Effects on adult cognitive function after neonatal exposure to clinically relevant doses of ionising radiation and ketamine in mice.

BACKGROUND: Radiological methods for screening, diagnostics and therapy are frequently used in healthcare. In infants and children, anaesthesia/sedation is often used in these situations to relieve the patients' perception of stress or pain. Both ionising radiation (IR) and ketamine have been shown to induce developmental neurotoxic effects and this study aimed to identify the combined effects of these in a murine model. METHODS: Male mice were exposed to a single dose of ketamine (7.5 mg kg-1 body weight) s.c. on postnatal day 10. One hour after ketamine exposure, mice were whole body irradiated with 50-200 mGy gamma radiation (137Cs). Behavioural observations were performed at 2, 4 and 5 months of age. At 6 months of age, cerebral cortex and hippocampus tissue were analysed for neuroprotein levels. RESULTS: Animals co-exposed to IR and ketamine displayed significant (P≤0.01) lack of habituation in the spontaneous behaviour test, when compared with controls and single agent exposed mice. In the Morris Water Maze test, co-exposed animals showed significant (P≤0.05) impaired learning and memory capacity in both the spatial acquisition task and the relearning test compared with controls and single agent exposed mice. Furthermore, in co-exposed mice a significantly (P≤0.05) elevated level of tau protein in cerebral cortex was observed. Single agent exposure did not cause any significant effects on the investigated endpoints. CONCLUSION: Co-exposure to IR and ketamine can aggravate developmental neurotoxic effects at doses where the single agent exposure does not impact on the measured variables. These findings show that estimation of risk after paediatric low-dose IR exposure, based upon radiation dose alone, may underestimate the consequences for this vulnerable population.

Dose and time dependent apoptotic response in a human melanoma cell line exposed to accelerated boron ions at four different LET.

The aim was to investigate and compare the influence of linear energy transfer (LET), dose and time on the induction of apoptosis in a human melanoma cell line exposed to accelerated light boron ((10)B) ions and photons. Cells were exposed in vitro to doses up to 6 Gy accelerated boron ions (40, 80, 125 and 160 eV nm(-1)) and up to 12 Gy photons (0.2 eV nm(-1)). The induction of apoptosis was measured up to 9 days after irradiation using morphological characterization of apoptotic cells and bodies. In parallel, measurements of cell-cycle distribution, monitored by DNA flow cytometry, and cell survival based on the clonogenic cell survival assay, were performed. In addition, the induction and repair of DNA double-strand breaks (DSB), using pulsed-field gel electrophoresis (PFGE) were studied. Accelerated boron ions induced a significant increase in apoptosis as compared with photons at all time points studied. At 1-5 h the percentage of radiation-induced apoptotic cells increased with both dose and LET. At the later time points (24-216 h) the apoptotic response was more complex and did not increase in a strictly LET-dependent manner. The early premitotic apoptotic cells disappeared at 24 h following exposure to the highest LET (160 eV nm(-1)). A postmitotic apoptotic response was seen after release of the dose-, time- and LET-dependent G2/M accumulations. The loss of clonogenic ability was dose- and LET-dependent and the fraction of un-rejoined DSB increased with increasing LET. Despite the LET-dependent clonogenic cell killing, it was not possible to measure quantitatively a LET-dependent apoptotic response. This was due to the different time course of appearance and disappearance of apoptotic cells.

Chromatin- and temperature-dependent modulation of radiation-induced double-strand breaks.

PURPOSE: To investigate the influence of chromatin organization and scavenging capacity in relation to irradiation temperature on the induction of double-strand breaks (DSB) in structures derived from human diploid fibroblasts. MATERIALS AND METHODS: Agarose plugs with different chromatin structures (intact cells+/-wortmannin, permeabilized cells with condensed chromatin, nucleoids and DNA) were prepared and irradiated with X-rays at 2 or 37 degrees C and lysed using two different lysis protocols (new ice-cold lysis or standard lysis at 37 degrees C). Induction of DSB was determined by constant-field gel electrophoresis. RESULTS: The dose-modifying factor (DMF(temp)) for irradiation at 37 compared with 2 degrees C was 0.92 in intact cells (i.e. more DSB induced at 2 degrees C), but gradually increased to 1.5 in permeabilized cells, 2.2 in nucleoids and 2.6 in naked DNA, suggesting a role of chromatin organization for temperature modulation of DNA damage. In addition, DMF(temp) was influenced by the presence of 0.1 M DMSO or 30 mM glutathione, but not by post-irradiation temperature. CONCLUSION: The protective effect of low temperature was correlated to the indirect effects of ionizing radiation and was not dependent on post-irradiation temperature. Reasons for a dose modifying factor <1 in intact cells are discussed.

