• Energy Research

High atomic number and high-energy (HZE) particles in deep space are of low abundance but substantially contribute to the biological effects of space radiation. Shielding is so far the most effective way to partially protect astronauts from these highly penetrating particles. However, simulated calculations and measurements have predicted that secondary particles resulting from the shielding of cosmic rays produce a significant fraction of the total dose and dose equivalent. In this study, we investigated the biological effects of secondary radiation with two cell types, and with cells exposed in different phases of the cell cycle, by comparing the biological effects of a 200 MeV/u iron beam with a shielded beam in which the energy of the iron ion beam was decreased from 500 MeV/u to 200 MeV/u with PMMA, polyethylene (PE), or aluminum. We found that beam shielding resulted in increased induction of 53BP1 foci and micronuclei in a cell-type-dependent manner compared with the unshielded 200 MeV/u Fe ion beam. These findings provide experimental proof that the biological effects of secondary particles resulting from the interaction between HZE particles and shielding materials should be considered in shielding design.

DNA double strand break (DSB) repair pathway choice following ionizing radiation (IR) is currently an appealing research topic, which is still largely unclear. Our recent paper indicated that the complexity of DSBs is a critical factor that enhances DNA end resection. It has been well accepted that the RPA-coated single strand DNA produced by resection is a signaling structure for ATR activation. Therefore, taking advantage of high linear energy transfer (LET) radiation to effectively produce complex DSBs, we investigated how the complexity of DSB influences the function of ATR pathway on the G2/M checkpoint regulation. Human skin fibroblast cells with or without ATM were irradiated with X rays or heavy ion particles, and dual-parameter flow cytometry was used to quantitatively assess the mitotic entry at early period post radiation by detecting the cells positive for phosphor histone H3. In ATM-deficient cells, ATR pathway played a pivotal role and functioned in a dose- and LET-dependent way to regulate the early G2/M arrest even as low as 0.2Gy for heavy ion radiation, which indicated that ATR pathway could be rapidly activated and functioned in an ATM-independent, but DSB complexity-dependent manner following exposure to IR. Furthermore, ATR pathway also functioned more efficiently in ATM-proficient cells to block G2 to M transition at early period of particle radiation exposure. Accordingly, in contrast to ATM inhibitor, ATR inhibitor had a more effective radiosensitizing effect on survival fraction following heavy ion beams as compared with X ray radiation. Taken together, our results reveal that the complexity of DSBs is a crucial factor for the activation of ATR pathway for G2/M checkpoint regulation, and ATM-dependent end resection is not essential for the activation.

Abstract The quality of the sublethal damage (SLD) after irradiation with high–linear energy transfer (LET) ion beams was investigated with low-LET photons. Chinese hamster V79 cells and human squamous carcinoma SAS cells were first exposed to a priming dose of different ion beams at different LETs at the Heavy Ion Medical Accelerator in the Chiba facility. The cells were kept at room temperature and then exposed to a secondary test dose of X-rays. Based on the repair kinetics study, the surviving fraction of cells quickly increased with the repair time, and reached a plateau in 2–3 h, even when cells had received priming monoenergetic high-LET beams or spread-out Bragg peak beams as well as X-ray irradiation. The shapes of the cell survival curves from the secondary test X-rays, after repair of the damage caused by the high-LET irradiation, were similar to those obtained from cells exposed to primary X-rays only. Complete SLD repairs were observed, even when the LET of the primary ion beams was very high. These results suggest that the SLD caused by high-LET irradiation was repaired well, and likewise, the damage caused by the X-rays. In cells where the ion beam had made a direct hit in the core region in an ion track, lethal damage to the domain was produced, resulting in cell death. On the other hand, in domains that had received a glancing hit in the low-LET penumbra region, the SLD produced was completely repaired.

Our cell survival data showed the obvious dependence of RBE on photon energy: The RBE value for 200 kV X-rays was approximately 10% greater than those for mega-voltage photon beams. In radiation therapy using mega-voltage photon beams, the photon energy distribution outside the field is different with that in the radiation field because of a large number of low energy scattering photons. Hence, the RBE values outside the field become greater. To evaluate the increase in RBE, the method of deriving the RBE using the Microdosimetric Kinetic model (MK model) was proposed in this study. The MK model has two kinds of the parameters, tissue-specific parameters and the dose-mean lineal energy derived from the lineal energy distributions measured with a Tissue-Equivalent Proportional Counter (TEPC). The lineal energy distributions with the same geometries of the cell irradiations for 200 kV X-rays, (60)Co γ-rays, and 6 MV X-rays were obtained with the TEPC and Monte Carlo code GEANT4. The measured lineal energy distribution for 200 kV X-rays was quite different from those for mega-voltage photon beams. The dose-mean lineal energy of 200 kV X-rays showed the greatest value, 4.51 keV/µm, comparing with 2.34 and 2.36 keV/µm for (60)Co γ-rays and 6 MV X-rays, respectively. By using the results of the TEPC and cell irradiations, the tissue-specific parameters in the MK model were determined. As a result, the RBE of the photon beams (y(D): 2~5 keV/µm) in arbitrary conditions can be derived by the measurements only or the calculations only of the dose-mean lineal energy.

