Spectrometry of linear energy transfer with track-etched detectors in carbon ion beams, MONO and SOBP

Microdosimetry for a carbon ion beam using track-etched detectors

Track-etched detectors (TED) have been used as linear energy transfer (LET) spectrometers in heavy ion beams for many years. LET spectra and depth-dose distribution of a carbon ion beam were measured behind polymethylmethacrylate degraders at Heavy Ion Medical Accelerator in Chiba, Japan. The measurements were performed along monoenergetic beam with energy 290 MeV u(-1) in different positions: (1) at beam extraction area, (2) at beginning, (3) maximum and (4) behind the Bragg peak region (0, 117, 147 and 151 mm of water-equivalent depth, respectively). The LET spectra inside and outside of the primary ion beam have been evaluated. TED record only heavy charged particles with LET above 8-10 keV µm(-1), while electrons and ions with lower LET are not detected. The Geant4 simulation toolkit version 4.9.6.P01 has been used to estimate the contribution of non-detected particles to absorbed dose. Presented results demonstrate the applicability of TED for microdosimetry measurements in therapeutic carbon ion beams.

Gamma-H2AX foci in cells exposed to a mixed beam of X-rays and alpha particles

Background

Little is known about the cellular effects of exposure to mixed beams of high and low linear energy transfer radiation. So far, the effects of combined exposures have mainly been assessed with clonogenic survival or cytogenetic methods, and the results are contradictory. The gamma-H2AX assay has up to now not been applied in this context, and it is a promising tool for investigating the early cellular response to mixed beam irradiation.

Purpose

To determine the dose response and repair kinetics of gamma-H2AX ionizing radiation-induced foci in VH10 human fibroblasts exposed to mixed beams of ²⁴¹Am alpha particles and X-rays.

Results

VH10 human fibroblasts were irradiated with each radiation type individually or both in combination at 37°C. Foci were scored for repair kinetics 0.5, 1, 3 and 24 h after irradiation (one dose per irradiation type), and for dose response at the 1 h time point. The dose response effect of mixed beam was additive, and the relative biological effectiveness for alpha particles (as compared to X-rays) was of 0.76 ± 0.52 for the total number of foci, and 2.54 ± 1.11 for large foci. The repair kinetics for total number of foci in cells exposed to mixed beam irradiation was intermediate to that of cells exposed to alpha particles and X-rays. However, for mixed beam-irradiated cells the frequency and area of large foci were initially lower than predicted and increased during the first 3 hours of repair (while the predicted number and area did not).

Conclusions

The repair kinetics of large foci after mixed beam exposure was significantly different from predicted based on the effect of the single dose components. The formation of large foci was delayed and they did not reach their maximum area until 1 h after irradiation. We hypothesize that the presence of low X-ray-induced damage engages the DNA repair machinery leading to a delayed DNA damage response to the more complex DNA damage induced by alpha particles.

Characterisation of a setup for mixed beam exposures of cells to 241Am alpha particles and X-rays

Exposure of humans to mixed fields of high- and low-linear energy transfer (LET) radiation occurs in many situations-for example, in urban areas with high levels of indoor radon as well as background gamma radiation, during airplane flights or certain forms of radiation therapy. From the perspective of health risk associated with exposure to mixed fields, it is important to understand the interactions between different radiation types. In most cellular investigations on mixed beams, two types of irradiations have been applied sequentially. Simultaneous irradiation is the desirable scenario but requires a dedicated irradiation facility. The authors have constructed a facility where cells can be simultaneously exposed to (241)Am alpha particles and 190-kV X-rays at 37°C. This study presents the technical details and the dosimetry of the setup, as well as validates the performance of the setup for clonogenic survival in AA8 Chinese hamster ovary cells. No significant synergistic effect was observed. The relative biological effectiveness of the alpha particles was 2.56 for 37 % and 1.90 for 10 % clonogenic survival.

Micronuclei in human peripheral blood lymphocytes exposed to mixed beams of X-rays and alpha particles

The purpose of this study was to analyse the cytogenetic effect of exposing human peripheral blood lymphocytes (PBL) to a mixed beam of alpha particles and X-rays. Whole blood collected from one donor was exposed to different doses of alpha particles ((241)Am), X-rays and a combination of both. All exposures were carried out at 37 °C. Three independent experiments were performed. Micronuclei (MN) in binucleated PBL were scored as the endpoint. Moreover, the size of MN was measured. The results show that exposure of PBL to a mixed beam of high and low linear energy transfer radiation led to significantly higher than expected frequencies of MN. The measurement of MN size did not reveal any differences between the effect of alpha particles and mixed beam. In conclusion, a combined exposure of PBL to alpha particles and X-rays leads to a synergistic effect as measured by the frequency of MN. From the analysis of MN distributions, we conclude that the increase was due to an impaired repair of X-ray-induced DNA damage.

Spectra of linear energy transfer and other dosimetry characteristics as measured in C290 MeV/n MONO and SOBP ion beams at HIMAC-BIO (NIRS, Japan) with different detectors

Active mobile dosimetry unit (Liulin), passive plastic nuclear track detectors (PNTD) and thermoluminescent detectors (TLD) were exposed in a C290 MeV/n beam at HIMAC-BIO (NIRS, Japan). Two different types of beam configuration were used--monoenergetic beam (MONO) and spread-out Bragg peak (SOBP); the detectors were placed at several depths from the entrance up to the depths behind the Bragg peak. Relative response of TLDs in beams has been studied as a function of the depth, and it was re-proved that it can depend on the linear energy transfer (LET). Liulin measures energy deposition in Si; the spectra of energy deposited in Si can be transformed to the spectra of lineal energy or LET. PNTDs are able to determine the LET of registered particles directly. The limitation of both methods is in the range in which they can determine the LET-Liulin is able to measure perpendicularly incident charged particles up to ∼35 keV/µm (in water), PNTD can measure from ∼7 to 400 keV/µm, independently of the registration angle. The results from both methods are compared and combined for both beams' configuration, and a good agreement is observed.