A practical alpha particle irradiator for studying internal alpha particle exposure

Experimental Setups for In Vitro Studies on Radon Exposure in Mammalian Cells-A Critical Overview

Naturally occurring radon and its short lived progeny are the second leading cause of lung cancer after smoking, and the main risk factor for non-smokers. The radon progeny, mainly Polonium-218 (²¹⁸Po) and Polonium-214 (²¹⁴Po), are responsible for the highest dose deposition in the bronchial epithelium via alpha-decay. These alpha-particles release a large amount of energy over a short penetration range, which results in severe and complex DNA damage. In order to unravel the underlying biological mechanisms which are triggered by this complex DNA damage and eventually give rise to carcinogenesis, in vitro radiobiology experiments on mammalian cells have been performed using radon exposure setups, or radon analogues, which mimic alpha-particle exposure. This review provides an overview of the different experimental setups, which have been developed and used over the past decades for in vitro radon experiments. In order to guarantee reliable results, the design and dosimetry of these setups require careful consideration, which will be emphasized in this work. Results of these in vitro experiments, particularly on bronchial epithelial cells, can provide valuable information on biomarkers, which can assist to identify exposures, as well as to study the effects of localized high dose depositions and the heterogeneous dose distribution of radon.

Computational modelling of γ-H2AX foci formation in human cells induced by alpha particle exposure

In cellular experiments, radiation-induced DNA damage can be quantified by counting the number of γ-H2AX foci in cell nucleus by using an immunofluorescence microscope. Quantification of DNA damage carries uncertainty, not only due to lack of full understanding the biological processes but also limitations in measurement techniques. The causes of limited certainty include the possibility of expressing foci in varying sizes responding individual DSBs and the overlapping of foci on the two-dimensional (2D) immunofluorescence microscopy image of γ-H2AX foci, especially when produced due to high-LET radiation exposure. There have been discussions on those limitations, but no successful studies to overcome them. In this paper, a practical modelling has been developed to simulate the occurrences of double-strand breaks (DSBs) and the formations of γ-H2AX foci in response to individual DSB formations, in cell nucleus due to exposure to alpha particles. Cell irradiation and DSB production were simulated using a user-written code that utilizes Geant4-DNA physics models. A C +  + code was used to simulate the formation γ-H2AX foci, which were spatially correlated to the loci of DBSs, and to calculate the number of individual foci from the observed 2D image of the cell nucleus containing the overlapping γ-H2AX foci. The average size of focal images was larger from alpha particle exposure than that from X-ray exposure, whereas the number of separate focal images were comparable except at doses up to 0.5 Gy. About 40% of separate focal images consisted of overlapping γ-H2AX foci at 1 Gy of alpha particle exposure. The foci overlapping ratios were obtained by simulation for individual size groups of focal images at varying doses. The size distributions of foci at varying doses were determined with experimentally obtained separate focal images. The correction factor for foci number was calculated using the foci overlapping ratio and foci size distribution, which are specific to dose from alpha particle exposure. The number of individual foci formations induced by applying the correction factor to the experimentally observed number of focal images better reflected the quality of alpha particles in causing DNA damage. Consequently, the conventional γ-H2AX assay can be better implemented by employing this computational modelling of γ-H2AX foci formation.

Small-scale dosimetry for alpha particle 241Am source cell irradiation and estimation of γ-H2AX foci distribution in prostate cancer cell line PC3

Background

The development of new targeted alpha therapies motivates improving alpha particle dosimetry. For alpha particles, microscopic targets must be considered to estimate dosimetric quantities that can predict the biological response. As double-strand breaks (DSB) on DNA are the main cause of cell death by ionizing radiation, cell nuclei are relevant volumes necessary to consider as targets. Since a large variance is expected of alpha particle hits in individual cell nuclei irradiated by an uncollimated alpha-emitting source, the damage induced should have a similar distribution. The induction of DSB can be measured by immunofluorescent γ-H2AX staining. The cell γ-H2AX foci distribution and alpha particle hits distribution should be comparable and thereby verify the necessity to consider the relevant dosimetric volumes.

Methods

A Monte Carlo simulation model of an ²⁴¹Am source alpha particle irradiation setup was combined with two versions of realistic cell nuclei phantoms. These were generated from DAPI-stained PC3 cells imaged with fluorescent microscopy, one consisting of elliptical cylinders and the other of segmented mesh volumes. PC3 cells were irradiated with the ²⁴¹Am source for 4, 8 and 12 min, and after 30 min fixated and stained with immunofluorescent γ-H2AX marker. The detected radiation-induced foci (RIF) were compared to simulated RIF.