Difference in the induction, but not in the repair, of X-ray- and nitrogen ion-induced DNA single-strand breaks as measured using human cell extracts.

PURPOSE: To compare the repair efficiency of X-ray (low linear energy transfer [LET]) and nitrogen ion (high LET)-induced single-strand breaks (SSB) in a human cell-free end-joining system. MATERIALS AND METHODS: SSB were introduced into a bacterial plasmid, pBR322, by X-rays (4 MeV photons) and nitrogen ions with an LET=125 keV micro m(-1). Repair efficiency was studied under incubation with the protein extracts from human squamous carcinoma cells, UT-SCC-5. RESULTS: A several fold higher dose of nitrogen ion radiation compared with X-ray radiation was needed to induce a similar loss of supercoiled plasmid DNA. There was no difference in the repair efficiency of SSB induced by these two types of radiation. CONCLUSION: The data indicate that X-rays at 25 Gy and nitroging ions at 100 Gy radiation doses, under condition of low scavenging capacity (10 mM Tris), induce SSB of similar complexity or, alternatively, differences in SSB complexity do not alter the repair rate.

Relative biological effectiveness of boron ions on human melanoma cells.

PURPOSE: To compare the difference in relative biological effectiveness (RBE) between (10)B ions and a (60)Co gamma-ray beam for human melanoma cells using in vitro cell survival based on a clonogenic assay. MATERIALS AND METHODS: Cells were irradiated in vitro under aerobic conditions with (60)Co and (10)B ions with different linear energy transfer (LET) (40, 80 and 160 eV nm(-1)). The dose to the cells was determined using ferrous sulphate dosimetry and an ionisation chamber. The standard linear-quadratic model and the newly proposed repairable conditionally repairable damage (RCR) model were used to calculate the RBE. RESULTS: The RBE at 10% cell survival for 40, 80 and 160 eV nm(-1) boron ions compared with (60)Co were 1.98 (1.83-2.22), 2.85 (2.64-3.11) and 3.37 (3.17-3.58), respectively, of almost independence of the model used in the calculation. CONCLUSIONS: Different cell survival models may generate different RBE, especially at low doses and high cell survival levels.

Induction and repair of DNA damage in human cells at different stages of differentiation.

Use of cellular systems capable of undergoing in vitro differentiation can give useful information on the basic mechanisms of cellular radiation sensitivity. During differentiation the cellular organisation, including the nuclear structure and the intracellular concentration of several compounds and enzymes change drastically. Accordingly, radiation response to ionising radiation is also expected to change. The human proerythroblastoid cell line K562 can be induced to pseudoerythroid differentiation. This process has been characterised and studies have been carried out on DNA single strand break and double strand break induction and repair before and after differentiation commitment. Rejoining studies have been performed for both types of damage and correct double strand break rejoining has been also measured in particular genomic locations. An overview is presented of these results together with preliminary data recently obtained on radiation induced DNA fragmentation as a function of radiation quality.

Radiation quality dependence of DNA damage induction.

Analysis of DNA fragmentation and repair in relation to radiation quality may give important information about the role of break complexity and correlated double strand breaks (DSBs). DNA fragment analysis was performed by pulsed-field gel electrophoresis after exposure to different radiation qualities. Normal human fibroblasts were irradiated with boron ions (40, 80 and 160 keV.micron-1), nitrogen ions (80, 125, 175 and 225 keV.micron-1) and neon ions (225 and 300 keV.micron-1). The amount of DNA less than 1.1 Mbp decreased with increasing linear energy transfer (LET) for all three ions. When theoretical random distributions were subtracted from the experimental data for 225 keV.micron-1 nitrogen ions in all size intervals (5-5700 kbp), there was a significant non-random distribution of DSBs for sizes up to 1-3 Mbp. This non-random distribution of breaks, probably produced by intra-track correlated DSBs, may constitute a substantial portion of the high-LET induced DSBs.

