Air kerma reference field with high energy photons using a 15 MeV electron beam from a clinical linear accelerator.

In the medical and nuclear fields, there are environments where exposure to photons with energies above several MeV can result in problems. The National Metrology Institute of Japan has developed a high-energy photon field using a 15 MeV electron beam of a clinical linear accelerator with a copper target and an aluminium filter unit to facilitate dosimeter calibration in terms of air kerma. To determine the air kerma rate, the energy fluence distribution at a reference point was calculated, and both calculations and experiments evaluated the effective energy and spatial dose distribution. Moreover, to validate the air kerma measurement, two commercial cavity chambers were calibrated in a developed photon field. The results obtained exhibited a 4% difference compared with those in a Co-60γ-ray reference field.

Measuring and assessing individual external doses during the rehabilitation phase in Iitate village after the Fukushima Daiichi nuclear power plant accident.

After the Fukushima Daiichi nuclear power plant accident in 2011, Iitate village was placed under an evacuation order because the level of radioactive materials drifting from the nuclear plant to the village was above a government-set level for allowing residents to live in the area. The evacuation advisory for most of the village was lifted on 31 March 2017. For displaced residents deciding whether or not to return to their homes, it is important to correctly understand and estimate the realistic individual external doses they will receive after returning to the village. In this study, with the support of residents of the village, we used a personal dosimeter (D-Shuttle) coupled with a global positioning system device to measure and thus understand realistic individual external doses while the residents were in Iitate village and to project the individual external doses for different administrative districts as of 1 April 2017. The measured individual external doses measured by D-Shuttle for 38 study participants showed that the doses measured during time spent inside the village were higher and more widely distributed than the doses measured during time spent outside the village. The exposure ratio (ER) was defined as the ratio of additional individual external dose measured by D-Shuttle to the additional ambient dose based on an airborne monitoring survey. The medians of the average ERs were 0.13 (min-max 0.06-0.27) for time spent at home and 0.18 (min-max 0.08-0.36) for time spent outdoors. Projected additional annual individual external doses as of 1 April 2017 for different administrative districts in the village were calculated using ERs obtained in this study. Assuming that individuals spent 8 h per day on outdoor activities and 16 h on indoor activities, additional annual individual external doses were estimated to be below 3 mSv using the mean of the average ERs for most districts in the village, and these values were well below the individual external doses estimated using the approach taken by the central government. The results of this study provide valuable information both for understanding realistic radiological situations in the village and for those who want to know their future individual external dose in order to make a decision on whether or not to live in the village.

Current situations and discussions in Japan in relation to the new occupational equivalent dose limit for the lens of the eye.

Since the International Commission on Radiological Protection recommended reducing the occupational equivalent dose limit for the lens of the eye in 2011, there have been extensive discussions in various countries. This paper reviews the current situation in radiation protection of the ocular lens and the discussions on the potential impact of the new lens dose limit in Japan. Topics include historical changes to the lens dose limit, the current situation with occupational lens exposures (e.g., in medical workers, nuclear workers, and Fukushima nuclear power plant workers) and measurements, and the current status of biological studies and epidemiological studies on radiation cataracts. Our focus is on the situation in Japan, but we believe such information sharing will be useful in many other countries.

DEVELOPMENT OF ABSOLUTE MEASUREMENT SYSTEM FOR 𝑾AIR IN 60CO GAMMA RAYS.

In the ICRU Report 90, the uncertainty of the recommendation for ${{W}}_{{air}}$ was changed from 0.15 to 0.35%. The purpose of this study is to develop an absolute measurement system for ${{W}}_{{air}}$ in electrons with lower uncertainty using 60Co gamma rays. ${{W}}_{{air}}$ was determined by the ratio of the energy deposition to the number of ion pairs created in a given volume of air using a graphite calorimeter and a graphite-wall ionisation chamber. The obtained value for ${{W}}_{{air}}$ was 33.91 eV with a relative standard uncertainty of 0.08 eV (k = 1), and was in good agreement with the recommended value of the ICRU Report 90 (33.97 eV). Additionally, the uncertainty for ${{W}}_{{air}}$ obtained in this investigation is 0.23% and is comparable with that reported by Burns et al., which is the lowest uncertainty in recent studies determining ${{W}}_{{air}}$ In the future, the proposed system can be used to measure ${{W}}_{{air}}$ in high-energy electron beams.

