Estimation of radiation tolerance to high LET heavy ions in an anhydrobiotic insect,Polypedilum vanderplanki

Purpose: Anhydrobiotic larvae of Polypedilum vanderplanki are known to show an extremely high tolerance against a range of stresses. We have recently reported that this insect withstands exposure to high doses of gamma-rays (linear energy transfer [LET] 0.2 keV/μm). However, its tolerance against high LET radiation remains unknown. The aim of this study is to characterize the tolerance to high-LET radiations of P. vanderplanki.

Materials and methods: Larval survival and subsequent metamorphoses were compared between anhydrobiotic (dry) and non-anhydrobiotic (wet) samples after exposure to 1 – 7000 Gy of three types of heavy ions delivered from the azimuthally varying field (AVF) cyclotron with LET values ranging from 16.2 – 321 keV/μm. The tolerance against ⁴He ions was also compared among three chironomid species.

Results: At all LET values measured, dry larvae consistently showed greater radiation tolerance than hydrated larvae, perhaps due to the presence of high concentrations of the disaccharide trehalose in anhydrobiotic animals, and the radiation-induced damage became evident at lower doses as development progressed. Relative biological effectiveness (RBE) values based on the median inhibitory doses reached a maximum at 116 keV/μm (¹²C), and the maximum RBE clearly increased as development progressed. Lower D₀ (dose to reduce survival from relative value 1.00 – 0.37 on the exponential part of the survival curve), and higher D_q (quasi-threshold dose) were found in individuals exposed to ⁴He ions, compared to gamma-rays, and in P. vanderplanki larvae compared to non-anhydrobiotic chironomids.

Conclusion: Anhydrobiosis potentiates radiation tolerance in terms of larval survival, pupation and adult emergence of P. vanderplanki exposed to high-LET radiations as well as to low-LET radiation. P. vanderplanki larvae might have more efficient DNA damage repair after radiation than other chironomid species.     

Biological effects of anhydrobiosis in an African chironomid, Polypedilum vanderplanki on radiation tolerance

PURPOSE: Anhydrobiotic organisms are known to have an extremely high tolerance against a range of stresses. However, the functional role of anhydrobiosis in radiation tolerance is poorly understood, especially in development following irradiation. The present study aims to evaluate effects of anhydrobiosis on radiation tolerance in an anhydrobiotic insect, Polypedilum vanderplanki. MATERIALS AND METHODS: Larval survival (48 h), anhydrobiotic ability, metamorphosis and reproduction after exposure to 1-9000 Gy of gamma-rays at the larval stage were compared between anhydrobiotic (dry) and normal (wet) phases. RESULTS: Wet larvae were killed in a dose-dependent manner at doses higher than 2000 Gy, and all died within 8 h after 4000 Gy exposure. In contrast, dry larvae survived even 5000 Gy, and some of them still tolerated 7000 Gy and were alive at 48 h after rehydration. Moreover, greater radiotolerance of dry larva, compared to wet ones, was demonstrated in terms of metamorphoses. However, anhydrobiosis did not protect against radiation damage in terms of producing viable offspring. CONCLUSION: These results indicate that anhydrobiosis enhances radiotolerance, resulting in increases of successful metamorphoses.     

Modelling of cell killing due to sparsely ionizing radiation in normoxic and hypoxic conditions and an extension to high LET radiation

Abstract

Purpose: An approach for describing cell killing with sparsely ionizing radiation in normoxic and hypoxic conditions based on the initial number of randomly distributed DNA double-strand breaks (DSB) is proposed. An extension of the model to high linear energy transfer (LET) radiation is also presented.

Materials and methods: The model is based on the probabilities that a given DNA giant loop has one DSB or at least two DSB. A linear combination of these two classes of damage gives the mean number of lethal lesions. When coupled with a proper modelling of the spatial distribution of DSB from ion tracks, the formalism can be used to predict cell response to high LET radiation in aerobic conditions.

