Regulatory implementation of the occupational equivalent dose limit for the lens of the eye and underlying relevant efforts in Japan

Yokoyama, Sumi*; Hamada, Nobuyuki*; Tsujimura, Norio; Kunugita, Naoki*; Nishida, Kazutaka*; Ezaki, Iwao*; Kato, Masahiro*; Okubo, Hideki*

International Journal of Radiation Biology, 99(4), p.604 - 619, 2023/04

https://doi.org/10.1080/09553002.2022.2115160

In April 2011, the International Commission on Radiological Protection recommended reducing the occupational equivalent dose limit for the lens. Such a new occupational lens dose limit has thus far been implemented in many countries, and there are extensive discussions toward its regulatory implementation in other countries. In Japan, discussions in the Japan Health Physics Society (JHPS) began in April 2013 and in Radiation Council in July 2017, and the new occupational lens dose limit was implemented into regulation in April 2021. To share our experience, we have published a series of papers summarizing situations in Japan: the first paper based on information available by early 2017, and the second paper by early 2019. This paper (our third paper of this series) aims to review updated information available by mid-2022, such as regarding regulatory implementation of the new occupational lens dose limit, recent discussions by relevant ministries based on the opinion from the council, establishment process of safety and health management systems, the JHPS guidelines on lens dose monitoring and radiation safety, voluntary countermeasures of the licensees, development of lens dose calibration method, and recent studies on exposure of the lens in nuclear workers and biological effect on the lens.

Microdosimetric modeling of relative biological effectiveness for skin reactions; Possible linkage between in vitro and in vivo data

Sato, Tatsuhiko; Matsuya, Yusuke; Hamada, Nobuyuki*

International Journal of Radiation Oncology, Biology, Physics, 114(1), p.153 - 162, 2022/09

https://doi.org/10.1016/j.ijrobp.2022.05.010

The microdosimetric kinetic model, which was originally developed for estimating cell surviving fractions for various radiations, was improved to be capable of estimating the mean and uncertainty of RBE for skin reactions. The parameter used in the model was independently determined from in vitro measurements of dermal cell survival and in vivo measurements of skin reactions taken from 8 and 23 papers, respectively. Our model quantitatively revealed that RBE for skin reactions tend to be higher than that for dermal cell survival. RBE of various mono-energetic radiations calculated from this model confirmed that the past evaluations made by ICRP and NCRP a few decades ago are still supported by recent experimental data. Conclusions: Our model can play important roles not only in medical physics for avoiding unnecessary skin reactions in particle therapy and BNCT but also in radiation protection for future decision making of the recommended RBE values.

Implementation of simplified stochastic microdosimetric kinetic models into PHITS for application to radiation treatment planning

Sato, Tatsuhiko; Hashimoto, Shintaro; Inaniwa, Taku*; Takada, Kenta*; Kumada, Hiroaki*

International Journal of Radiation Biology, 97(10), p.1450 - 1460, 2021/10

https://doi.org/10.1080/09553002.2021.1956003

The stochastic microdosimetric kinetic (SMK) model is one of the most sophisticated and precise models used in the estimation of the relative biological effectiveness of carbon-ion radiotherapy (CRT) and boron neutron capture therapy (BNCT). Through the introduction of Taylor expansion (TE) or fast Fourier transform (FFT), we developed two simplified SMK models and implemented them into the Particle and Heavy Ion Transport code System (PHITS). This study enables the instantaneous calculation of the equieffective dose for CRT and BNCT, considering their cellular-scale dose heterogeneities. Treatment-planning systems that use the improved PHITS as a dose-calculation engine are under development.

Evaluation of DNA damage induced by Auger electrons from Cs

Watanabe, Ritsuko; Hattori, Yuya; Kai, Takeshi

International Journal of Radiation Biology, 92(11), p.660 - 664, 2016/11

https://doi.org/10.3109/09553002.2015.1136853

To understand the effect of internal exposure of Cs, we focus on estimation of microscopic energy deposition pattern and DNA damage induced by directly emitted electrons (beta-rays, internal conversion electrons, Auger electrons) from Cs. Monte Carlo track simulation method was used to calculate the microscopic energy deposition pattern. To simulate the energy deposition by directly emitted electrons, we considered the multiple ejections of electrons after internal conversion. Induction process of DNA strand breaks and base lesions was modeled and simulated using Monte Carlo methods for cell mimetic condition. The yield and spatial distribution of simple and complex DNA damage were calculated for the cases of -rays and electrons from Cs. The simulation showed that significant difference of DNA damage spectrum was not caused by the difference between secondary electron spectrum by -rays and directly ejected electron spectrum. The result support that the existing evaluation that internal exposure and external exposure are almost equivalent.