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/micrometers 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.

Induction and rejoining of DNA double-strand breaks in normal human skin fibroblasts after exposure to radiation of different linear energy transfer: possible roles of track structure and chromatin organization.

DNA double-strand breaks are nonrandomly induced by high-LET radiation. Differences in the induction and rejoining of DSBs after irradiation with ions having different LET were detected by fragment analysis. The data obtained indicate that the track structure of the traversing particle and its interaction with the different chromatin structures of the cellular DNA influence the yield as well as the distribution of the induced damage. The induction and rejoining of clustered DSBs induced by the same nitrogen ion fluence at LETs of 80-225 keV/microm were investigated by a detailed analysis of the DNA fragmentation patterns in normal human fibroblasts. The DSBs in the cells were allowed to rejoin during incubations for 0-20 h. Two separate pulsed-field gel electrophoresis protocols were used, optimized for separation of fragments in the size ranges 1-6 Mbp and 5 kbp-1.5 Mbp. A strong influence of LET on the level of DSB induction was evident. The DSB yield increased from 4.5 +/- 0.2 to 10.0 +/- 0.3 DSBs per particle traversal through the cell nucleus when LET increased from 80 to 225 keV/microm. Further, the size distribution of the DNA fragments showed a significant dependence on radiation quality, with an excess of fragments at 50-200 kbp and around 1 Mbp. Differences in repair kinetics were also evident, with slower rejoining for increasing LET, and the initial nonrandom fragment distributions were still present after 1 h of repair.

DNA fragmentation induced in K562 cells by nitrogen ions.

This study was aimed at investigating the radiation induced DNA fragmentation pattern as a function of cellular differentiation and radiation quality. DNA double strand breaks (DSB) induced by gamma-rays were analyzed in K562 human proerythroblasts before (AP cells) and after (D cells) differentiation induction while DNA DSB induced by 125 keV/micrometers N-ions have been studied in AP cells. Pulsed-Field Gel Electrophoresis (PFGE) of cellular DNA was used to determine the DSB yield by analysis of the Fraction of Activity Released (FAR) and of the fragmentation pattern in a specific size range (5.7-0.225 Mbp). The results so far obtained show that the DSB induction by gamma-rays is different if evaluated with the FAR or with the fragmentation analysis. The DSB yield obtained with the former method is about 1.4 times higher in AP respect to D cells while the latter method indicates that more fragments are produced in D cells. Comparison between gamma-rays and N-ions in AP cells shows that no significant differences are detected by the FAR analysis; otherwise fragmentation analysis demonstrates a higher effectiveness of nitrogen ions.

DNA damage induced by radiation of different linear energy transfer: initial fragmentation.

PURPOSE: To investigate DNA fragmentation as a function of linear energy transfer (LET) after exposure to accelerated ions in the LET range 40-225 keV/microm. MATERIALS AND METHODS: Fragmentation patterns of double-stranded DNA in the range 5 kilobasepairs (kbp) to 5.7 megabasepairs (Mbp) were analysed after irradiation of low-passage GM 5758 normal human fibroblast cells with 60Co-photons, helium ions at 40 keV/microm and high-LET nitrogen ions between 80 and 225 keV/microm. Two separate pulsed-field gel electrophoresis protocols were used, optimized for separation of 1-6 Mbp and 5 kbp to 1.5 Mbp fragments. RESULTS: An increased probability of formation of short and medium-sized DNA fragments was revealed following high-LET irradiation. The DNA double-strand break (dsb) induction yields were, respectively, 5.8 and 6.9-8.8 x 10(-9) dsb bp(-1) Gy(-1) for 60Co-photons and ions. The ion yields were some 80-110% higher than those calculated according to a conventional approach, disregarding the fragment distributions. For photons, the yield was 13% higher. The corresponding relative biological effectiveness (RBE) of dsb induction was in the range 1.2-1.5. CONCLUSIONS: A significant non-random contribution to the number of dsb after irradiation with high-LET was confirmed by detailed fragment analysis using pulsed-field gel electrophoresis. The LET had a strong influence on the initial DNA fragment distribution, and hence also on the induction yields measured. However, when the LET was increased to the highest values studied for nitrogen ions, the yield decreased slightly.

