma rays. The linear energy transfers (LET infinity s) for the radiations were 90 +/- 5, 35 +/- 3 and 0.3 keV/micrometer, respectively. Single acute exposures to all three radiations caused only limited changes in the electroretinogram (ERG) until a critical dose was approached. At that dose, which was 45 +/- 3 Gy for all three types of radiation, a sudden loss in the b-wave of the ERG occurred. From less accurate measurements, it appeared that the response of the a-wave of the ERG was similar. The critical dose for ERG loss was reflected also in the rejoining of strand breaks in the DNA of retinal photoreceptor cells. Above the critical dose, the rejoining of DNA strand breaks was limited; below the critical dose, the rejoining of DNA strand breaks approached completion even though it took days to do so. In the latter respect, the response of photoreceptor cells to heavy-ion irradiation in situ is different from that reported for proliferating cells in culture.

Animals↗

PubMed1981

Effects of heavy ions on rabbit tissues: alopecia.

During extended investigations of the radiation responses of optic and proximate tissues of New Zealand white rabbits (Oryctolagus cuniculus), the skin surrounding the eyes was exposed to the Bragg plateau regions of 530 MeV/amu Ar ions and 365 MeV/amu Ne ions, and also to 60Co gamma rays. The linear energy transfers (LET infinity s) for the radiations were 90 +/- 5, 35 +/- 3, and 0.3 keV/micron, respectively. Alopecia was measured after irradiation with subjective indices of baldness (0-5) that were calibrated against hair loss in other rabbits irradiated with Ne and Ar ions at the central region of the ear. The patterns of post-irradiation alopecia, which conformed to the model proposed by Casarett (1963) for the long-term expression of radiation damage in proliferating animal tissues, were modified by radiation quality in the following ways: (1) For acute (early) losses, the magnitude of the recovery phase decreased as the LET infinity of the incident radiation increased; (2) In the intermediate or plateau phase of radiation response, the values of relative biological effectiveness (r.b.e) approached those reported for proliferating cells in culture, but differences were found that may be related to the changes in the magnitude of the recovery phase; (3) For a given level of intermediate damage, the onset of the late degenerative phase of radiation response, which was expressed years after irradiation, occurred earlier the higher the LET infinity of the incident radiation. Taken overall, the most important finding of these experiments was that the r.b.e.s for alopecia varied with post-irradiation time.

Alopecia↗

PubMed1982

Effects of heavy ions on rabbit tissues: cataractogenesis.

As part of an investigation of the responses of optic and proximate tissues to heavy-ion irradiation, the lenses of New Zealand white rabbits were exposed to the Bragg plateau regions of 530 MeV/amu Ar ions and 365 MeV/amu Ne ions and also to 60Co gamma-photons. The linear energy transfers (LET infinity s) for the radiations were 90 +/- 5, 35 +/- 3, and 0.3 keV/micrometer, respectively. After irradiation, lenticular opacities were monitored through their incipient and/or clinical stages (less than or equal to 5 years) by slit-lamp biomicroscopy and scored with subjective, but well-defined, indices. Cataractogenesis, which progressed according to the model proposed by Rubin and Casarett (1968), was modified by radiation quality in the following ways. (1) The rate of development of the early (acute) stage increased with the LET infinity of the incident radiation; (2) at the intermediate (plateau) stage, the values for the relative biological effectiveness (r.b.e.) of the heavy ions were similar to those reported for proliferating cells in culture; (3) for a given intermediate level, the onset of late cataractogenesis occurred earlier the higher the LET affinity of the radiation involved. As with alopecia, the r.b.e.s for cataractogenesis varied with post-irradiation time.

Animals↗

PubMed1983

Survival responses of cell subpopulations isolated from a heterogeneous human colon tumour after combinations of hyperthermia and X-irradiation.

