Might intrinsic radioresistance of human tumour cells be induced by radiation?

Survival measurements were made on six human tumour cell lines in vitro after irradiation with single doses of X rays. Doses up to 5 Gy were used giving surviving fractions down to 20%, but the majority of the measurements were made at doses < 1 Gy. These six cell lines have very different intrinsic radiosensitivities: HT29, Be11, and RT112 are radioresistant with surviving fractions at 2 Gy (SF2) between 60 and 74%, while MeWo, SW48, and HX142 are radiosensitive (SF2 = 3-29%). For all the cell lines, response over the dose range 2-5 Gy showed a good fit to a Linear-Quadratic (LQ) model. However, HT29, Be11, and RT112 cells showed a significant increase in X-ray radiosensitivity at doses below < 1 Gy compared with the prediction extrapolated from a LQ model fitted to the data at higher doses. The LQ model also slightly underpredicted the effect of low-dose X rays in MeWo cells, but the response of SW48 and HX142 cells was well described by the LQ model at all doses, with no evidence of increased low-dose effectiveness. The most plausible explanation for this phenomenon is that it reflects an induced radioresistance so that low doses of X-rays in vitro are more effective per Gy than higher doses, because only at higher doses is there sufficient damage to trigger repair systems or other radioprotective mechanisms. It follows that variation in the amount of inducible radioresistance might explain, in part, differences in intrinsic radiosensitivity above > 1 Gy between cell lines: cells would be intrinsically radiosensitive because they have a diminished inducible response.     

Radioresistant MTp53-expressing rat embryo cell transformants exhibit increased DNA-dsb rejoining during exposure to ionizing radiation

Recent data suggest that aberrant function of the wild type p53 protein (WTp53) may alter cellular survival following DNA damage through cellular pathways involving apoptosis and cell-cycle checkpoints, but little is known concerning it's possible role in DNA repair. In the present study, the ionizing radiation sensitivity was determined for a series of rat embryo fibroblast (REF) cell lines transfected with an activated form of the H-ras oncogene alone, or in combination with a variety of missense-mutant p53 (MTp53) alleles. Transformed REF clones which expressed exogenous MTp53 and p21ras proteins (CLASS II clones) were generally radioresistant in culture as determined by higher values for the surviving fraction after 2 Gy (SF2 value) and the radiation dose required to reduce survival to a fraction of 0.1 (D10 value), compared either to transformed REF clones expressing p21ras protein alone (CLASS I clones), or to non-transfected REF control cell lines expressing baseline endogenous levels of p21ras and WTp53 protein. The increased radioresistance observed in the CLASS II clones (following both HDR- and LDR-irradiation), was significantly correlated with increased expression of MTp53 protein, and a decreased radiation-induced G1 arrest response. The variability observed in clonogenic radiosensitivity among REF clones was not explained by differential radiation-induced apoptosis. Using the Comet assay performed after continuous low dose-rate (LDR)-irradiation, MTp53-expressing REF clones were also found to be more proficient at the rejoining of DNA double-strand breaks (DNA-dsb), compared to WTp53-expressing REF clones. These results suggest that an enhanced DNA and cellular repair capacity may, in part, explain the increased radiation survival observed in some MTp53-expressing transformed fibroblasts and tumours.     

DNA damage from oxidants: influence of lesion complexity and chromatin organization

