Vol.5 No.4 2013

Research paper : Dose standards for safe and secure breast cancer screening (T. Tanaka et al.)−231−Synthesiology - English edition Vol.5 No.4 (2013) Both the ionization chamber and semiconductor dosimeters have been developed as dosimeters for mammography X-rays (or low-energy X-rays); however, the energy dependence of the sensitivity (hereinafter energy dependence) cannot be avoided because of the physical structure of the dosimeters. Therefore, for the measurement of the X-ray energy region used in medical practice, academic societies recommend calibration of the dosimeters using a reference X-ray field with accurately determined characteristics.2.5 International trendBecause the rapid increase in the number of patients with breast cancer was seen earlier in Europe and the US than in Japan, the construction of quality control systems started much earlier in Europe and the US. In the US, the American College of Radiology (ACR) established a quality standard for mammography in 1986 and began work on quality control. In 1992, the Mammography Quality Standard Act was established as a federal law, and mammography screening became legislated.[8] Under the mammography screening law, all facilities conducting mammography screenings must receive certification from inspection institutes (ACR or state governments) accredited by the Food and Drug Administration (FDA) and are required to undergo medical inspection and receive clearance from the FDA. This law states that the dosimeter must be calibrated once every 2 years, and traceability to the national standard must be guaranteed. The National Institute of Standards and Technology (NIST), which is responsible for the American national standard, disseminates the mammography air kerma standard based on Mo/Mo radiation qualities. The Quality Control Manual[9] published by ACR was adopted into a quality control manual for Japan.The European Reference Organization for Quality Assured Breast Screening and Diagnostic Services (EUREF) played a central role in creating the guideline for mammography quality control in Europe.[10] This guideline states that the dose must be evaluated once every 6 months. European countries conduct quality control using individual methods that are based on this guideline. The air kerma standard based on Mo/Mo qualities was disseminated mainly in Germany and the UK. However, several European institutes calibrate mammography dosimeters using the W/A1 qualities, and there were concerns that differences in radiation quality may affect the calibration results. Therefore, institutes belonging to the European Collaboration in Measurement Standards (EUROMET, currently known as the European Association of National Metrology Institutes; EURAMET) performed an international comparison for the calibration of mammography dosimeters.In this international comparison, several ionization chamber and semiconductor dosimeters were circulated and calibrated with the radiation qualities (either Mo/Mo or W/A1) used by the institutes, and the calibration results were compared. The results indicated that for the dosimeters with a small energy dependence, such as the ionization chambers for mammography (soft X-rays), the effect of radiation quality on the calibration result was not a major issue in practice. However, dosimeters with a large energy dependence, such as the semiconductor dosimeters, should be calibrated under radiation quality conditions similar to that of mammography X-rays (such as Mo/Mo).[11]With this background, the Consultative Committee for Ionizing Radiation in the International Bureau of Weights and Measures (Bureau International des Poids et Mesures; BIPM) made a resolution that the BIPM establish the air kerma standard for mammography based on the Mo/Mo radiation qualities. This means that the dosimeters should be calibrated using the air kerma standard for mammography radiation qualities. BIPM prepared the reference field for mammography X-rays based on Mo/Mo radiation qualities, and an international comparison with BIPM began in 2009.[12] 2.6 Societal need for the mammography X-ray air kerma standardWith low-energy X-rays such as those used in mammography, the energy dependence of the dosimeter (both ionization chamber and semiconductor types) is large, and academic societies recommended that the calibration be performed at an energy close to that of the X-rays used in medical practice. Conventionally, the dose standard of X-rays in the mammography energy region was disseminated with W/A1 radiation qualities. Figure 3 illustrates an example of the energy dependence of the ionization chambers (range of Dosimeter ADosimeter BDosimeter C(all ionization chamber type)Calibration coefficientHalf-value layer(mmAI)Region mainly used in mammography2. 3 Example of the energy dependence of the ionization chamber


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