[Technical Report: Quality Assurance for 125I Seed Sources in Permanent Prostate Brachytherapy].

This technical report provides useful information on the current status and issues of quality control in 125I seed source strength measurement for Permanent Prostate Brachytherapy in Japan.With the spread of 125I seed brachytherapy, the traceability of source strength measurements with the single-seed assay was established in Japan. This allows medical facilities to measure reference air kerma rate of 125I seeds with their own well-type of ionization chamber. However, it is difficult to maintain the traceability chain because the 125I reference air kerma rate standards have been hardly utilized by medical facilities so far. Meanwhile, some serious incidents of contamination of the different source strengths and dead seeds were reported in Japan.To address the specific issues in Japan, JASTRO Brachytherapy Subcommittee established a working group (WG) in 2021. The goal of this WG is to investigate the management methods of source strength measurement used in medical facilities, and to discuss the ideal and practicable methods of source management such as verifying the number of seeds and source strength. Initially, a questionnaire survey was conducted to facilities offering 125I seed brachytherapy in Japan. Sixty-seven out of 95 facilities responded (response rate 70.5%). This survey revealed that 41% of facilities did not perform either confirmation of the number of seeds or measurement of source strength. There are several reasons why the source strength was not measured in those facilities. For example, 125I seeds are provided under the sterilized conditions; quality assurance by source suppliers is reliable; and there is not sufficient staff.The single-seed assay is regarded as an internationally standardized and the most reliable measurement method. Therefore, it is an essential measurement technique to ensure traceability of source strength measurements. However, our survey found that most Japanese facilities do not perform single-seed assays. Meanwhile, some facilities have performed batch assay as an alternative method, in which all of the multiple sources in a batch are measured while loaded into sterilized cartridges. Although the measurement by the batch assay is less accurate than the one by the single-seeded assay, the batch assay does not require re-sterilization of the source and can be performed quickly. It might be useful to detect unexpected errors such as differences in the number of sources and abnormalities in source strength.In this report, we will introduce several methods of source strength measurement that have been implemented in medical facilities. The quality assurance of 125I seed sources in prostate interstitial brachytherapy should be provided not only by the source suppliers but also by the medical facilities that use sources to treat patients. We hope that medical facilities will refer to this technical report and use it as an aid to quality assurance in their own facilities.

The NMIJ air kerma primary standard for high energy x-ray beams in 300-450 kV.

Accurate radiation dosimetry is required for radiation protection in various environments. Therefore, dosemeters and dose-rate meters must be calibrated in standard radiation fields. The National Metrology Institute of Japan (NMIJ) expands the energy range of x-ray reference field measurement up to 450 kV using a cylindrical graphite-walled cavity ionization chamber. Departure from the condition of the Spencer-Attix cavity theory was evaluated by comparing the measurement results obtained using the cavity ionization and the free-air ionization chambers, which are used as the primary standard up to a tube voltage of 250 kV. The calibration coefficients found using the spherical ionization chamber were in good agreement with those obtained by the free-air ionization chamber within relative standard uncertainties (k = 1) for N-200 and N-250 x-ray fields. Consistent calibration coefficients were obtained in the energy range 300-450 kV.

SIMPLIFIED PRACTICAL TEST METHOD FOR PORTABLE DOSE METERS USING SEVERAL SEALED RADIOACTIVE SOURCES.

