We report the discovery of abnormal fluctuations in the output obtained when measuring a water phantom and adjustments that reduce these outliers. Using a newly developed three-dimensional scanning water phantom system, we obtained the depth dose and off-axis dose ratio required for the beam data of a medical linear accelerator (LINAC). The field and reference detectors were set such that the measured values could be viewed in real time. We confirmed the scanning data using the field detector and the change in the output using the reference detector while measuring by using the water phantom. Prior to output adjustment of the LINAC, we observed output abnormalities as high as 18.4%. With optimization of accelerator conditions, the average of the output fluctuation width was reduced to less than ±0.5%. Through real-time graphing of reference detector measurements during measurement of field detector, we were able to rapidly identify abnormal fluctuations. Although beam data collected during radiation treatment planning are corrected for output fluctuations, it is possible that sudden abnormal fluctuations actually occur in the output. Therefore, the equipment should be tested for output fluctuations at least once a year. Even after minimization of fluctuations, we recommend determining the potential dose administered to the human body taking into account the width of the output fluctuation.

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A primary standard for the absorbed dose rate to water in a ⁶⁰Co gamma-ray field was established at National Metrology Institute of Japan (NMIJ) in fiscal year 2011. Then, a ⁶⁰Co gamma-ray standard field for therapy-level dosimeter calibration in terms of absorbed dose to water was developed at National Institute of Radiological Sciences (NIRS) as a secondary standard dosimetry laboratory (SSDL). The results of an IAEA/WHO TLD SSDL audit demonstrated that there was good agreement between NIRS stated absorbed dose to water and IAEA measurements. The IAEA guide based on the ISO standard was used to estimate the relative expanded uncertainty of the calibration factor for a therapy-level Farmer type ionization chamber in terms of absorbed dose to water (N_D,w) with the new field. The uncertainty of N_D,w was estimated to be 1.1% (k=2), which corresponds to approximately one third of the value determined in the existing air kerma field. The dissemination of traceability of the calibration factor determined in the new field is expected to diminish the uncertainty of dose delivered to patients significantly.

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A massive earthquake of magnitude 9 terribly happened far out at sea of Tohoku area on 11 March, 2011. After this earthquake the hugest tsunami in the history came to the hundreds km of the seashore of Tohoku area. Due to this tsunami all of the four nuclear power plants of Fukushima Daiichi lost every electric power and, soon after this, loss nuclear fuels from number 1 to 3 reactors melt through their power containers. According to this phenomena, large amount of the radio-activities have been released in the air. There were some releases but major contaminations happened at the time of the two releases in the morning of 15 March, 2011. Due to this, to the direction of the northwest until the Iitate Village over 30 km zone was contaminated. In this paper I explain the time course of the accidents and that how contaminated.

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It is well established that those who were exposed to ionizing radiation have increased risks of developing malignancies. The magnitude of the risk varies depending on not only the dose but also age at the time of exposure, gender, background incidence rate etc. In the case of atomic bomb survivors, the relative risk of cancer is linearly related to the dose, and the sex averaged relative risk (exposure age is 30, risk calculation is when they reached age 70) is 1.5 at 1 Gy. Because the increased risks below 100 to 200 mGy are too small and not statistically significant, there are arguments in interpreting the risks at the low dose range.

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We explained the spirit of the clinical research guideline and the present condition of the clinical research in Japan. And then, the responsibility of the principal investigators was described.

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