DNA double-strand break signalling: X-ray energy dependence of residual co-localised foci of γ-H2AX and 53BP1

Energy spectrum measurement of scattered X-rays during IVR procedure

With the increase of the number of interventional radiology (IVR) procedures, the occupational exposure of operators and medical staff has attracted keen attention. The energy of scattered radiation in medical clinical sites is important for estimating the biological effects of occupational exposure. Recent years have seen many reports on the dose of scattered radiation by IVR, but few on the energy spectrum. In this study, the energy spectrum of scattered X-rays was measured by using a cadmium telluride (CdTe) semiconductor detector during IVR on several neurosurgical and cardiovascular cases. The cumulated spectra in each case were compared. The spectra showed little changes among neurosurgical cases and relatively large changes among cardiovascular cases. This was assumed to be due to the change of X-ray tube voltage and tube angle was larger in cardiovascular cases. The resulting energy spectra will be essential for the assessment of detailed biological effects of occupational exposure.

Peripheral blood lymphocytes differ in DNA damage response after exposure to X-rays with different physical properties

Introduction: In radiology, low X-ray energies (<140 keV) are used to obtain an optimal image while in radiotherapy, higher X-ray energies (MeV) are used to eradicate tumor tissue. In radiation research, both these X-ray energies being used to extrapolate in vitro research to clinical practice. However, the energy deposition of X-rays depends on their energy spectrum, which might lead to changes in biological response. Therefore, this study compared the DNA damage response (DDR) in peripheral blood lymphocytes (PBLs) exposed to X-rays with varying beam quality, mean photon energy (MPE) and dose rate.Methods: The DDR was evaluated in peripheral blood lymphocytes (PBLs) by the ɣ-H2AX foci assay, the cytokinesis-block micronucleus assay and an SYTOX-based cell death assay, combined with specific cell death inhibitors. Cell cultures were irradiated with a 220 kV X-ray research cabinet (SARRP, X-Strahl) or a 6 MV X-ray linear accelerator (Elekta Synergy). Three main physical parameters were investigated: beam quality (V), MPE (eV) and dose rate (Gy/min). Additional copper (Cu) filtration caused variation in the MPE (78 keV, 94 keV, 118 keV) at SARRP; dose rates were varied by adjusting tube current for 220 kV X-rays (0.33-3 Gy/min) or water-phantom depth in the 6 MV set-up (3-6 Gy/min).Results: The induction of chromosomal damage and initial (30 min) DNA double-stranded breaks (DSBs) were significantly higher for 220 kV X-rays compared to 6 MV X-rays, while cell death induction was similar. Specific cell death inhibitors for apoptosis, necroptosis and ferroptosis were not capable of blocking cell death after irradiation using low or high-energy X-rays. Additional Cu filtration increased the MPE, which significantly decreased the amount of chromosomal damage and DSBs. Within the tested ranges no specific effects of dose rate variation were observed.Conclusion: The DDR in PBLs is influenced by the beam quality and MPE. This study reinforces the need for consideration and inclusion of all physical parameters in radiation-related studies.

An inter-comparison between radiobiological characteristics of a commercial low-energy IORT system by Geant4-DNA and MCDS Monte Carlo codes

INTRODUCTION

The need for accurate relative biological effectiveness (RBE) estimation for low energy therapeutic X-rays (corresponding to 50 kV nominal energy of a commercial low-energy IORT system (INTRABEAM)) is a crucial issue due to increased radiobiological effects, respect to high energy photons. Modeling of radiation-induced DNA damage through Monte Carlo (MC) simulation approaches can give useful information. Hence, this study aimed to evaluate and compare RBE of low energy therapeutic X-rays using Geant4-DNA toolkit and Monte Carlo damage simulation (MCDS) code.

MATERIALS AND METHODS

RBE calculations were performed considering the emitted secondary electron spectra through interactions of low energy X-rays inside the medium. In Geant4-DNA, the DNA strand breaks were obtained by employing a B-DNA model in physical stage with 10.79 eV energy-threshold and the probability of hydroxyl radical's chemical reactions of about 0.13%. Furthermore, RBE estimations by MCDS code were performed under fully aerobic conditions.

