Vol.5 No.4 2013
Research paper : Dose standards for safe and secure breast cancer screening (T. Tanaka et al.)−239−Synthesiology - English edition Vol.5 No.4 (2013) development of this standard, there were no major problems. We determined that the issues could be solved by applying the dosimetry technology for low-energy X-rays that had been previously developed at AIST. Specifically, for the primary standard, the dosimetry technology for existing soft X-rays (W/A1 radiation quality) was used, and correction factors for the primary standard were newly evaluated using the radiation quality of mammography X-rays. Using the Monte Carlo calculation, we confirmed that there would be no major negative effects on the measurement results or uncertainties in the early stages of development. Therefore, we employed this method for standard development.Other than the above method, there are other ways of developing a new primary standard specifically for mammography X-rays. For example, a free air ionization chamber that is optimized (with small corrections) for mammography radiation quality can be developed. This is a method wherein the correction factors are calculated using the Monte Carlo method (or other methods) and are fed back to the design of the free air ionization chamber for optimal results. BIPM developed the dose standard for mammography using this method. Of course, the uncertainty will be smaller if an optimized primary instrument is developed (at the 95 % confidence level, the uncertainty is 0.6 % for AIST and 0.4 % for BIPM). However, the development period will be longer. While this is an extreme case, BIPM started development in 2001 and embarked on an international key comparison in 2009. In cases where it is necessary to quickly meet the social demand, as in our case, the greatest merit is the shortest possible development period.Another technological issue was the improvement in the reliability of mammography machine dose evaluation in medical practice. Because the irradiation geometries are different between the reference field of AIST, overseas metrology institutes, and the actual mammography machine, the uncertainty of dose evaluation in medical practice becomes higher. Because mammography X-rays are of low energy, differences in irradiation geometries (e.g., the irradiation distance and presence of compression plates) greatly affect the uncertainty of dose evaluation in addition to the difference in radiation quality. Therefore, we developed a dose standard in which the irradiation geometry was similar to that of a mammography machine, and we tried to reduce the uncertainty in dose evaluation in medical practice. The irradiation distance and compression plate are taken into account by AIST, although they are not considered in overseas standards. However, we were careful to maintain the international compatibility of the standard, and at the same time, we ensured that the radiation quality complied with the IEC standard.The differences between the calibration coefficients of dosimeters A, B, and C (Fig. 3) may be mainly attributable to differences in the X-ray entrance window material in the ionization chambers as well as the internal structures of the chambers. With low-energy X-rays, a large difference is noted in the energy dependency of the calibration coefficient—even in the ionization chamber dosimeter, which is known to be highly precise. I have added an explanation in the paper.4 New efforts to establish a standard dissemination systemQuestion (Naoto Kobayashi)On the calibration service system, you write that the standard was disseminated smoothly by having the calibration laboratories bring the client’s dosimeter to AIST to do the calibration (subcontracted irradiation test), without fabricating a new calibration facility. You give the example of the glass dosimeter. Were there any new efforts and attempts unseen before connected to this? It seems that it was routine and there was hardly any problem, but how was it actually?Answer (Takahiro Tanaka)The most notable feature of this study was that reliability improvement was performed via evaluation of the glass dosimeter, which is conventionally used in medical practice, in addition to the conventional calibration service system through the calibration of dosimeters.In the initial plan for standard development, we were thinking about the conventional traceability mediated by dosimeter calibration. I was thinking of the following flow: 1) the dosimeter owned by the calibration laboratory is calibrated at AIST and 2) the calibrated dosimeter is used as a secondary standard to calibrate the user’s dosimeter at the X-ray reference field of the calibration laboratory. However, because the radiation quality of mammography X-rays is different from that of the X-rays used in calibration, step 1 with an X-ray field of mammography radiation quality is insufficient, and an X-ray field of mammography radiation quality is needed for step 2. However, even if there are demands for the standard, the calibration laboratories were of the opinion that the facility investment required to introduce an X-ray irradiation machine would be too expensive. Therefore, we devised a way to disseminate the standard smoothly by having the calibration laboratories use the irradiation facility at AIST.Although it is estimated that there are approximately 1000 mammography dosimeters distributed throughout Japan, there are only a few calibration laboratories, and I thought that further planning would be needed for widespread standard dissemination. Therefore, in the development phase of this standard, we began looking at the mammography glass dosimeter that was being widely used for dose evaluation in medical practice. By evaluating glass dosimeters according to AIST’s dose standard, the reliability of the dose evaluation for many mammography machines will improve. However, there is one problem with evaluating glass dosimeters using this standard: a special reader is needed to read the accumulated dose information from the glass dosimeters. Thus, the dose cannot be determined immediately upon irradiation. Because the irradiation of glass dosimeters at the reference field of AIST and the reading and analysis of the irradiation data were separated, time was required to overcome issues such as the evaluation of uncertainty. Our mammography glass dosimeter is a unique Japanese dosimeter, and I think it has excellent potential.5 A comparison of situations in other countriesComment (Naoto Kobayashi)In this international key comparison, good results have been obtained as shown in Fig. 9. AIST was the first to participate and obtained good results which show that the international equivalency of the national standard was verified, and this is extremely significant. I think the high quality of AIST will become clearer if you discuss the situation of other countries that participated in the comparison (such as the type and performance of the detectors).Answer (Takahiro Tanaka)In this international key comparison, the transfer ionization chamber was employed by all institutes including AIST. AIST was original in its choice of the transfer standard. Institutes other than AIST selected only 1 transfer standard with a flat energy characteristic in the mammography X-ray energy range. AIST selected 3 types of transfer ionization chambers with different energy dependences to conduct a thorough comparison. As a result, sufficient compatibility was obtained with the BIPM values for all 3 dosimeters.