Results

The mesh volume phantom detected a higher mean of alpha particle hits and energy imparted (MeV) per cell nuclei than the elliptical cylinder phantom, but the mean specific energy (Gy) was very similar. The mesh volume phantom detected a slightly larger variance between individual cells, stemming from the more extreme and less continuous distribution of cell nuclei sizes represented in this phantom. The simulated RIF distribution from both phantoms was in good agreement with the detected RIF, although the detected distribution had a zero-inflated shape not seen in the simulated distributions. An estimate of undetected foci was used to correct the detected RIF distribution and improved the agreement with the simulations.

Conclusion

Two methods to generate cell nuclei phantoms for Monte Carlo dosimetry simulations were tested and generated similar results. The simulated and detected RIF distributions from alpha particle-irradiated PC3 cells were in good agreement, proposing the necessity to consider microscopic targets in alpha particle dosimetry.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40658-022-00475-x.

Characterization of γ-H2AX foci formation under alpha particle and X-ray exposures for dose estimation

DNA double-strand break (DSB) induction is one of the phenotypes of cellular damage from radiation exposure and is commonly quantified by γ-H2AX assay with the number of excess fluorescent foci per cell as the main component. However, the number of foci alone may not fully characterize the state of DNA damage following exposures to different radiation qualities. This study investigated the feasibility of utilizing the focus size distribution and dephosphorylation rate of γ-H2AX to identify the type of causative radiation and dose. Human lung epithelial cells and mouse vascular endothelial cells were used to observe the expression changes of γ-H2AX foci due to alpha particle and X-ray exposures. Results showed that the average number of excess foci per cell linearly increased with the dose. The focus size distribution showed a consistent pattern depending on the causative radiation type. Three criteria for the identification of causative radiation type were derived from experimental focus size distributions and were validated in blind testing with correct identification of 27 out of 32 samples. The dose could be estimated based on the proportionality constant specific to the identified radiation type with a difference of less than 15% from the actual value. The different dephosphorylation rates of γ-H2AX produced from alpha particle and X-ray exposures were effectively utilized to determine the individual dose contributions of alpha particles and X-rays under mixed beam exposure. Individual doses were estimated to have differences of less than ~ 12% from actual values.

A high-throughput alpha particle irradiation system for monitoring DNA damage repair, genome instability and screening in human cell and yeast model systems

Ionizing radiation (IR) is environmentally prevalent and, depending on dose and linear energy transfer (LET), can elicit serious health effects by damaging DNA. Relative to low LET photon radiation (X-rays, gamma rays), higher LET particle radiation produces more disease causing, complex DNA damage that is substantially more challenging to resolve quickly or accurately. Despite the majority of human lifetime IR exposure involving long-term, repetitive, low doses of high LET alpha particles (e.g. radon gas inhalation), technological limitations to deliver alpha particles in the laboratory conveniently, repeatedly, over a prolonged period, in low doses and in an affordable, high-throughput manner have constrained DNA damage and repair research on this topic. To resolve this, we developed an inexpensive, high capacity, 96-well plate-compatible alpha particle irradiator capable of delivering adjustable, low mGy/s particle radiation doses in multiple model systems and on the benchtop of a standard laboratory. The system enables monitoring alpha particle effects on DNA damage repair and signalling, genome stability pathways, oxidative stress, cell cycle phase distribution, cell viability and clonogenic survival using numerous microscopy-based and physical techniques. Most importantly, this method is foundational for high-throughput genetic screening and small molecule testing in mammalian and yeast cells.

Combined effect of alpha particles and cigarette smoke on human lung epithelial cells in vitro

Purpose: The combined toxicity of alpha particles and cigarette smoke to the critical cells in the lungs was investigated to assess the risk of smoking workers who handle naturally occurring radioactive materials. Materials and methods: The toxicity of alpha particles and cigarette smoke extract (CSE) was evaluated in terms of DNA double-strand break (DSB) induction and clonogenic cell death of human lung epithelial cells in vitro. The cells were exposed to alpha particles at doses of up to 0.25 Gy for gamma-H2AX assay and from 1.25 Gy to 5 Gy for clonogenic assay. CSE exposure of the cells was facilitated in the culture medium at CSE concentrations ranging from 1% to 12%. Additional experiments were performed using mouse endothelial cells for comparison. Results: The increases in the levels of DNA DSBs were linearly dependent on radiation dose and CSE concentration. The CSE-treated cells also responded with a linearly increasing number of DNA DSBs to the radiation dose. Both human lung epithelial cells and mouse endothelial cells showed exponential decreases in clonogenic surviving fraction as the dose from alpha particle exposure increased. Both cells responded with the clonogenic surviving fractions decreasing in a linear proportion to the CSE concentration in the culture medium. Conclusion: In our experimental in vitro setup, CSE treatment and alpha particle exposure affected the cells in an additive manner either for DNA DSB production or for clonogenic cell death induction.