RESULTS

Acquired results by two considered MC codes showed that the same trend is found for RBEDSB and RBESSB variations. Totally, a reasonable agreement between the calculated RBE values (both RBESSB and RBEDSB) existed between the two considered MC codes. The mean differences of 9.2% and 1.8% were obtained between the estimated RBESSB and RBEDSB values by two codes, respectively.

CONCLUSION

Based on the obtained results, it can be concluded that a tolerable accordance is found between the calculated RBEDSB values through MCDS and Geant4-DNA, a fact which appropriates both codes for RBE evaluations of low energy therapeutic X-rays, especially in the case of RBEDSB where lethal damages are regarded.

Inflammatory Signaling and DNA Damage Responses after Local Exposure to an Insoluble Radioactive Microparticle

Simple Summary

A cesium-bearing microparticle (Cs-BMP) is an insoluble radioactive microparticle possessing high specific radioactivity, which was discovered after the incident at the Fukushima nuclear power plant. Due to their insoluble nature, such Cs-BMPs are assumed to adhere in the long term to normal tissue, leading to chronic local exposure. However, radiation risk due to the intake of internal exposure to radioactive cesium is conventionally estimated from the organ dose given by uniform exposure to soluble cesium. As such, it is critical to clarify the normal tissue effects posed by heterogeneous exposure to Cs-BMPs. This in vitro study reports on the relationship between the inflammatory responses and DNA damage induction during local exposure to a Cs-BMP.

Abstract

Cesium-bearing microparticles (Cs-BMPs) can reach the human respiratory system after inhalation, resulting in chronic local internal exposure. We previously investigated the spatial distribution of DNA damage induced in areas around a Cs-BMP; however, the biological impacts have not been fully clarified due to the limited amount of data. Here, we investigated the inflammatory signaling and DNA damage responses after local exposure to a Cs-BMP in vitro. We used two normal human lung cell lines, i.e., lung fibroblast cells (WI-38) and bronchial epithelial cells (HBEC3-KT). After 24 h exposure to a Cs-BMP, inflammation was evaluated by immunofluorescent staining for nuclear factor κB (NF-κB) p65 and cyclooxygenase 2 (COX-2). The number of DNA double-strand breaks (DSBs) was also detected by means of phospholylated histone H2AX (γ-H2AX) focus formation assay. Cs-BMP exposure significantly increased NF-κB p65 and COX-2 expressions, which were related to the number of γ-H2AX foci in the cell nuclei. Compared to the uniform (external) exposure to ¹³⁷Cs γ-rays, NF-κB tended to be more activated in the cells proximal to the Cs-BMP, while both NF-κB p65 and COX-2 were significantly activated in the distal cells. Experiments with chemical inhibitors for NF-κB p65 and COX-2 suggested the involvement of such inflammatory responses both in the reduced radiosensitivity of the cells proximal to Cs-BMP and the enhanced radiosensitivity of the cells distal from Cs-BMP. The data show that local exposure to Cs-BMP leads to biological effects modified by the NF-κB pathway, suggesting that the radiation risk for Cs-BMP exposure can differ from that estimated based on conventional uniform exposure to normal tissues.

High versus low dose irradiation for tumor immune reprogramming

Local administration of ionizing radiation to tumors can promote anticancer immune responses that lead to the abscopal regression of distant metastases, especially in patients receiving systemic immune-checkpoint inhibitors. Growing preclinical evidence indicates that high-dose irradiation administered locally to destroy malignant lesions, can promote the release of danger-associated molecular patterns that lead to the recruitment of immune cells, thus inducing a systemic response against tumor antigens that protects against local disease relapse and also mediates distant antineoplastic effects. An accumulating body of preclinical evidence supports also the implementation of low-dose irradiation to induce tumor immune reprogramming. Here, we provide the rationale for a clinical research agenda to refine future clinical practice based on innovative combinations of radiation-immunotherapy.