Rejoining of DNA fragments produced by radiations of different linear energy transfer.

PURPOSE: To analyse the rejoining of DNA double-strand breaks (dsb) produced by high-linear energy transfer (LET) ions, with the specific focus on the influence, on the rejoining estimates, of the way dsb are distributed along chromosomes. MATERIAL AND METHODS: Low passages of normal human fibroblasts (GM5758) were irradiated with 60Co photons, 40 keV/microm helium ions or nitrogen ions with LETs of 80, 125, 175 and 225 keV/microm. The double-stranded DNA fragment distributions, ranging from 5 kbp to 5.7 Mbp, were assayed by pulsed-field gel electrophoresis after repair incubation for 0-22 h. RESULTS: The rejoining was biphasic and the half-times of the two phases were 15 min and 2-3h, respectively, and were independent of LET. Although the majority of breaks were rejoined by the fast phase, both the fraction of dsb rejoined by the slow phase and the fraction of unrejoined dsb at 20-22h increased with increasing LET. CONCLUSIONS: DNA fragment analysis detected LET-dependent differences in the amount of rejoining while the half-times were independent of LET. The majority of dsb were rapidly rejoined even after high-LET irradiation. If fragment-size distribution is not taken into account, both the fraction of breaks rejoined by slow kinetics, and the fraction of unrejoined breaks, can be overestimated when the LET is increased.

Induction and rejoining of large DNA fragments after ion irradiation.

Radiation-induced DNA double-strand breaks (DSBs) were analyzed by separating large DNA fragments by pulsed-field gel electrophoresis. Human U-343MG glioma and K562 erythroleukemia cells were irradiated with 60Co gamma rays or nitrogen ions with high linear energy transfer (125 keV/microm). By comparing the fraction of DNA released into the gel below different size thresholds, corresponding to megabase-pair-sized DNA fragments, the relative effectiveness of the nitrogen ions was found to be dependent on both dose and the threshold size used in the evaluation. This dose dependence was most evident for the smallest threshold (6 Mbp) and was due to a linear dose response for release of the fragments for the ions compared to the curvilinear response for the gamma rays. The two curves intersected, and the relative yield of fragments (nitrogen ions/gamma rays) decreased from more than 3 below 1.5 Gy to 0.8 at 30 Gy. For the larger sizes (6-10.5 Mbp), the relative yield was constant at around 0.7. Thus the ion-induced fragments were shifted to smaller sizes compared to the 60Co gamma rays, and the data for nitrogen ions could not be fitted to random fragment distributions at doses < or =20 Gy. From these results, we conclude that a substantial fraction of the DSBs induced by heavy ions were nonrandomly distributed, correlated with DSBs within a region of < or =2 Mbp. After a dose of 20 Gy, the rejoining curves for ion-induced DSBs were different for each fragment size, resulting in different levels of unrejoined breaks after 6 h.

A review of dsb induction data for varying quality radiations.

PURPOSE: This short review summarizes the data obtained with various techniques for measuring the yields of double strand breaks (dsb) produced by particle radiations of differing linear energy transfer (LET) in order to obtain relative biological effectiveness (RBE) values. RESULTS AND CONCLUSIONS: Studies aimed at understanding the interactions of different types of radiation with cellular DNA have monitored the yields of DNA dsb versus radiation quality. Several techniques have been used to measure dsb yields in mammalian cells, and these include: neutral sedimentation gradients, filter elution and more recently pulsed field gel electrophoresis techniques (PFGE). Recent developments in PFGE have allowed the measurement of both the yields and the distribution of breaks within the genome, which go part of the way to explaining the RBE values close to 1.0 previously measured using other approaches with various radiation qualities. It is clear that future studies to determine the effectiveness of radiations of differing LET must use techniques that determine both yields and distributions of dsb, and assays need to be developed to allow these measurements at biologically relevant doses.