Results: Survival data for sparsely ionizing radiation of cell lines in normoxic/hypoxic conditions were satisfactorily fitted with the proposed parametrization. It is shown that for dose ranges up to about 10 Gy, the model describes tested experimental survival data as good as the linear-quadratic model does. The high LET extension yields a reasonable agreement with data in aerobic conditions.

Conclusions: A new survival model has been introduced that is able to describe the most relevant features of cellular dose-response postulating two damage classes.     

The response of a human malignant melanoma cell line to high LET radiation.

The response of a human malignant melanoma cell line in vitro to high linear energy transfer radiation was studied utilizing the neutrons produced by the reaction of 16 and 50 MeV deuterons on beryllium. The relative biological effectiveness (RBE) relative to cobalt-60 gamma radiation was determined under conditions of complete oxygenation. The data indicate that the radioresistance ascribed to malignant melanoma in vivo is not an intrinsic quality of the cell but rather may be mediated by the in vivo environment.     

Biological effectiveness of 12C and 20Ne ions with very high LET

Purpose: To determine the relationship between the relative biological effectiveness (RBE) for cell inactivation and linear energy transfer (LET) in the Bragg peak region of ¹²C and ²⁰Ne ions.

Materials and methods: Chinese hamster ovary (CHO-K1) cells were exposed to high LET ¹²C (33.2 MeV, 20.3 MeV, 9.1 MeV at cell entrance) and ²⁰Ne ions (56.2 MeV, 34.7 MeV, 15 MeV at cell entrance) and to low LET x-rays. Technical details of the irradiation facility are presented which is based on the Monte Carlo simulation of the lateral spread of heavy ions as a result of the multiscattering small-angle process in physical conditions of the experimental set-up.

Results: RBE has been measured for LET values close to the Bragg peak maximum, i.e., 440–830 keV/μm for ¹²C and for 1020–1600 keV/μm for ²⁰Ne ions. RBE values at several levels of survival were estimated and were found to decrease with increasing LET. The inactivation cross sections were calculated from the final slope of dose-response curves and were found to increase with increasing LET.

Conclusions: The RBE decreases with increasing LET in the range between 440 and 1600 keV/μm for the two types of radiations forming a single line when plotted together, pointing towards LET as the single determinant of RBE. The inactivation cross section describing the killing efficiency of a single particle at the end of particle range comes close to the size of the cell nucleus.     

Response to high LET radiation 12C (LET, 295 keV/microm) in M5 cells, a radio resistant cell strain derived from Chinese hamster V79 cells

PURPOSE: To study the effects of 12C-beam of 295 keV/microm (57.24 MeV) on M5 and Chinese hamster V79 cells by using cytogenetic assays like micronuclei (MN) induction, chromosomal aberrations (CA) and apoptosis. Additionally, the relative survival of these two cell lines was tested by the colony forming ability of the cells, with a view to understanding the mechanism of cellular damages that lead to difference in cell survival. MATERIALS AND METHODS: Confluent cells were irradiated with 12C-beam at various doses using 15UD Pelletron accelerator. Cell survival was studied by the colony forming ability of cells. MN assay was done by fluorescent staining. Different types of chromosomal aberrations in metaphase cells were scored at 12 h after irradiation. Apoptosis was measured at different post irradiation times as detected by nuclear fragmentation and DNA ladder was prepared after 48 h of incubation. RESULTS: Dose-dependent decrease in surviving fractions was found in both the cell lines. However, the surviving fractions were higher in M5 cells in comparison to V79 cells when exposed to the same radiation doses. On the other hand, induced MN frequencies, CA frequencies and apoptosis percentages were less in M5 cells than V79 cells. Very good correlations between surviving fractions and induced MN frequencies or induced total CA or induced apoptosis percentages were obtained in this study. CONCLUSIONS: The cell strain M5 showed relatively more radio-resistance to 12C-beam compared to Chinese hamster V79 cells in this study. As the MN formation, CA and apoptosis induction were less in M5 cells as compared to parental V79 cells, the higher cell survival in the former could possibly be attributed to their better repairing ability leading to higher cell survival.     