DNA damage by reactive oxygen species results in a spectrum of DNA lesions including single-strand breaks (ssb) and double-strand breaks (dsb). However, most damage is not lethal, and the location and nature of the DNA damage, in addition to total number of breaks, are likely to be critical in determining ultimate survival. Generally associated only with ionizing radiation, multiply damaged sites (i.e., complex lesions and clusters of complex lesions in DNA) are more likely to be lethal because they are less easily repaired. We examined five drugs known to cause DNA adducts, strand breaks, and reactive oxygen species for their ability to produce complex lesions: 4-nitroquinoline-1-oxide (4NQO), H2O2, doxorubicin, Tirapazamine, and etoposide. As indicators of lesion complexity we compared 1) the ratio of ssb to dsb, 2) the rate of rejoining of single-strand breaks, 3) the relative lethality of the breaks (number of breaks per mean lethal dose), and 4) the ability to produce complex lesions. Tirapazamine, etoposide, and doxorubicin gave dsb/ssb ratios similar to that for X-rays, whereas 4NQO and H2O2 showed dsb/ssb ratios of 200 and 3250, respectively. The number of dsb per LD50 varied from 2.5 to 500 for different drugs. There was no apparent relation between ssb rejoining half-time (3.5-85 min) and relative lethality or lesion complexity. A modified (nonionic detergent) filter elution method confirmed that tirapazamine, like ionizing radiation, produced multiple dsb within single chromatin domains. These data indicate that complex lesions can be produced by a number of different chemicals and suggest that the damage that results in killing by these drugs may be related to production of multiply damaged sites in DNA.     

0.3 keV carbon K ultrasoft X-rays are four times more effective than gamma-rays when inducing oncogenic cell transformation at low doses

Oncogenic transformation and inactivation were investigated in C3H10T1/2 mouse embryo fibroblasts exposed to proton-induced 0.28 keV carbon K (CK)-characteristic X-rays and 60Co gamma-rays as reference radiation at high dose-rate (2-3 and 0.7 Gy/min respectively). Both oncogenic cell transformation and cell inactivation followed a linear-quadratic relationship with dose. At low doses where the linear component dominates CK ultrasoft X-rays were more effective, by a factor of 4, at inducing oncogenic cell transformation and cell inactivation compared with 60Co gamma-rays. For both endpoints the RBE of CK ultrasoft X-rays gradually decreased with increasing dose mainly due to the greater quadratic component for 60Co gamma-rays compared with CK ultrasoft X-rays. Our experimental data are in agreement with the hypothesis that single DNA double-strand breaks (dsbs), which are induced by 0.28-keV ultrasoft CK X-rays, may lead to oncogenic cell transformation. With increasing absorbed dose, i.e. with decreasing mean distance between dsbs induced by 0.28-keV ultrasoft X-rays, oncogenic cell transformation and cell inactivation may also be induced by interaction between those dsbs.     

Apoptosis in the subependyma of young adult rats after single and fractionated doses of X-rays

Ionizing radiation is commonly used in the treatment of brain tumors but can cause significant damage to surrounding normal brain. The pathogenesis of this damage is uncertain, and understanding the response of potential target cell populations may provide information useful for developing strategies to optimize therapeutic irradiation. In the mammalian forebrain, the subependyma is a mitotically active area that is a source of oligodendrocytes and astrocytes, and it has been hypothesized that depletion of cells from this region could play a role in radiation-induced white matter injury. Using a distinct morphological pattern of nuclear fragmentation and an immunohistochemical method to specifically label the 3'-hydroxyl termini of DNA strand breaks (terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling), we quantified apoptosis in the subependyma in the young adult rat brain after single and fractionated doses of X-rays. Significant increases in apoptotic index (percentage of cells showing apoptosis) were detected 3 h after irradiation, and the peak apoptotic index was detected at 6 h. Six h after irradiation, the dose response for apoptosis was characterized by a steep increase in apoptotic index between 0.5 and 2.0 Gy and a plateau from 2-30 Gy. The fraction of cells susceptible to apoptosis was estimated to be about 40%, and treatment of rats with cycloheximide inhibited apoptosis. When daily 1.5-Gy fractions of X-rays were administered, the first three fractions were equally effective at decreasing the cell population via apoptosis. There was no additional apoptosis or decrease in cellularity in spite of one to four additional doses of X-rays. Those data suggested some input of cells into the subependymal population during fractionated treatment, and subsequent studies showed that there was a significant rise in 5-bromo-2' deoxyuridine labeling index 2-3 days after irradiation, indicating increased cellular proliferation. The proliferative response after depletion of cells via apoptosis may represent the recruitment of a relatively quiescent stem cell population. It is possible that the radiation response of subependymal stem cells and not the apoptotic-sensitive population per se are critical elements in the response of the brain to radiation injury.     