Effects of Pb2+, Ni2+, Hg2+ and Se4+ on cultured cells. Analysis of uptake, toxicity and influence on radiosensitivity.

Effects of Pb2+, Ni2+, Hg2+ and Se4+ on cultured human glioma U-343MG cells were investigated considering uptake, toxicity and, in combination with radiation, clonogenic cells survival. The cells were exposed to 0-100 microM of the metals for a week before the evaluation. The tests showed a tendency to toxicity with 10 microM nickel although not significant (P > 0.05). Selenium, lead and mercury exerted a significant toxicity (P < 0.05) at 2.5 microM, 10 microM and 1 microM, respectively. To challenge the clonogenic cell survival capacity, the cells were irradiated with 60Co photons after being exposed to the highest nontoxic concentration of the different metals. The clonogenic cell survival tests, after irradiation, showed no significant change if the cells were exposed to 5 microM nickel, 0.5 microM selenium or 5 microM lead compared with those not exposed. Mercury, 0.1 microM, gave a relative reduction in survival compared with only irradiated cells of 58 +/- 17%. Thus, only mercury affected the radiation-induced damage and/or repair. When exposed to the highest nontoxic concentrations of the different metals, the cultures did not display a significant uptake ratio (metal concentration ratio of exposed cells to control cells) of nickel (3.1 +/- 3.3), only a small uptake ratio of selenium (4.0 +/- 0.4), while there was a large uptake ratio of both lead (2.6 +/- 1.7) x 10(2) and mercury (1.5 +/- 0.2) x 10(1). The results indicated that nickel was neither especially toxic nor influenced the clonogenic cell survival after irradiation. Mercury was more toxic and also influenced the radiation sensitivity. Lead was taken up strongly but did not influence the radiation sensitivity. Selenium accumulated but gave no detectable effect on the radiation sensitivity.

Rejoining of DNA double-strand breaks induced by accelerated nitrogen ions.

Rejoining of radiation-induced DNA double-strand breaks (dsb) was measured in cultured cells with pulsed-field gel electrophoresis after radiation doses in the range of 5-30 Gy. Human glioma, U-343MG and Chinese hamster, V79, cells were irradiated with either accelerated nitrogen ions of high linear energy transfer, LET approximately 125 keV/ microns, or photons from 60Co. The induction frequencies of dsb were similar for the two radiation qualities with a relative biological effectiveness, RBE, of 0.90 and 0.89 for the human and hamster cell lines respectively. The biphasic rejoining kinetics differed significantly between the two radiation qualities when studied in the human glioma cells. The difference was seen within the first hour after irradiation and after 6 h there were considerable differences in both the total amount of unrejoined dsb and the fraction of dsb rejoined during the slow phase. When rejoining was analysed 20-22 h after irradiation, the nitrogen ions gave 2.5-2.9 times more residual dsb than the gamma photons. The results for the hamster V79 cells were, up to 2h after irradiation, similar, but the difference between the two radiation qualities was less accentuated. In summary, similar initial yields of dsb after exposure of cells to high or low LET resulted in both radiation quality and cell type-dependent differences when the rejoining of these breaks were compared.

Irregular variations in radiation sensitivity when the linear energy transfer is increased.

Seven cell lines were analyzed for clonogenic survival after irradiation with photons (60Co) or accelerated helium or nitrogen ions. The cell lines showed different sensitivity to photon radiation and most of the differences decreased after irradiation with helium ions with a linear energy transfer (LET) of about 40 keV/microns. However, all cell types had individual LET sensitization patterns and the mean relative biological effectiveness (RBE) at 10% survival ranged from 1.46 +/- 0.12 to 2.41 +/- 0.26 for the helium ions. This difference was significant and the differences increased further when higher survival levels were considered. There was only a weak tendency towards a relation between photon and helium ion sensitivity when the linear component of the survival curves, the alpha-values, were compared, and no relation at all for other parameters. It was not possible to predict the response to an increased LET from the photon responses obtained. Three of the cell lines were also irradiated with nitrogen ions with an LET of 125 keV/microns. These cells were, as expected, sensitized further and the average RBE at 10% survival was 3.67 +/- 0.67. However, one cell line was more resistant than the others in this case. Furthermore, the quadratic component of the survival curves, the beta values, were higher after irradiation with nitrogen than with helium ions. Thus, several irregular and unexpected results were seen when the LET was increased.