The Bystander cell-killing effect mediated by nitric oxide in normal human fibroblasts varies with irradiation dose but not with radiation quality

Yokota, Yuichiro; Funayama, Tomoo; Muto, Yasuko*; Ikeda, Hiroko; Kobayashi, Yasuhiko

International Journal of Radiation Biology, 91(5), p.383 - 388, 2015/05

https://doi.org/10.3109/09553002.2015.1021960

We investigated the dependence of the bystander cell-killing effect on radiation dose and quality, and related molecular mechanisms. Human fibroblasts were irradiated with -rays or carbon ions and co-cultured with non-irradiated cells. Survival rates of non-irradiated cells decreased and nitrite concentrations in culture medium increased with increasing doses. Their dose responses were similar between -rays and carbon ions. Treatment of the specific nitric oxide (NO) radical scavenger prevented reductions in survival rates of non-irradiated cells. Negative relationships were observed between survival rates and nitrite concentrations. From these results, it was concluded that the bystander cell-killing effect mediated by NO radicals in human fibroblasts depends on irradiation doses, but not on radiation quality. NO radical production appears to be an important determinant of -ray- and carbon-ion-induced bystander effects.

Development of a simulation method for dynamics of electrons ejected from DNA molecules irradiated with X-rays

Kai, Takeshi; Higuchi, Mariko; Fujii, Kentaro; Watanabe, Ritsuko; Yokoya, Akinari

International Journal of Radiation Biology, 88(12), p.928 - 932, 2012/12

https://doi.org/10.3109/09553002.2012.708086

Lifetime of the unpaired electron species in calf thymus DNA thin films induced by nitrogen and oxygen K-shell photoabsorption

Oka, Toshitaka; Yokoya, Akinari; Fujii, Kentaro

International Journal of Radiation Biology, 88(12), p.884 - 887, 2012/12

https://doi.org/10.3109/09553002.2012.695099

no abstracts in English

Applications of the microdosimetric function implemented in the macroscopic particle transport simulation code PHITS

Sato, Tatsuhiko; Watanabe, Ritsuko; Sihver, L.*; Niita, Koji*

International Journal of Radiation Biology, 88(1-2), p.143 - 150, 2012/01

https://doi.org/10.3109/09553002.2011.611216

The microdosimetric function of PHITS has been applied to the biological dose estimation for charged-particle therapy and the risk estimation for astronauts. The former application was performed in combination with the microdosimetric kinetic model, while the latter was done with the radiation quality factor expressed as a function of lineal energy. Owing to the unique features of the microdosimetric function, the improved PHITS has a potential to establish more sophisticated systems for the radiological protection in space as well as the treatment planning of charged-particle therapy.

Studies of soft X-ray-induced Auger effect on the induction of DNA damage

Yokoya, Akinari; Fujii, Kentaro; Shikazono, Naoya; Akamatsu, Ken; Urushibara, Ayumi; Watanabe, Ritsuko

International Journal of Radiation Biology, 84(12), p.1069 - 1081, 2008/12

https://doi.org/10.1080/09553000802534950

The role of Auger effect in inducing DNA damage has been studied using soft X-ray irradiation, which mainly cause photoelectric effect of DNA constituent atoms. As a consequence of Auger decay process, ejected low energy photo- or Auger-electrons might impact on proximately chemical group in the molecule. These highly localized collision events are expected to lead to a clustered DNA damage site within a few nano-meter. We have revealed that soft X-ray (60 keV) induced lesions visualized by the enzymatic probes show much higher yields than those induced by low LET -ray irradiation, and the yields decreased with decreasing soft X-ray energy (below a few keV). These results indicate that the complexity of damage site strongly depends on photo- or Auger electron range. The recent progress that has been made in the study of the process of DNA-radicals as precursors using an EPR apparatus combined with a synchrotron soft X-ray source is also presented.

Characterization of lesions induced in linear-formed plasmid DNA by valence ionization and Auger decay at carbon, nitrogen and oxygen

Akamatsu, Ken; Fujii, Kentaro; Yokoya, Akinari

International Journal of Radiation Biology, 84(12), p.1082 - 1092, 2008/12

https://doi.org/10.1080/09553000802499220

The aim of this study is to clarify the difference of DNA damage induced by USX from that by Co-60 -rays. We have recently attended to unaltered base release, strand break terminus and base lesion, focusing on the effect of nitrogen and oxygen K-shell electron ionization. The yields of unaltered bases released were determined by HPLC. To quantify and characterize the strand break termini, the digestion rates of the irradiated DNA pretreated with or without calf intestinal alkaline phosphatase by snake venom phosphodiesterase (SVPD) were measured. This experimental method revealed that the production of the termini, which can not be digested by SVPD, were predominant in comparison with that of SVPD- digestive ones, e.g., with 3'OH, in both USX and -rays. Furthermore, piperidine-treated irradiated DNA was also analyzed by the same method above to quantify piperidine-labile base lesions.