Dieldrin induces peroxisomal enzymes in fish (Sparus aurata) liver

We improved alkaline sucrose density gradient sedimentation to detect very long single-strand DNA at the megabase level (from less than 1 to about 4 Mb). Hitherto, these have not sedimented correctly due to some artifacts. One artifact was aggregation of sticky DNA and proteins formed in the gradient. Then, in some gradients, biphasic distribution was observed, the major peak of which was reasonable as a result of random scission by X-rays, but the minor, fast-sedimenting population was another artifact resulting from incomplete denaturation of the DNA. We mainly reduced the centrifugal force and used a solution for cell lysis with a high concentration of salt. By means of this procedure, DNA single-strand breaks induced by relatively low doses of X-rays and subsequent repair processes can be measured in human fibroblasts. The protocol is also applicable to the study of DNA damage accompanied by strand scission, such as by UV or dimethyl sulfate as well as their repair. The technique is sensitive enough to detect even single-strand breaks induced by 0.1 J/m2 UV and sufficiently reproducible that breaks induced by increasing UV dosages were dose dependent. Thus, this technique was proven to be very sensitive, reliable and simple to perform. Therefore, this improvement will be extremely useful to investigators studying DNA repair.     

Effect of humeral head component size on hemiarthroplasty translations and rotations

PURPOSE: To study the effects of carbon K ultrasoft X-rays, which produce a single photoelectron with a track length of < 7 nm, on the production of structural chromosome-type changes. MATERIALS AND METHODS: Untransformed human fibroblasts (HF12) were irradiated in G1 phase. Aberrations were analysed using fluorescence in situ hybridization using multi-coloured chromosome specific DNA probes for chromosomes 1 and 2 and an alpha-satellite pan-centromeric probe. RESULTS: CK X-rays have a high efficiency per unit absorbed dose for producing simple and complex exchanges. Mean absorbed doses of 0.33-1.31 Gy produce simple exchanges with a predominantly linear dose dependency, and visibly complex exchanges increased by more than the power 2 of the dose, with no evidence of a linear component. The proportion of exchanges that are visibly complex ranged from 9% to 46%. CONCLUSIONS: The linear response for simple exchanges provides further support to the hypothesis that damaged DNA may be able to interact with undamaged DNA. The high proportion of complex exchanges may be due to the increased efficiency of double-strand break induction and to the high density of tracks per unit absorbed dose targeting pre-existing sites, some of which may be close to the incident nuclear membrane.     

Determination of the radiation sensitivity of the stromal cells in the murine long-term bone marrow culture by measuring the induction and rejoining of interphase chromosome breaks

PURPOSE: To determine the radiosensitivity of bone marrow stromal cells, the rate of interphase chromosome breakage and rejoining of stromal cells in the murine long term bone marrow culture and of human skin fibroblasts were compared. METHODS AND MATERIALS: The cells were irradiated with doses up to 6 Gy and repair times up to 6 hr were investigated. After induction of premature chromosome condensation by fusing the cells with mitotic HeLa cells, the number of interphase chromosome fragments was counted. RESULTS: The number of radiation induced breaks was found to be not significantly different for both cell types with 6.16 +/- 0.26 breaks per Gray for the fibroblasts and 5.96 +/- 0.20 breaks per Gray for the stromal cells. A significant difference was observed in the repair rate. The fibroblasts rejoined 39.6% of the breaks induced initially during the first hour after irradiation and 5.6 +/- 1.84 breaks remained unrejoined after 6 hr, while the stromal cells were able to rejoin 63.2% in 1 hr and had 2.05 +/- 0.07 breaks unrejoined after 6 hr. CONCLUSION: If the well substantiated assumption is made, that the capacity to repair DNA double strand breaks or interphase chromosome breaks is correlated with the cellular radiosensitivity, this finding indicate, that murine bone marrow stromal cells are more radioresistant than human skin fibroblasts.     