Spectral and spatial shaping of laser-driven proton beams using a pulsed high-field magnet beamline

Intense laser-driven proton pulses, inherently broadband and highly divergent, pose a challenge to established beamline concepts on the path to application-adapted irradiation field formation, particularly for 3D. Here we experimentally show the successful implementation of a highly efficient (50% transmission) and tuneable dual pulsed solenoid setup to generate a homogeneous (laterally and in depth) volumetric dose distribution (cylindrical volume of 5 mm diameter and depth) at a single pulse dose of 0.7 Gy via multi-energy slice selection from the broad input spectrum. The experiments were conducted at the Petawatt beam of the Dresden Laser Acceleration Source Draco and were aided by a predictive simulation model verified by proton transport studies. With the characterised beamline we investigated manipulation and matching of lateral and depth dose profiles to various desired applications and targets. Using an adapted dose profile, we performed a first proof-of-technical-concept laser-driven proton irradiation of volumetric in-vitro tumour tissue (SAS spheroids) to demonstrate concurrent operation of laser accelerator, beam shaping, dosimetry and irradiation procedure of volumetric biological samples.

Track Structure Study for Energy Dependency of Electrons and X-rays on DNA Double-Strand Break Induction

Radiation weighting factor wR for photons and electrons has been defined as unity independently of the energy of the particles. However, the biological effects depend on the incident energies according to in vitro experimental data. In this study, we have quantified the energy concentration along electron tracks in terms of dose-mean lineal energy (yD) on chromosome (micro-meter) and DNA (nano-meter) order scales by Monte Carlo simulations, and evaluated the impact of photon energies on DNA double-strand break (DNA-DSB) induction from an experimental study of irradiated cells. Our simulation result shows that the yD values for diagnostic X-rays (60-250 kVp) are higher than that for therapeutic X-rays (linac 6 MV), which agrees well with the tissue equivalent proportional counter (TEPC) measurements. The relation between the yD values and the numbers of γ-H2AX foci for various photon energy spectra suggests that low energy X-rays induce DNA-DSB more efficiently than higher energy X-rays even at the same absorbed dose (e.g., 1.0 Gy). The relative biological effectiveness based on DNA-DSBs number (RBEDSB) is proportionally enhanced as the yD value increases, demonstrating that the biological impact of the photon irradiation depends on energy concentration along radiation tracks of electrons produced in the bio-tissues. Ultimately, our study implies that the value of wR for photons varies depending on their energies.

DNA damage induction during localized chronic exposure to an insoluble radioactive microparticle

Insoluble radioactive microparticles emitted by the incident at the Fukushima nuclear power plant have drawn keen interests from the viewpoint of radiation protection. Cs-bearing particles have been assumed to adhere in the long term to trachea after aspirated into respiratory system, leading to heterogeneous dose distribution within healthy tissue around the particles. However, the biological effects posed by an insoluble radioactive particle remain unclear. Here, we show cumulative DNA damage in normal human lung cells proximal and distal to the particle (β-ray and γ-ray-dominant areas, respectively) under localized chronic exposure in comparison with uniform exposure. We put a Cs-bearing particle into a microcapillary tip and placed it onto a glass-base dish containing fibroblast or epithelial cells cultured in vitro. A Monte Carlo simulation with PHITS code provides the radial distribution of absorbed dose-rate around the particle, and subsequently we observed a significant change in nuclear γ-H2AX foci after 24 h or 48 h exposure to the particle. The nuclear foci in the cells distal to the particle increased even under low-dose-rate exposure compared with uniform exposure to 137Cs γ-rays, which was suppressed by a treatment with a scavenger of reactive oxygen species. In contrast, such focus formation was less manifested in the exposed cells proximal to the particle compared with uniform exposure. These data suggest that the localized exposure to a Cs-bearing particle leads to not only disadvantage to distal cells but also advantage to proximal cells. This study is the first to provide quantitative evaluation for the spatial distribution of DNA double strand breaks after the heterogeneous chronic exposure to a Cs-bearing particle in comparison with uniform Cs exposure.