Differences between biological effects of high LET and low LET radiations in relation to their application in radiotherapy

The application of fast neutrons, negative pions or heavy ions will only provide an advantage for the radiotherapy of cancer if, in comparison with conventional radiation, e.g., X-rays, gamma rays and electrons, better depth-dose and collimation characteristics or specific radiobiological dose-response relationships result in greater local control probabilities for tumours without increased frequencies of severe normal tissue damage. Differences in intrinsic radiosensitivity and the presence of hypoxic cells are considered to be the main factors which can cause values of the relative biological effectiveness (RBE) for responses of tumours to be larger than RBE values for normal tissue tolerance. Clinical studies on lung metastases irradiated with single doses of 15 MeV neutrons indicate that RBE values for tumour growth delay can vary between 1.2 and 4.0, while RBE values for damage to several normal tissues are estimated from data on a few animal tissues to be approximately 2.5.     

Role of light and heavy minerals on natural radioactivity level of high background radiation area,Kerala, India

Natural radionuclides (²³⁸U, ²³²Th and ⁴⁰K) concentrations and eight different radiological parameters have been analyzed for the beach sediments of Kerala with an aim of evaluating the radiation hazards. Activity concentrations (²³⁸U and ²³²Th) and all the radiological parameters in most of the sites have higher values than recommended values. The Kerala beach sediments pose significant radiological threat to the people living in the area and tourists going to the beaches for recreation or to the sailors and fishermen involved in their activities in the study area. In order to know the light mineral characterization of the present sediments, mineralogical analysis has been carried out using Fourier transform infrared (FTIR) spectroscopic technique. The eight different minerals are identified and they are characterized. Among the various observed minerals, the minerals such as quartz, microcline feldspar, kaolinite and calcite are major minerals. The relative distribution of major minerals is determined by calculating extinction co-efficient and the values show that the amount of quartz is higher than calcite and much higher than microcline feldspar. Crystallinity index is calculated to know the crystalline nature of quartz present in the sediments. Heavy mineral separation analysis has been carried out to know the total heavy mineral (THM) percentage. This analysis revealed the presence of nine heavy minerals. The minerals such as monazite, zircon, magnetite and illmenite are predominant. Due to the rapid and extreme changes occur in highly dynamic environments of sandy beaches, quantities of major light and heavy minerals are widely varied from site to site. Granulometric analysis shows that the sand is major content. Multivariate statistical (Pearson correlation, cluster and factor) analysis has been carried out to know the effect of mineralogy on radionuclide concentrations. The present study concluded that heavy minerals induce the ²³⁸U and ²³²Th concentrations. Whereas, light mineral (calcite) controls the ⁴⁰K concentration. In addition to the heavy minerals, clay content also induces the important radioactive variables.     

Analysis of death pattern in cancer cells by using different kinds of LET radiation

We investigated the death pattern of cancer cells by using different kinds of linear energy transfer (LET) radiation. We used two human squamous cell carcinoma cell lines with an identical genotype except for the p53 gene. SAS/mp53 cells were established by transfection with the mp53 gene to SAS cells having functional p53 (wtp53). As the control, a neovector was transfected to the SAS cells (SAS/neo cells). Both types of cells were exposed to X-rays (1.5 KeV/micron) or accelerated C-beams (30-100 KeV/micron). The frequency of cell death (apoptosis and necrosis) was measured by acridine orange/ethidium bromide(AO/EB) double staining for fluorescence microscopy. We found that (1) low-LET radiation induced apoptosis only in SAS/neo cells; (2) high-LET radiation at an iso-survival dose induced apoptosis not but necrosis in SAS/neo cells at a higher frequency; (3) high-LET radiation induced p53-independent apoptosis even in SAS/mp53 cells. Our findings suggest that high-LET radiotherapy is expected to (1) have validity in its application to patients carrying mutated p53 cancer cells and (2) reduce injury to adjacent normal tissue for high-frequency-induced apoptosis without inflammatory response. We propose that elucidation of p53-independent apoptosis-related genes might provide new insights into radiotherapy for cancer.     