Plasmid DNA strand breaks induced by laser irradiation of 193 nm wavelength

DNA of plasmid pBR322 irradiated with laser at a wavelength of 193 mm was treated with an extract containing proteins from E.coli K12 AB1157 (wild-type). The enzymes were found to produce single- and double-strand DNA breaks, which was interpreted as a transformation of a portion of cyclobutane pyrimidine dimers and (6-4) photoproducts into nonrepairable single-strand DNA breaks. The products resulted from ionization of DNA, in particular, single-strand breaks, transform to double-strand breaks. A comparison of these data with the data on survival of plasmid upon transformation of E.coli K12 AB1157 enables one to assess the biological significance of single- and double-strand breaks. The inactivation of the plasmid (in AB1157) is mainly determined by the number of directly formed laser-induced single-strand breaks, whereas the contribution of enzymatically produced single- and double-strand breaks is insignificant.     

Allelic loss on chromosome 22 in oral cancer: possibility of the existence of a tumor suppressor gene on 22q13

PURPOSE: To model the influence of hypoxic radioprotection in fractionated treatments over a range of fraction sizes. To determine whether there is a "therapeutic window" of dose per fraction where hypoxic radioresistance could be reduced, and if so, where it occurs in different cell lines. MATERIALS AND METHODS: A mathematical model has been used to simulate the response of cells to low doses of radiation, in the region of clinical interest. We have used the inducible repair variant of the linear quadratic (LQ) equation, with a hypersensitive region (alphaS) at low doses that gradually transforms to the accepted "resistance" in the shoulder region (alphaR). It contains two new parameters, the ratio alphaS/alphaR, and D(C). We have accepted that the "induction dose" D(C) is modified by anoxia to the same extent as the other parameters. We have initially modeled using theoretical parameters and then checked the conclusions with 14 sets of published experimental data for cell lines investigated for inducible repair. RESULTS: We have computed the clinical hypoxic protection (OER') as a function of dose per fraction in simulations of clinical fractionated schedules. We have identified a therapeutic window in terms of dose per fraction at about 0.5 Gy, where the OER' is minimized, regardless of the precise cell survival curve parameters. The minimum OER' varies from one cell line to another, falling to about 1.0 if alphaS/alphaR = 6-10 and even far below 1.0 if alphaS/alphaR > or = 20. DISCUSSION: Hyperfractionation using 0.5 Gy fractions may therefore be more effective than oxygen mimetic chemical sensitizers, since it could even make some tumor cells more sensitive than oxic normal tissues. The tumor lines that benefit most from this type of sensitization are those with the highest intrinsic oxic radioresistance, i.e. those with high SF2 values.     

The effect of low-intensity laser radiation on functional potential of leukocytes

We examined whether X radiation induces a particular deletion in the mitochondrial DNA (mtDNA) of the cells of two human squamous cell carcinoma lines with different sensitivity to radiation and in a radiosensitive ataxia telangiectasia (AT) cell line. We used polymerase chain reaction (PCR) to quantify the accumulation of a particular 4977-bp deletion (delta mtDNA4977). PCR products of delta mtDNA4977 were detectable after exposure to 10 Gy in the radioresistant squamous cell carcinoma cells, 2 Gy in the radiosensitive squamous cell carcinoma cells and 1 Gy in the radiosensitive AT cells. These observations suggest that ionizing radiation induces the delta mtDNA4977 in human cells and that the radiation doses required to induce this deletion reflect the sensitivity of cells to radiation.     

Inhibition of the mitogen activated protein (MAP) kinase cascade potentiates cell killing by low dose ionizing radiation in A431 human squamous carcinoma cells