Relation between DNA double-strand breaks and energy spectra of secondary electrons produced by different X-ray energies

Purpose: In a radiological examination, low-energy X-radiation is used (<100 keV). For other radiological procedures, the energy used is several MeV. ICRP in publication 103 has currently considered that photons irrespective of their energy have the same radiation weighting factor. Nevertheless, there are topological differences at the nanoscale of X-ray energy deposition as a function of its energy spectrum, meaning that the different interactions with living matter could vary in biological efficacy. Materials and methods: To study these differences, we characterized our irradiation conditions in terms of initial photon energies, but especially in terms of energy spectra of secondary electrons at the cell nucleus level, using Monte Carlo simulations. We evaluated signaling of DNA damage by monitoring a large number of γH2A.X foci after exposure of G0/G1-phase synchronized human primary endothelial cells from 0.25 to 5 Gy at 40 kV, 220 kV and 4 MV X-rays. Number and spatial distribution of γH2A.X foci were explored. In parallel, we investigated cell behavior through cell death and ability of a mother cell to produce two daughter cells. We also studied the missegregation rate after cell division. Results: We report a higher number of DNA double-strand breaks signaled by γH2A.X for 40 kVp and/or 220 kVp compared to 4 MVp for the highest tested doses of 2 and 5 Gy. We observed no difference between the biological endpoint studies with 40 kVp and 220 kVp X-ray spectra. This lack of difference could be explained by the relative similarity of the calculated energy spectra of secondary electrons at the cell monolayer. Conclusion: The energy spectrum of secondary electrons seems to be more closely related to the level of DNA damage measured by γH2A.X than the initial spectrum of photon energy or voltage settings. Our results indicate that as the energy spectrum of secondary electrons increases, the DNA damage signaled by γH2A.X decreases and this effect is observable beyond 220 kVp.

DNA double strand breaks induced by low dose mammography X-rays in breast tissue: A pilot study

Breast tissue is very sensitive to ionizing radiation due to the presence of reproductive hormones, including estrogen. In the present pilot study, the efficiency of mammography X-rays to induce DNA double strand breaks (DSB) in mammary epithelial cells was investigated. For this, freshly resected healthy breast tissue was irradiated with 30 kV mammography X-rays in the dose range 0-500 mGy (2, 4, 10, 20, 40, 100 and 500 mGy). Breast specimens were also irradiated with identical doses of ⁶⁰Co γ-rays as a radiation quality standard. With the γH2AX-foci assay, the number of DNA DSB induced by radiation were quantified in the mammary epithelial cells present in breast tissue. Results indicated that foci induced by 30 kV X-rays and γ-rays followed a biphasic linear dose-response. For 30 kV X-rays, the slope in the low dose region (0-20 mGy) was 8.71 times steeper compared with the slope in the higher dose region (20-500 mGy). Furthermore, compared with γ-rays, 30 kV X-rays were also more effective in inducing γH2AX-foci. This resulted in a relative biological effectiveness (RBE) value of 1.82 in the low dose range. In the higher dose range, an RBE close to 1 was obtained. In conclusion, the results indicated the existence of a low dose hypersensitive response for DSB induction in the dose range representative for mammography screening, which is probably caused by the bystander effect. This could affect the radiation risk calculations for women participating in mammography screening.

Estimation of the radiation-induced DNA double-strand breaks number by considering cell cycle and absorbed dose per cell nucleus

