Vol.5 No.4 2013
Research paper : Dose standards for safe and secure breast cancer screening (T. Tanaka et al.)−235−Synthesiology - English edition Vol.5 No.4 (2013) reference field at AIST to perform calibration. The fees for the calibration/testing service are calculated according to the number of days the irradiation facility is used. Therefore, the expenditure that must be borne by the calibration labs was reduced, and a smooth dissemination of the standard was expected.6 Validation of the calibration6.1 Confirmation of the international equivalence of the national standardTo confirm international equivalence, all standards must be compared with the standards of other countries. As mentioned earlier, BIPM developed the dose standard for mammography X-rays and has performed key comparisonsTerm 7 since 2009. AIST participated in the international key comparison in 2009.There are 2 methods for international comparison of a radiation dose standard. The first method is a direct comparison between the primary standards of the countries involved. For example, the primary standard instrument (free air ionization chamber) at AIST is taken to the reference field at BIPM, and the absolute values of the dose are compared. This method is limited to cases in which the primary standard instrument is transferrable. Another method is an indirect comparison, in which the transfer instruments are calibrated using the primary standard at the institutes involved and the calibration results (calibration coefficient) are compared. This method is useful in cases where the primary standard instrument cannot be transported easily because of its size.The free air ionization chamber, which is the standard instrument used for measuring mammography X-rays at AIST, is also used to determine the conventional soft X-rays (W/Al) dose. This standard instrument was compared directly with the BIPM standard in 2004, and its equivalence was confirmed. Therefore, in this key comparison, the latter method of the indirect comparison was selected. Three ionization chamber dosimeters with different energy characteristics were selected for a detailed comparison. Figure 8 shows a comparison of the calibration results obtained for the 3 types of dosimeters measured at the 2 institutes.As shown in Figure 8, the calibration coefficients of the dosimeters showed good agreement between institutes. The calibration uncertainty of BIPM (error bar in Fig. 8) was smaller than that of AIST for all 3 types of dosimeters because BIPM has a newly developed free air ionization chamber that has been optimized (i.e., the correction coefficient for equation (1) is small) for the radiation quality of mammography X-rays.Figure 9 shows the results of the international key comparison for mammography dose standards. Germany (the PhysikalischTechnische Bundesanstalt, PTB), the US (NIST), W/Mo 50 kVW/Mo 30 kVW/Mo 23 kVMo/Mo 35 kVMo/Mo 30 kVMo/Mo 28 kVMo/Mo 25 kVNMIJ/AIST(Japan)PTB(Germany)NIST(USA)NRC(Canada)-5-10-150510150.5 %Relative deviation (per mille) of the measured values of each country and the reference value (BIPM)AISTBIPMRadiation quality(Mo/Mo)Calibration coefficient(Gy/μC)25 kV28 kV30 kV35 kV116118120122848586874.654.704.754.80(a)(b)(c)Fig. 9 International key comparison for the mammography dose standardThe vertical axis shows the deviation (per mille) from the BIPM reference value. For each data point, the vertical bar shows the uncertainty at the 95 % confidence level.Fig. 8 Comparison between AIST and BIPM calibration coefficients of 3 types of dosimeters (a, b, and c)The energy characteristic of the dosimeter is (a) flat, (b) declining, and (c) increasing.