The molecular mechanism(s) by which tumor cells survive after exposure to ionizing radiation are not fully understood. Exposure of A431 cells to low doses of radiation (1 Gy) caused prolonged activations of the mitogen activated protein (MAP) kinase and stress activated protein (SAP) kinase pathways, and induced p21(Cip-1/WAF1) via a MAP kinase dependent mechanism. In contrast, higher doses of radiation (6 Gy) caused a much weaker activation of the MAP kinase cascade, but a similar degree of SAP kinase cascade activation. In the presence of MAP kinase blockade by the specific MEK1 inhibitor (PD98059) the basal activity of the SAP kinase pathway was enhanced twofold, and the ability of a 1 Gy radiation exposure to activate the SAP kinase pathway was increased approximately sixfold 60 min after irradiation. In the presence of MAP kinase blockade by PD98059 the ability of a single 1 Gy exposure to cause double stranded DNA breaks (TUNEL assay) was enhanced at least threefold over the following 24-48 h. The increase in DNA damage within 48 h was also mirrored by a similar decrease in A431 cell growth as judged by MTT assays over the next 4-8 days following radiation exposure. This report demonstrates that the MAP kinase cascade is a key cytoprotective pathway in A431 human squamous carcinoma cells which is activated in response to clinically used doses of ionizing radiation. Inhibition of this pathway potentiates the ability of low dose radiation exposure to induce cell death in vitro.     

Resistance to radiation-induced apoptosis in Burkitt's lymphoma cells is associated with defective ceramide signaling

Increased sensitivity to ionizing radiation has been shown to be due to defects in double-strand break repair and mutations in the proteins that detect DNA damage. However, it is now recognized that the cellular radiation response is complex and that radioresistance/radiosensitivity may also be regulated at different levels in the radiation signal transduction pathway. Here, we describe a direct relationship between resistance to radiation-induced apoptosis and defective ceramide signaling. Radiation sensitivity in human tumor cells correlated with the immediate accumulation of the second messenger ceramide. In the BL30A Burkitt's lymphoma line, ceramide increased 4-fold by 10 min postirradiation (10 Gy), and in the moderately sensitive HL-60 leukemia cells, ceramide accumulated 2.5-fold above basal levels. In contrast, in all radioresistant tumor cells examined, including several Burkitt's lymphoma lines (BL30K, BL29, and BL36) and the MO59K glioma cell line, ceramide did not accumulate postirradiation. The ability to abrogate ceramide production by pretreatment with the tumor promoter, 12-O-tetradecanoylphorbol 13-acetate, conferred resistance to radiation-induced apoptosis in the sensitive BL30A cells. An isogenic subline of BL30A, BL30K, was resistant to both C8-ceramide (20 microM) and ionizing radiation-induced apoptosis. Bypassing the block in radiation-induced ceramide production by the addition of exogenous ceramide was not sufficient to induce apoptosis; this suggests the existence of a second ceramide-associated signaling defect in these radioresistant cells that confers resistance to ceramide-induced apoptosis. Thus, these results provide compelling evidence that ceramide is an essential mediator of radiation-induced apoptosis and that defective ceramide signaling confers an apoptosis-resistant phenotype in tumor cells.     

Bax and Bcl-2 expressions predict response to radiotherapy in human cervical cancer

PURPOSE: The ratio of Bcl-2 to Bax expression determines survival or death following an apoptotic stimulus. In order to establish a new predictor of the outcome of treatment for human cervical carcinoma, we investigated the relationship between the expressions of the Bax and Bcl-2 proteins and the response to radiotherapy after the administration of 10.8 Gy. METHODS: A total of 44 patients with histologically proven carcinoma of the uterine cervix, including three with recurrent cervical stump carcinomas, were treated with definitive radiotherapy. The presence of mutations in exons 5-8 of the p53 gene was analyzed by a single-strand conformation polymorphism analysis and DNA sequencing. RESULTS: Forty patients were found to have wild-type p53, and the remaining four had mutant p53. The Bax and Bcl-2 protein expressions prior to radiotherapy did not correlate with response and survival. However, the Bax and Bcl-2 protein expressions after radiotherapy correlated with both response and survival. Bax-positive tumors showed significantly better responses than the Bax-negative tumors after 10.8 Gy radiation (P = 0.0002). In contrast, the Bcl-2-positive tumors showed significantly poorer responses than the Bcl-2-negative tumors after radiation (P = 0.002). Increased Bax expression after the 10.8 Gy radiotherapy was found to be correlated with good survival (P = 0.04). In contrast, increased Bcl-2 expression after such radiotherapy was correlated with poor survival (P = 0.002). CONCLUSION: The levels of Bax and Bcl-2 expression after 10.8 Gy radiotherapy are useful prognostic markers in patients with human cervical carcinoma.     