DNA double-strand breaks (DSBs) are thought to be the main cause of cell death after irradiation. In this study, we estimated the probability distribution of the number of DSBs per cell nucleus by considering the DNA amount in a cell nucleus (which depends on the cell cycle) and the statistical variation in the energy imparted to the cell nucleus by X-ray irradiation. The probability estimation of DSB induction was made following these procedures: (i) making use of the Chinese Hamster Ovary (CHO)-K1 cell line as the target example, the amounts of DNA per nucleus in the logarithmic and the plateau phases of the growth curve were measured by flow cytometry with propidium iodide (PI) dyeing; (ii) the probability distribution of the DSB number per cell nucleus for each phase after irradiation with 1.0 Gy of 200 kVp X-rays was measured by means of γ-H2AX immunofluorescent staining; (iii) the distribution of the cell-specific energy deposition via secondary electrons produced by the incident X-rays was calculated by WLTrack (in-house Monte Carlo code); (iv) according to a mathematical model for estimating the DSB number per nucleus, we deduced the induction probability density of DSBs based on the measured DNA amount (depending on the cell cycle) and the calculated dose per nucleus. The model exhibited DSB induction probabilities in good agreement with the experimental results for the two phases, suggesting that the DNA amount (depending on the cell cycle) and the statistical variation in the local energy deposition are essential for estimating the DSB induction probability after X-ray exposure.

Comparison of clonogenic cell survival and DNA damage induced by 188Re and X-rays in rat thyroid cells

AIM

Ionizing radiation produces DNA lesions among which DNA double strand breaks (DSB) are the most critical events. Radiation of various energy types might differ in their biological effectiveness. Here, we compared cell survival and DNA damage induced by ¹⁸⁸Re and X-rays using γH2AX foci as a measure of DSB. The correlation between survival and residual foci was also analyzed.

METHODS

PCCl3 cells were irradiated with 200 kV X-rays (1.2 Gy/min) or 0.5-25 MBq/ml ¹⁸⁸Re (1 h irradiation) achieving doses up to 10 Gy. By blocking of sodium iodide symporter (NIS) essentially extracellular activity could be guaranteed. Survival fractions (SF) were detected by colony forming assay. Initial and residual γH2AX foci (15 min and 24 h after irradiation) were assessed by immunostaining. The relationship between SF and residual radiation induced γH2AX foci (RIF) was evaluated by Spearman and Pearson correlation tests.

RESULTS

We did not find significant differences between the survival curves in terms of the radiation quality. The D₃₇ values were 4.6 Gy and 4.2 Gy for ¹⁸⁸Re or X-ray, respectively. The initial foci numbers were in the same range for ¹⁸⁸Re and X-ray, but higher levels of residual foci persisted after X-rays in comparison to ¹⁸⁸Re (1 Gy_X-ray 6.5 ± 0.2; 1 Gy_Re-188 4.8 ± 0.2 RIF). Accordingly, for ¹⁸⁸Re a higher extent of DSB repair was found. The Spearman test revealed a significant (p < 0.01) correlation between SF and residual RIF for both radiation modalities.

CONCLUSION

No differences in terms of radiation were found for SF and initial foci. However, residual foci were lower for ¹⁸⁸Re than for X-rays. A prediction of SF by residual foci should consider the properties of the radiation qualities that influence foci removal and DSB repair.

Radiobiological influence of megavoltage electron pulses of ultra-high pulse dose rate on normal tissue cells

Regarding the long-term goal to develop and establish laser-based particle accelerators for a future radiotherapeutic treatment of cancer, the radiobiological consequences of the characteristic short intense particle pulses with ultra-high peak dose rate, but low repetition rate of laser-driven beams have to be investigated. This work presents in vitro experiments performed at the radiation source ELBE (Electron Linac for beams with high Brilliance and low Emittance). This accelerator delivered 20-MeV electron pulses with ultra-high pulse dose rate of 10(10) Gy/min either at the low pulse frequency analogue to previous cell experiments with laser-driven electrons or at high frequency for minimizing the prolonged dose delivery and to perform comparison irradiation with a quasi-continuous electron beam analogue to a clinically used linear accelerator. The influence of the different electron beam pulse structures on the radiobiological response of the normal tissue cell line 184A1 and two primary fibroblasts was investigated regarding clonogenic survival and the number of DNA double-strand breaks that remain 24 h after irradiation. Thereby, no considerable differences in radiation response were revealed both for biological endpoints and for all probed cell cultures. These results provide evidence that the radiobiological effectiveness of the pulsed electron beams is not affected by the ultra-high pulse dose rates alone.