Sealed radioactive sources which have small activity were employed for the determination of response and tests for non-linearity and energy dependence of detector responses. Close source-to-detector geometry (at 0.3 m or less) was employed to practical tests for portable dose meters to accumulate statistically sufficient ionizing currents. Difference between response in the present experimentally studied field and in the reference field complied with ISO 4037 due to non-uniformity of radiation fluence at close geometry was corrected by use of Monte Carlo simulation. As a consequence, corrected results were consistent with the results obtained in the ISO 4037 reference field within their uncertainties.

Relationship between Individual External Doses, Ambient Dose Rates and Individuals' Activity-Patterns in Affected Areas in Fukushima following the Fukushima Daiichi Nuclear Power Plant Accident.

The accident at Fukushima Daiichi Nuclear Power Plant on March 11, 2011, released radioactive material into the atmosphere and contaminated the land in Fukushima and several neighboring prefectures. Five years after the nuclear disaster, the radiation levels have greatly decreased due to physical decay, weathering, and decontamination operations in Fukushima. The populations of 12 communities were forced to evacuate after the accident; as of March 2016, the evacuation order has been lifted in only a limited area, and permanent habitation is still prohibited in most of the areas. In order for the government to lift the evacuation order and for individuals to return to their original residential areas, it is important to assess current and future realistic individual external doses. Here, we used personal dosimeters along with the Global Positioning System and Geographic Information System to understand realistic individual external doses and to relate individual external doses, ambient doses, and activity-patterns of individuals in the affected areas in Fukushima. The results showed that the additional individual external doses were well correlated to the additional ambient doses based on the airborne monitoring survey. The results of linear regression analysis suggested that the additional individual external doses were on average about one-fifth that of the additional ambient doses. The reduction factors, which are defined as the ratios of the additional individual external doses to the additional ambient doses, were calculated to be on average 0.14 and 0.32 for time spent at home and outdoors, respectively. Analysis of the contribution of various activity patterns to the total individual external dose demonstrated good agreement with the average fraction of time spent daily in each activity, but the contribution due to being outdoors varied widely. These results are a valuable contribution to understanding realistic individual external doses and the corresponding airborne monitoring-based ambient doses and time-activity patterns of individuals. Moreover, the results provide important information for predicting future cumulative doses after the return of residents to evacuation order areas in Fukushima.

Angular dependence of the wall correction factor for air kerma measurements using cylindrical cavity chambers.

The dependence of the results of air kerma measurements on the orientation of the cylindrical cavity chamber used for beam standardizations is reexamined. The wall attenuation correction factors for the (60)Co and (136)Cs air kerma measurements were obtained for various irradiation angles by extrapolation of the dependences of experimental data and by Monte Carlo simulations. Applying the correction based on extrapolation does not provide an angle-invariant air kerma rate, as desired. In contrast, Monte Carlo simulations, which take into account various affecting phenomena, produce a wall correction factor that makes the corrected air kerma independent of the incidence angle.

Measurement of anisotropic angular distributions of photon energy spectra for I-125 brachytherapy sources.

The angular distribution of photon energy spectra emitted from an I-125 brachytherapy source was measured using a specially designed jig in the range of ±70° in the plane of the long axis of the source. It is important to investigate the angular dependence of photon emissions from these sources for the calibration of the air kerma rate. The results show that the influence of the distributions between 0° and ±8° is small enough to allow a calibration using current primary instruments which have a large entrance window.

Effect of the diaphragm of free-air ionisation chamber for X-ray air-kerma measurements.

Free-air ionisation chambers are widely used at standards laboratories as primary standards for absolute measurements of air kerma in X-ray fields. The area of the diaphragm aperture of a free-air ionisation chambers is an important factor for absolute measurements because it defines the size of the X-ray beam incident on the free-air chamber. In this study, correction factors for the contribution of X rays transmitted through the diaphragm of a free-air ionisation chamber and those scattered from the surface of the diaphragm aperture are obtained by Monte Carlo simulation for two different sized free-air ionisation chambers and for various diaphragm aperture sizes, X-ray energies and source-to-chamber distances.