Radiation-induced DNA damage and repair in radiosensitive and radioresistant human tumour cells measured by field inversion gel electrophoresis

Radiation-induced DNA damage induction and repair was measured in two human squamous carcinoma cell lines with differing radiosensitive. Experiments were carried out with field inversion gel electrophoresis (FIGE), adapted to measure DNA double strand break (DSB) induction and repair in unlabelled cells. The sensitivity of the method was increased by introducing a hybridization membrane into the agarose gel. Damaged DNA accumulated on one spot on the membrane resulting in high local concentrations. This DNA was quantified using radioactively-labelled total human DNA as a probe. Dose response experiments for damage induction correlated well with the results using prelabelled cells. Linear DNA damage induction curves were observed with a sensitivity for the post-labelling method of 1 Gy. No differences in DSB induction were found, however, between the radiosensitive SCC61 and the radioresistant SQ20B cell line. Repair experiments were carried out with trypsinized cells with different doses and repair temperatures. The 10, 25 and 50 Gy doses resulted in 6, 13 and 50% of the DNA migrating out of the plug at 0 h. For both the cell lines 75-85% of the initial damage was repaired within 1 h at 37 degrees C at all three radiation doses, i.e. no significant differences were observed in repair rates or extent between the two cell lines. At 24 degrees C repair was slower than at 37 degrees C, and at 0 degree C no repair was observed. In summary, radiosensitivity differences at physiological temperatures could not be explained by differences in either induction or repair of DNA damage as measured by pulsed field gel electrophoresis.     

Comparison of bond strengths of three denture base resins to treated nickel-chromium-beryllium alloy

The success of radiotherapy in eradicating tumours depends on the total radiation dose, but what limits this dose is the tolerance of the normal tissues within the treatment volume. Studies involving fibroblast survival have demonstrated the theoretical feasibility of a predictive assay of radiation sensitivity, but such an assay is still far from clinical application. Using pulsed-field gel electrophoresis (PFGE), we have quantified the initial "apparent" number of DNA double-strand breaks (dsb) induced by the radiation as an alternative measure of sensitivity in 2 different normal cell types from the same patients, epidermal skin cells and lymphocytes. We found significant inter-individual variation in the measured dsb (1-5 dsb/Gy/DNA unit). We also found a linear correlation between molecular damage in lymphocytes and skin samples from the same patient (slope = 0.83; r = 0.694; p = 0.0001). These results suggest that the initial number of dsb could be used as an indicator of the in vivo response to radiation.     

Effects of low radiation doses on Chinese hamster cells

The induction of cytogenetic damages after irradiation with single dose of gamma-rays (0.1-2 Gy) have been studied. It is shown non-linear curve for the induction of chromosome aberrations, detected by anaphase method. After irradiation in S-stage of the cell cycle at dose below 0.2 Gy the cells were more radiosensitive than after irradiation with doses 0.3-2 Gy. Between the phases of high radiosensitivity and radioresistance the reversal dose-effect relation was observed. This phenomenon was not marked for the cells after irradiation in G2-stage of the cell cycle. It is possible, this results could reflect an induced radioresistance at low dose of irradiation.     

Kinetics of the formation of chromosome aberrations in X-irradiated human lymphocytes, using PCC and FISH

