Vol.5 No.4 2013
10/70

Research paper : Dose standards for safe and secure breast cancer screening (T. Tanaka et al.)−234−Synthesiology - English edition Vol.5 No.4 (2013) 4.2 Development of the mammography X-ray reference fieldA reference field for the radiation quality of mammography X-rays was developed starting mainly from the dominant radiation qualities used in practice. As mentioned earlier, the radiation quality is determined by the target material of the X-ray tube, the tube voltage, and the material and thickness of the additional filter. We ensured that we were using a radiation quality that could be used overseas as well as in Japan, as exemplified by the International Organization for Standardization (ISO) and IEC standards (for mammography, IEC 61267).[13] Such a radiation quality is necessary for international key comparisons that are conducted to confirm the equivalence of the dose standard around the world. However, a radiation standard other than that of the IEC is used in the Japanese quality control manuals, and compliance with the Japanese standard would lead to a smooth dissemination of the standard. Consideration of both overseas and domestic situations when establishing a standard is the first step toward disseminating a dose standard that is internationally equivalent.Because the linear absorption coefficient for air that is used in mammography is larger than that used in other diagnostic X-ray modalities, the radiation quality changes significantly because of the calibration distance (distance from the focus point of the X-ray tube to the reference plane of the free air ionization chamber); therefore, it is important to set the calibration distance. In countries such as Germany and the US that have begun disseminating standards before Japan, the calibration distance is 1 m. However, AIST set the calibration distance at 60 cm considering the irradiation distance of the mammography machine. Later, BIPM set the calibration distance at 60 cm.A dose standard based on the radiation quality through the compression paddle was originally developed in Japan. In actual mammography screening, the breasts are irradiated with X-rays through the compression paddle. The low-energy X-rays used in mammography tend to be absorbed by the compression paddle, and the radiation quality changes greatly. Thus, an original radiation quality was developed considering the quality of radiation exposure to the breasts. This radiation quality was required for the calibration of the dosimeter used in mammography quality control.5 Construction of the calibration service system for mammographyThe construction of a calibration service is essential for improving the reliability of dose evaluation in medical practice. Therefore, multiple calibration services were established with the cooperation of industries and academic societies.5.1 Performance test of the glass dosimeter used for mammography quality controlBefore mammography was utilized for breast cancer screening in Japan, mammography quality control was performed mainly by related academic societies. A mammography glass dosimeter was developed to easily evaluate the dose and the radiation quality necessary for mammography quality control.The glass dosimeter is an integrated-type radiophotoluminescenceTerm 6 dosimeter, and silver-activated phosphate glass is used as the fluorescent glass element. Figure 7 shows a photograph of the mammography glass dosimeter.The mammography glass dosimeter consists of a fluorescent glass element and aluminum filters with 4 different thicknesses covering the surface of the element. The thicknesses of the aluminum filters are 0.3, 0.4, 0.6, and 1.0 mm, and an attenuation curve can be obtained from one irradiation. The half-value layer and tube voltage can be calculated from this attenuation curve, and the air kerma can be calculated from the amount of fluorescence at the open window (without the Al filter) glass element. Thus, the AGD required for mammography quality control can be evaluated with only one irradiation. The performance of this glass dosimeter was evaluated using the reference field.Our results confirmed that the half-value layer, tube voltage, and air kerma values are in good agreement with the values measured using the free air ionization chamber (within 2 % uncertainty at the 95 % confidence level).[14]5.2 Use of the irradiation facility at AISTA mammography X-ray source is necessary for calibration laboratories to perform calibration using the radiation quality of mammography X-rays. However, such an X-ray source costs a minimum of tens of millions of yen; therefore, the calibration service would not be viable as a business. To solve the issue, AIST initiated a calibration/testing service in which the calibration laboratory is allowed to bring the reference dosimeter (assumed to be traceable to the primary standard) and the dosimeter to be calibrated to the mammography 45 mm6 mm13 mm1.0 mmAI0.60.40.3OWFig. 7 Glass dosimeter developed for quality control of mammography (with cooperation from Chiyoda Technol Corporation)

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