Large Field Alpha Irradiation Setup for Radiobiological Experiments

The use of alpha particles irradiation in clinical practice has gained interest in the past years, for example with the advance of radionuclide therapy. The lack of affordable and easily accessible irradiation systems to study the cell biological impact of alpha particles hampers broad investigation. Here we present a novel alpha particle irradiation set-up for uniform irradiation of cell cultures. By combining a small alpha emitting source and a computer-directed movement stage, we established a new alpha particle irradiation method allowing more advanced biological assays, including large-field local alpha particle irradiation and cell survival assays. In addition, this protocol uses cell culture on glass cover-slips which allows more advanced microscopy, such as super-resolution imaging, for in-depth analysis of the DNA damage caused by alpha particles. This novel irradiation set-up provides the possibility to perform reproducible, uniform and directed alpha particle irradiation to investigate the impact of alpha radiation on the cellular level.

Design and testing of a microcontroller that enables alpha particle irradiators to deliver complex dose rate patterns

There is increasing interest in using alpha particle emitting radionuclides for cancer therapy because of their unique cytotoxic properties which are advantageous for eradicating tumor cells. The high linear energy transfer (LET) of alpha particles produces a correspondingly high density of ionizations along their track. Alpha particle emitting radiopharmaceuticals deposit this energy in tissues over prolonged periods with complex dose rate patterns that depend on the physical half-life of the radionuclide, and the biological uptake and clearance half-times in tumor and normal tissues. We have previously shown that the dose rate increase half-time that arises as a consequence of these biokinetics can have a profound effect on the radiotoxicity of low-LET radiation. The microcontroller hardware and software described here offer a unique way to deliver these complex dose rate patterns with a broad-beam alpha particle irradiator, thereby enabling experiments to study the radiobiology of complex dose rate patterns of alpha particles. Complex dose rate patterns were created by precise manipulation of the timing of opening and closing of the electromechanical shutters of an α-particle irradiator. An Arduino Uno and custom circuitry was implemented to control the shutters. The software that controls the circuits and shutters has a user-friendly Graphic User Interface (GUI). Alpha particle detectors were used to validate the programmed dose rate profiles. Circuit diagrams and downloadable software are provided to facilitate adoption of this technology by other radiobiology laboratories.

Measurement of activity distribution in an Am-241 disc source using peeled-off Gafchromic EBT3 films

Commercial alpha-emitting sources are fabricated mainly in a disc type. An alpha particle irradiator in Radiation Bioengineering Laboratory at Seoul National University was installed with an Am-241 disc source. Commercial Am-241 disc sources are fabricated by incorporating the radioactive element into a thin substrate layer. Those disc sources are utilized assuming that the radioactive element is uniformly distributed in the active layer of disc sources. In this study, we employed peeled-off Gafchromic EBT3 films to investigate the uniformity of areal radioactivity density over the disc source and to measure the effect of non-uniform activity distribution on dose distribution at the bottom of the cell culture dish positioned in a varying distance from the source. The measurements with the peeled-off EBT3 films informed that the areal activity density in the disc source differed by up to approximately 45% from the average. However, the inhomogeneous Am-241 distribution in a disc source did not affect the radial distribution of fluence rate at the inner bottom of cell dish when the dish is apart from the source sufficiently. The dose distribution measured with an EBT3 film nearly accorded with that obtained by Monte Carlo simulation assuming the uniform Am-241 activity distribution in the active layer of the disc source. Finally, the dose to a single-cell layer of 5 μm in a nominal thickness was obtained by Monte Carlo simulation assuming a uniform Am-241 activity distribution in the disc source at distances of 20 and 30 mm from the source. The cellular dose estimates were higher than the film dose estimates at all radial distances. The cellular dose decreased with an increasing radial distance from the center to a smaller extent than the EBT3 film dose did.