Hypofractionated carbon-ion therapy has been applied to treatment of several tumours. In this case, relative biological effectiveness (RBE) at high dose region must be considered, however, the RBE calculated physically has been not verified biologically. In this study, spheroid technique was adopted to estimate RBE in wide dose range. Cells were irradiated with X-rays and heavy-ions with LET of 13, 35, 100 and 300 keV/μm with monolayer and spheroid condition. Surviving fractions in wide dose range (0-15 Gy) were obtained to combined monolayer with spheroid survival data. The linear-quadratic and multi-target single-hit equation fitted well in survival data at low dose, and high dose region, respectively. A multi-process equation showed best fitting for survival data in wide dose range. RBE values of heavy-ions could be estimated by combination of monolayer and spheroid data. The values converged at 1.1-1.4 and varied by LET values at high and low dose region, respectively.

Purpose:Respiration‐gated irradiation for a moving target requires a longer time to deliver single fraction in proton radiotherapy (PRT). Ultrahigh dose rate (UDR) proton beam, which is 10–100 times higher than that is used in current clinical practice, has been investigated to deliver daily dose in single breath hold duration. The purpose of this study is to investigate the survival curve and relative biological effectiveness (RBE) of such an ultrahigh dose rate proton beam and their linear energy transfer (LET) dependence.Methods:HSG cells were irradiated by a spatially and temporally uniform proton beam at two different dose rates: 8 Gy/min (CDR, clinical dose rate) and 325 Gy/min (UDR, ultrahigh dose rate) at the Bragg peak and 1.75 (CDR) and 114 Gy/min (UDR) at the plateau. To study LET dependence, the cells were positioned at the Bragg peak, where the absorbed dose‐averaged LET was, and at the plateau, where it was . After the cell exposure and colony assay, the measured data were fitted by the linear quadratic (LQ) model and the survival curves and RBE at 10% survival were compared.Results:No significant difference was observed in the survival curves between the two proton dose rates. The ratio of the RBE for CDR/UDR was at the Bragg peak and at the plateau. On the other hand, Bragg peak/plateau RBE ratio was for UDR and for CDR.Conclusions:Present RBE can be consistently used in treatment planning of PRT using ultrahigh dose rate radiation. Because a significant increase in RBE toward the Bragg peak was observed for both UDR and CDR, further evaluation of RBE enhancement toward the Bragg peak and beyond is required.

Ionising radiation-induced bystander effects are well recognised, but its dependence on dose or linear energy transfer (LET) is still a matter of debate. To test this, 49 sites in confluent cultures of AG01522D normal human fibroblasts were targeted with microbeams of carbon (103 keV µm(-1)), neon (375 keV µm(-1)) and argon ions (1260 keV µm(-1)) and evaluated for the bystander-induced formation of micronucleus that is a kind of a chromosome aberration. Targeted exposure to neon and argon ions significantly increased the micronucleus frequency in bystander cells to the similar extent irrespective of the particle numbers per site of 1-6. In contrast, the bystander micronucleus frequency increased with increasing the number of carbon-ion particles in a range between 1 and 3 particles per site and was similar in a range between 3 and 8 particles per site. These results suggest that the bystander effect of heavy ions for micronucleus formation depends on dose.

We have established a single cell irradiation system, which allows selected cells to be individually hit with defined number of heavy charged particles, using a collimated heavy-ion microbeam apparatus at JAERI-Takasaki. This system has been developed to study radiobiological processes in hit cells and bystander cells exposed to low dose and low dose-rate high-LET radiations, in ways that cannot be achieved using conventional broad-field exposures. Individual cultured cells grown in special dishes were irradiated in the atmosphere with a single or defined numbers of 18.3 MeV/amu 12C, 13.0 MeV/amu 20Ne, and 11.5 MeV/amu 40Ar ions. Targeting and irradiation of the cells were performed automatically at the on-line microscope of the microbeam apparatus according to the positional data of the target cells obtained at the off-line microscope before irradiation. The actual number of particle tracks that pass through cell nuclei was detected with prompt etching of the bottom of the cell dish made of ion track detector TNF-1 (modified CR-39), with alkaline-ethanol solution at 37 degrees C for 15-30 minutes. Using this system, separately inoculated Chinese hamster ovary cells, confluent normal human fibroblasts, and single plant cells (tobacco protoplasts) have been irradiated. These are the first studies in which single-ion direct hit effect and the bystander effect have been investigated using a high-LET heavy particle microbeam.