In summary, this research has investigated the effects of combined modality treatment (i.e., low linear energy transfer ionizing radiation and hyperthermia at 42.5 degrees C) on the survival responses of two tumour subpopulations (designated clones A and D) obtained from a heterogeneous human colon adenocarcinoma. A constant hyperthermic exposure (2 hours at 42.5 degrees C) was given either 3 min before or 3 min after graded exposure to X-rays. An isobologram analysis (Steel and Peckham 1979) of the clonogenic survival responses of the two tumour subpopulations showed that the clone A responses were within the envelope of additivity for either sequence of application. In contrast, the responses of the clone D tumour subpopulation exhibited a supra-additive response to the combined treatments with the sequence of heat followed by X-irradiation being somewhat more effective than the sequence of X-irradiation followed by heat. These data indicate that the responses of tumour subpopulations obtained from heterogeneous solid tumours to combined modality treatments may vary in an, at present, unpredictable manner.

Cell Survival↗

PubMed1985

Characterization of hydroxyl radical-induced damage after sparsely and densely ionizing irradiation.

The extent of hydroxyl radical mediated cell inactivation was measured for a variety of particle beams ranging from 8.5 Me V/u neon ions to 570 Me V/u argon ions. In general, the fraction of the total radiosensitivity caused by OH decreases from close to 60 per cent at low ionization density or low linear energy transfer (low LET) to close to 25 per cent at high LET for aerobically irradiated mammalian cells. The extent of OH induced cell lethality can be explained in terms of LET infinity only for low energy or low atomic number particles where fragmentations and complicated track structures do not contaminate the characteristic particle LET. For example, at a calculated LET infinity of 100 ke V/micron, the OH mediated fraction of the total radiation damage is about 25 per cent for low energy carbon but close to 40 per cent for high energy carbon ions. For low energy charged nuclei of approximately the same energy, as the 5.4-13.4 MeV/u He, Li, C and Ne ions in this report, there is a predictable diminution of the OH mediated effect with increasing LET infinity; however, the biological effect cannot be predicted accurately from calculated LET infinity values for high energy particle irradiation, nor indeed from a variety of low energy charged particles of quite different energies (incident velocities). This illustrates the unsuitability of using LET as a unifying parameter, except under specific circumstances. As more is learned about the energy deposition for energized charged particles in terms of track structure (core and penumbra), it may be possible to characterize the radiobiological data with a better physical parameter than LET infinity.

Animals↗

PubMed1987

Heavy ion effects on cellular DNA: strand break induction and repair in cultured diploid lens epithelial cells.

The relative biological effectiveness (RBE) for the induction of DNA strand breaks and the efficiency of repair of these breaks in cultured diploid bovine lens epithelial cells was measured, using accelerated heavy ions in the linear energy transfer (LET)-range up to 16,200 keV/micron. At LET values above 800 keV/micron, the number of DNA strand breaks induced per particle increases both with the atomic number of the projectile and with its kinetic energy. About 90 per cent or more of the strand breaks induced by ions with an LET of less than 10,000 keV/micron are repaired within 24 h. Repair kinetics show a dependence on the particle fluence (irradiation dose). At higher particle fluences a higher proportion of non-rejoined breaks is found, even after prolonged periods of incubation. At any LET value, repair is much slower after heavy-ion exposure than after X-irradiation. This is especially true for low energetic particles with a very high local density of energy deposition within the particle track. At the highest LET value (16,200 keV/micron), no significant repair is observed.

Animals↗

PubMed1989

The irradiation of V79 mammalian cells by protons with energies below 2 MeV. Part I: Experimental arrangement and measurements of cell survival.

The relative biological effectiveness (RBE) has been determined for protons with mean energies of 1.9, 1.15 and 0.76 MeV, from measurements of the survival of V79 Chinese hamster cells. The cells are supported as a monolayer and are swept through a beam of scattered protons produced using a 4 MeV Van de Graaff accelerator. An estimation of the dose and unrestricted linear energy transfer (LET) variation within the sensitive volume of the cells is given for the three proton energies. The RBEs for cell survival (relative to 250 kVp X-rays) at the 10 per cent survival level are 1.6, 1.9 and 3.36 for protons with track-average LETs of 17, 24 and 32 keV microns-1 respectively, and the data suggest that protons are most effective at about 40-50 keV microns-1. It is shown that the proton RBEs can be reconciled with those of other light ions if plotted against z*2/beta 2 (where z* is the effective charge and beta is the relative velocity) rather than against LET.

Animals↗