Response of a cellulose nitrate plastic track detector to heavy ions

Samples of cellulose nitrate (Russian) plastic detectors exposed to different ions have been obtained from JINR, Dubna (U.S.S.R.). The bulk etch rate and track etch rate are measured for different temperatures and hence the activation energies are determined. The dependence of track etch rate on the energy loss, dE/dx of different ions is presented. It is observed that the normalized track etch rate V = V_t/V_b depends on dE/dx as well as on etching temperature. The maximum etched track lengths of these ions are compared with theoretically computed ranges. The experimental results give the evidence that there is no sharp threshold, at least in CN(R).     

异常高氡温泉辐射水平调查

目的 对降扎温泉周围的辐射水平进行调查,分析污染的来源。方法 采用固体径迹探测器、氡及子体连续测量装置,γ剂量率仪测量了温泉生活区及周边环境的氡、氡子体浓度及γ照射量率。结果 温泉水中氡浓度为23~764 Bq/L,生活区室内、室外及浴场中氡的浓度分别为254~876799、688~709和3590~15299 Bq/m³;室内外γ照射量率分别为205~28718和4104~18254 nGy/h。结论 降扎温泉是目前发现的异常高氡和高天然辐射区,其辐射水平和健康影响应引起关注。     

Formation and repair of clustered damaged DNA sites in high LET irradiated cells

Purpose: Radiation with high linear energy transfer (LET) produces clustering of DNA double-strand breaks (DSB) as well as non-DSB lesions. Heat-labile sites (HLS) are non-DSB lesions in irradiated cells that may convert into DSB at elevated temperature during preparation of naked DNA for electrophoretic assays and here we studied the initial formation and repair of these clustered damaged sites after irradiation with high LET ions.

Materials and methods: Induction and repair of DSB were studied in normal human skin fibroblast (GM5758) after irradiation with accelerated carbon and nitrogen ions at an LET of 125 eV/nm. DNA fragmentation was analyzed by pulsed-field gel electrophoresis (PFGE) and by varying the lysis condition we could differentiate between prompt DSB and heat-released DSB.

Results: Before repair (t = 0 h), the 125 eV/nm ions produced a significant fraction of heat-released DSB, which appeared clustered on DNA fragments with sizes of 1 Mbp or less. These heat-released DSB increased the total number of DSB by 30–40%. This increase is similar to what has been found in low-LET irradiated cells, suggesting that the relative biological effectiveness (RBE) for DSB induction will not be largely affected by the lysis temperature. After 1–2 hours repair, a large fraction of DSB was still unrejoined but there was essentially no heat-released DSB present.

Conclusions: These results suggest that high LET radiation, as low LET gamma radiation, induces a significant fraction of heat-labile sites which can be converted into DSB, and these heat-released DSB may affect both induction yields and estimates of repair.     

Locus specificity for mutation induction in human cells exposed to accelerated heavy ions

The relative efficiencies of two types of densely ionizing particles were compared for the induction of mutations at two distinct genetic loci in human cells. Mutations to 6-thioguanine resistance (hgprt locus) or to trifluorothymidine resistance (tk) locus were scored in TK6 human lymphoblastoid cells exposed to graded doses of 40Ar ions (470 MeV/amu, LET = 95-97 keV/microns) or 28Si ions (456 MeV/amu, 61 keV/microns). The autosomal tk locus was more efficiently mutated than the X-linked hgprt locus following heavy particle irradiations. This was predominantly due to the contribution of a class of slowly growing mutants scored at the tk locus. Silicon ions were more efficient per unit dose than argon ions for the induction of mutants at either locus. When the mutant yield for a particular ion was compared with particle fluence, similar numbers of hgprt mutants are induced by equal numbers of 40Ar or 28Si ions. Comparison of the number of tk mutants with particle fluence demonstrates an increased efficiency for 28Si ions over 40Ar. These data suggest that the LET-RBE relationship may be different for individual genetic loci in human cells.