Enhancement of DNA double-strand break induction and cell killing by K-shell absorption of phosphorus in human cell lines

PURPOSE

To investigate an enhancement of DNA double-strand break (DSB) induction and cell killing effect by K-shell ionization of phosphorus atoms and Auger electrons on human cell lines.

MATERIALS AND METHODS

Induction of DSB, DNA damage responses, cell cycle distributions, and cell killing effects were investigated after exposures of the cells with monochromatic synchrotron radiation soft X-rays of 2153 and 2147 eV, which were the resonance peak and off peak, respectively, of the K-shell photoabsorption of phosphorus.

RESULTS

Higher biological effects in the cells irradiated with soft X-rays at 2153 eV than at 2147 eV were observed in (i) the efficiency of 53BP1/γ-H2AX co-localized foci formation per dose and residual number of foci, (ii) prolonged phosphorylation levels of DSB repair and/or cell cycle checkpoint related proteins and G2 arrest, (iii) the cell killing effects at the 10% survival level of normal human fibroblasts, HeLa cells, and human glioblastoma M059K cells (1.2-1.5 times higher) and that of human ataxia telangiectasia mutated (ATM)-defective cells and glioblastoma DNA-dependent protein kinase catalytic subunit (DNA-PKcs)-defective cells (1.2 times).

CONCLUSION

The yield of DSB and partly less-reparable complex DNA damage induction in human cells was enhanced by K-shell photoabsorption of phosphorus and low-energy Auger electrons.

Radiobiological response to ultra-short pulsed megavoltage electron beams of ultra-high pulse dose rate

PURPOSE

In line with the long-term aim of establishing the laser-based particle acceleration for future medical application, the radiobiological consequences of the typical ultra-short pulses and ultra-high pulse dose rate can be investigated with electron delivery.

MATERIALS AND METHODS

The radiation source ELBE (Electron Linac for beams with high Brilliance and low Emittance) was used to mimic the quasi-continuous electron beam of a clinical linear accelerator (LINAC) for comparison with electron pulses at the ultra-high pulse dose rate of 10(10) Gy min(-1) either at the low frequency of a laser accelerator or at 13 MHz avoiding effects of prolonged dose delivery. The impact of pulse structure was analyzed by clonogenic survival assay and by the number of residual DNA double-strand breaks remaining 24 h after irradiation of two human squamous cell carcinoma lines of differing radiosensitivity.

RESULTS

The radiation response of both cell lines was found to be independent from electron pulse structure for the two endpoints under investigation.

CONCLUSIONS

The results reveal, that ultra-high pulse dose rates of 10(10) Gy min(-1) and the low repetition rate of laser accelerated electrons have no statistically significant influence (within the 95% confidence intervals) on the radiobiological effectiveness of megavoltage electrons.

Quantitative estimation of DNA damage by photon irradiation based on the microdosimetric-kinetic model

The microdosimetric-kinetic (MK) model is one of the models that can describe the fraction of cells surviving after exposure to ionizing radiation. In the MK model, there are specific parameters, k and yD, where k is an inherent parameter to represent the number of potentially lethal lesions (PLLs) and yD indicates the dose-mean lineal energy in keV/μm. Assuming the PLLs to be DNA double-strand breaks (DSBs), the rate equations are derived for evaluating the DSB number in the cell nucleus. In this study, we estimated the ratio of DSBs for two types of photon irradiation (6 MV and 200 kVp X-rays) in Chinese hamster ovary (CHO-K1) cells and human non-small cell lung cancer (H1299) cells by observing the surviving fraction. The estimated ratio was then compared with the ratio of γ-H2AX foci using immunofluorescent staining. For making a comparison of the number of DSBs among a variety of radiation energy cases, we next utilized the survival data in the literature for both cells exposed to other photon types, such as (60)Co γ-rays, (137)Cs γ-rays and 100 kVp X-rays. The ratio of DSBs based on the MK model with conventional data was consistent with the ratio of γ-H2AX foci numbers, confirming that the γ-H2AX focus is indicative of DSBs. It was also shown that the larger yD is, the larger the DSB number is. These results suggest that k and yD represent the characteristics of the surviving fraction and the biological effects for photon irradiation.