In order to study the initial frequencies and define kinetics of the formation of chromosomal exchanges in X-irradiated human lymphocytes, the premature chromosome condensation (PCC) technique was employed in combination with fluorescence in situ hybridization (FISH) with a composite probe for human chromosome 8 and a pan-centromeric probe for the whole genome. Human lymphocytes were X-irradiated (0.5, 1, 2, 3, 4 and 6 Gy), fused with mitotic Chinese hamster ovary (CHO) cells immediately or 1, 3, 6, 12 and 18 h after irradiation. Immediately after irradiation chromosomal breaks, dicentrics and translocations showed a linear dose-response. Unrejoined chromosome breaks were the most frequent types of aberrations (about 85%) observed. About 15% of total aberrations were chromosome exchanges of 65% of these were translocations and 35% were dicentrics. The chromosomal exchanges initially observed were mostly incomplete, with no complex exchanges at doses of 1 and 2 Gy, at higher doses (3-6 Gy) complex exchanges were observed and their frequencies increased with increasing post incubation time. Following different recovery times, repair kinetics of breaks for different doses of irradiation was studied. The shapes of the curves obtained for breaks as well as chromosome exchanges were linear-quadratic. The linear yield component, alpha, is formed entirely in the fast process that can be manifested in the early plateau, while component beta developed slowly in the subsequent hours. The kinetics of breaks rejoining was exponential, almost 50% of breaks rejoined after 1 h and at 18 h about 20% of breaks remained. At low doses of 1 and 2 Gy most of the exchanges were formed immediately and at higher doses, the frequency of exchanges increased with kinetics similar to that observed for the rejoining of breaks. However, the kinetics was different for different doses of irradiation. The frequency of dicentrics increased at doses above 2 Gy following 3 h recovery time, but for the translocations effect was pronounced even at 1 h recovery time. The frequency of incomplete exchanges (i.e., terminal translocations) decreased with post irradiation time and at 18 h was 30-40% less than the frequency obtained immediately after irradiation. The increase in the total translocations as a function of time between irradiation and fusion was due to a rapid increase in complete exchanges (i.e., reciprocal translocations). The frequency of ring chromosomes immediately after irradiation, also increased linearly, however, it was 3-5 times lower than dicentrics and remained almost constant in number for different doses and at different post-irradiation times.     

Substructure in the cell survival response at low radiation dose: effect of different subpopulations

In order to obtain more accurate measurements of cell survival after low doses of radiation, we have used the cell sorter assay, in which a cell sorter is used to accurately count out the number of cells plated for colony formation. This method, combined with data averaging, permits measurements of survival with superior precision, which have revealed that there is substructure in the radiation response of asynchronously dividing Chinese hamster cells. The substructure, observed at doses of a few Gy, has features of a 2-component response, consistent with the presence of subpopulations of cells of different cell-cycle-related radiosensitivity. The absence of any substructure in the radiation response of homogeneous (tightly synchronized) cell populations lends strong support to this subpopulation explanation of the substructure. This assay has also been used on a variety of human tumour cell lines, most of which exhibited substructure similar to that of Chinese hamster cells. This paper outlines the application of the cell sorter assay to three different problems: (i) radiosensitizer mechanisms-etanidazole and RB 6145 are shown to enhance primarily the beta term and alpha term, respectively, of tumour cell kill, indicating that sensitizer efficacy may be tumour-specific and predictable from tumour response parameters; (ii) accurate measurement of Relative Biological Effectiveness (RBE) in a modulated clinical proton beam shows that the RBE is both dose- and depth-dependent; and (iii) measurements at lower doses clearly demonstrate a second order of substructure, termed the hypersensitive response, at doses < 1 Gy.     

Thermoluminescence characteristics and dosimetric aspects of fluoroperovskites （NaMgF3：Eu2＋,Ce3＋）

The perovskite-like NaMgF3 polycrystalline powder samples, pure and rare earth doped, were synthesized by conventional solid state reaction method. The perovskite material was doped with 0.2 mol.% of EuF3 and CeF3 impurity. Phase purity of synthe-sized compounds was analyzed by powder X-ray diffraction technique. The thermoluminescence response of polycrystalline samples showed a linear response up to 12 Gy and then became sub linear at higher doses. The order of kinetics （b）, activation energy （E） and other trapping parameters were calculated using peak shape method and variable heating rate method. From glow curve analysis the symmetric factor was calculated. The glow curve showed two peaks at 404 and 488 K, and both of them confirmed the thermolumi-nescence in the phosphor, which obeyed second-order kinetics. The experimental resulted showed that this phosphor could have po-tential applications in radiation dosimetry.