In this paper, an outline of the biological and epidemiological evidence for radiation protection is presented together with the strengths and limitations of these studies in radiation risk evaluation. Various types of research can provide useful information to complement the risk estimates based on atomic-bomb survivor studies, particularly the effects of low-dose and lowdose rate radiation. Some basic issues in synthesizing scientific information for radiation risk evaluation are also discussed.
The objective of this study was to evaluate organ dose and effective dose to patients from examinations of the upper gastrointestinal (GI) tract. Absorbed doses of various tissues and organs were measured using novel photodiode dosimeters installed in an anthropomorphic phantom representing a standard Japanese adult body. The organ dose and the effective dose were assessed from the absorbed doses according to the definitions seen in the publications of the International Commission on Radiological Protection. Dose measurements were performed for each projection of the upper GI tract examination in seven procedures at four hospitals and in a mobile coach, and organ doses and effective doses were assessed for each procedure. Organ doses obtained in the observation areas such as the stomach, esophagus and colon were in the order of several to more than 60mGy, though they decreased to less than 1mGy for tissues and organs distant from the observation areas. Organ doses and effective doses differed largely according to tube voltage, filtration and tube current or mAs value of the x-ray generator used, and by examination protocol, number of images, fluoroscopy time, and imaging units such as screen/film, computed radiography, digital radiography and flat panel detector. The number of images and the fluoroscopy time were 7 and 1.5min for the examination in the mobile coach, and 18-22 and 2-6min in the hospitals. Evaluated effective dose for the examination in the mobile coach was 2.9mSv, and that in the hospitals ranged from 4.0-13.4mSv at a ratio of more than three.
In order to confirm the radiation safety for an active volcanic island, Miyake-jima, environmental radiation and radioactivity have been investigated. The average, minimum and maximum of the environmental gamma-ray dose rates were estimated to be 18.6nGy·h-1, 6.7nGy·h-1 and 29.2nGy·h-1, respectively. For the radioactivity in soils, the averages of 238U, 232Th and potassium concentrations were found from calculations to be 0.38μg·g-1, 0.43μg·g-1 and 0.38%, respectively. The radon exhalation rate was below the detection limit, and the average value of thoron exhalation rate was estimated to be 38mBq·m-2·s-1. These results are clear evidence that the radiation level and radioactivity on Miyake-jima are lower than the averages in other parts of Japan. It was confirmed that Miyake-jima is presently a safe island in terms of environmental radiation and radioactivity level.
An electronic dosimeter can be substituted for thermoluminescent dosimeters (TLDs) and radiophoto-luminescence glass dosimeters (RLPDs) in environmental radiation measurements. The dosimeter used here can record the time variation of the dose. Not only the integrated dose, but also the change of the dose rate can be grasped by means of the dosimeter. The measurement error of the dosimeters was examined. The instrumental error is small, being less than 4%. The systematic sensitivity difference was not recognized. The measurement error of 1μSv, which is the minimum dose for unit recording, was around 9%. At an integrated dose of 15μSv, the measurement error was less than 5%. Therefore, highly precise radiation monitoring is possible without checking individual sensitivity. The lower limit dose for detection, which can judge whether the dose exceeds the background, was 0.21μSv through integrating 1μSv. When irradiation was conducted at 1.05μSv/h for 2h, the data was obtained for an approximate time of irradiation as well as the total dose. The dosimeter is suitable for radiation monitoring around X-ray generators or accelerators, where radiation is generated intermittently.
A computer code named ACUTRI has been developed to assess tritium doses due to inhalation to the general public. The ACUTRI code can calculate the radiological impact of tritium gas (HT) and tritiated water (HTO) released accidentally to the atmosphere. The models in this code consist of a tritium transfer model including the oxidation of HT to HTO and the reemission of HTO from soil to the atmosphere and a dose calculation model. The atmospheric dispersion of the primary HT and HTO plumes and secondary HTO plume, which is reemitted from soil to the atmosphere, is calculated by using the Gaussian plume model. In this calculation, it is possible to analyze the meteorological data statistically, in the same way as a conventional dose assessment method according to the meteorological guideline of the Nuclear Safety Commission of Japan. Tritium concentrations in air and their resultant doses were calculated using the ACUTRI code under several conditions. In order to validate the model, calculations were compared with experimental results.