Cell type dependent effectiveness of tumour cell inactivation by radiation with increased ionisation density.

Radiation with increased ionization density, LET, will in the near future be applied in targeted radiotherapy and has already, to some degree, been applied in external radiotherapy with accelerated ions. There are indications from the literature that different types of cells are sensitized differently when the LET is increased. Radiation with three different ionisation densities was applied in the present study; 0.8 keV/microns photons from a reference 60Co source, 6 keV/microns helium ions from the entrance region of a monoenergetic helium ion beam and 25 keV/microns from a range modulated helium beam. Three types of cultured cells were analysed: human colon carcinoma LS-174T, human glioma U-343MG and hamster embryonic lung V79-379A. There were interesting differences between the cell types; the LS-174T cells showed strong sensitization already at an LET of 6 keV/microns and there was nearly no difference between the shape of the survival curves after irradiation with 6 or 25 keV/microns. The V79-379A cells were less sensitive to the 6 keV/microns radiation quality and the survival curve was, in this case, very similar to the reference 60Co survival curve. However, the V79-379A cells were sensitized when exposed to the 25 keV/microns helium ions. The U-343MG cells were intermediate in the sense that the 6 keV/microns curve fell in between the 60Co and the 25 keV/microns curves. These variations indicate that different types of cells react differently to changes in LET and that this is probably of special interest for intermediate LET radiation. Some cells might be easily sensitized by intermediate LET qualities while others might be more resistant and this is of importance for the future optimization of radiation treatment with both targeted agents and accelerated ions.

Clonogenic cell survival and rejoining of DNA double-strand breaks: comparisons between three cell lines after photon or He ion irradiation.

Three cell lines, human glioma U-343MG, Chinese hamster V79-379A and human colon carcinoma LS-174T, were analysed for clonogenic survival and with pulsed-field gel electrophoresis regarding induction and rejoining of double-strand breaks (dsb) in DNA. The cells were irradiated with either intermediate linear energy transfer (LET approximately 55 keV/microns) He ions or with 60Co photons and showed cell type-specific variations in their survival curves. The induction of dsb, per DNA unit, increased linearly with dose for both radiation qualities, and there were no cell type-dependent differences. However, the dsb induction frequency increased for all three cell lines with an RBE of 1.31 +/- 0.08 (SD) after He ion irradiation. All three cell lines showed biphasic dsb rejoining patterns with cell type-specific differences. The differences between the cell lines were somewhat smaller after irradiation with He ions, although the patterns were very similar to those seen after photon irradiation. The shoulders in the survival curves disappeared after He ion irradiations, however the cells retained the capacity to rejoin dsb. After 8-h rejoining the increase of LET to intermediate values did not give a higher fraction of residual dsb.

Relative biological effectiveness of intermediate energy protons. Comparisons with 60Co gamma-radiation using two cell lines.

Range modulated proton beams are used for radiotherapy of malignant tumours at several accelerator laboratories with the aim of introducing proton therapy as a clinical hospital-based therapy modality. Due to the finite range and the sharpness of the dose gradients, the dose to well defined target volumes can be raised without excessive irradiation of non-target tissue. The prescribed proton doses are determined in part on the basis of the relative biological effectiveness (RBE) of the particular radiation quality. In this study, RBE values were determined for a proton beam with a maximal range of 33 mm, which corresponds to an energy of approximately 67 MeV. The range modulated depth-dose distribution, with a 20 mm extended Bragg peak, was mainly designed for high precision treatment of small targets such as uveal melanomas. The tested cell lines, LS-174T and V79-379A, were chosen because of their suitability for clonogenic assays. The cells were irradiated with single doses in the range 2-10 Gy at different depths in the extended peak region of the range modulated proton beam. RBE values were determined by comparing the doses needed to obtain the same reduction in colony formation (0.5, 0.1 and 0.01) as with the reference 60Co gamma source. The mean RBE value was 1.22 with a standard deviation of 0.08. The variations depended on both cell type and on the survival levels considered.(ABSTRACT TRUNCATED AT 250 WORDS)