A formulation of cell surviving fraction after radiation exposure

Local energy transfer from electrons generated in biotissues that are exposed to ionizing radiation is fundamental to cell damage. Our aim in this investigation was to quantify the probability of cell mortality associated with the damage by electrons and the repair processes in the cell nucleus, envisaging a new interpretation of the cell surviving fraction (SF). We introduced a SF formula for cells exposed to X-rays, which is given as a linear combination of the Poisson distributions about the number of long-lived lesions per nucleus and their "non-lethal probabilities", to show the non-linearity of log SF as a function of dose. The model selection was rated by a statistical index, Akaike's information criterion (AIC). It was shown that the new formula is suitable for describing cell survival and explicitly takes account of the non-lethality in damage-processing pathways of the cells.

The effect of calyculin A on the dephosphorylation of the histone γ-H2AX after formation of X-ray-induced DNA double-strand breaks in human blood lymphocytes

PURPOSE

The purpose of this study was to investigate the effect of calyculin A on the number of γ-H2AX foci (phosphorylated histone variant 2AX) in lymphocytes after in vitro and in vivo irradiation with rather low doses as they are used in diagnostic and interventional radiology.

MATERIALS AND METHODS

For in vitro experiments blood samples of 14 healthy volunteers were irradiated with different doses (10, 50, 100 mGy) and incubated with (0.01, 0.1, 1, 10 nM) or without calyculin A for up to 2 hours. Non-irradiated samples with and without calyculin A served as controls. For in vivo evaluation blood samples were collected from seven patients undergoing computed tomography (CT) both with 1 nM calyculin A containing vials and vials without calyculin A. Foci were quantified in isolated lymphocytes using γ-H2AX immunofluorescence microscopy.

RESULTS

1 nM calyculin A led to a complete inhibition of γ-H2AX foci loss in irradiated samples whereas no inhibition of p53 binding protein 1 (53 BP1) foci was found. Lower concentrations of the phosphatase inhibitor did not have a sufficient effect on foci decrease. Calyculin A did not affect foci levels in non-irradiated samples. If no calyculin A was added into the vial before the blood draws detectable CT-induced foci levels were lower in all patients with a reduction of the medians of 35%.

CONCLUSIONS

Using γ-H2AX immunofluorescence microscopy calyculin A can be a useful tool to mark the induced γ-H2AX foci after low dose irradiation and to avoid an underestimation of the real deoxyribonucleic acid (DNA) damage in in vitro and in vivo experiments.

DNA double-strand breaks induced by mammographic screening procedures in human mammary epithelial cells

PURPOSE

To assess in vitro mammographic radiation-induced DNA damage in mammary epithelial cells from 30 patients with low (LR) or high (HR) family risk of breast cancer.

MATERIALS AND METHODS

Spontaneous and radiation-induced DNA double-strand breaks (DSB) were quantified by using immunofluorescence of the phosphorylated H2AX histone (γH2AX) in different conditions of mammography irradiation (2, 4, 2 + 2 mGy).

RESULTS

HR patients showed significantly more spontaneous γH2AX foci than LR patients (p = 0.014). A significant dose-effect was observed, with an exacerbation in HR patients (p = 0.01). The dose repetition (2 + 2 mGy) provided more induced and more unrepaired DSB than 2 mGy and 4 mGy, and was exacerbated in HR (p = 0.006).

CONCLUSIONS

This study highlights the existence of DSB induced by mammography and revealed by γH2AX assay with two major radiobiological effects occurring: A low-dose effect, and a LOw and Repeated Dose (LORD) effect. All these effects were exacerbated in HR patients. These findings may lead us to re-evaluate the number of views performed in screening using a single view (oblique) in women whose mammographic benefit has not properly been proved such as HR patients.