Vol.5 No.4 2013

Research paper : Dose standards for safe and secure breast cancer screening (T. Tanaka et al.)−233−Synthesiology - English edition Vol.5 No.4 (2013) smooth dissemination of the standard.This scenario was created to ensure rapid and widespread dissemination of the internationally equivalent dose standard to medical practitioners and to improve the reliability of dose evaluation.4 Development of the national primary standardTo develop a primary standard for mammography X-ray dose, a primary standard instrument for absolute dose measurement (measurement according to the definition of a unit) and a reference X-ray field with the same radiation quality as mammography X-rays must be developed. The development of a primary standard instrument and a mammography X-ray reference field will be described in these sections.4.1 Development of the primary standard instrumentIn the case of soft X-rays (here, a tube voltage of 50 kV or less), including those used in mammography, the dose standard (in Gy [or C/kg]) has already been disseminated for the air kermaTerm 4 (or exposureTerm 5) that has been physically defined.The free air ionization chamber, which is capable of absolute measurement of the radiation dose, is used worldwide as the primary standard instrument. Currently, this instrument is used as the primary standard instrument for the measurement of soft X-ray doses at AIST (Fig. 5).In the free air ionization chamber, the ion charge generated within the ionization volume is measured, and the radiation dose (or air kerma) is evaluated. When the mass of air within the ionization volume is set as m, the exposure rate (C·kg-1s-1) is obtained using equation (1).Ẋ kimI10i=1(1)Π=-• • •In this equation, I is the ionization current measured by the free air ionization chamber and ki is the product of the correction factors. The correction factors are used to correct for the deviation between the actual experimental condition and the ideal condition defined for the dose, and there are a total of 10 different correction factors. For mammography X-rays, the largest correction factor (approximately 1.5–2 %) is the correction for the attenuation of X-ray along the air path between the reference plane and center of the collective electrode. There is a minor correction (approximately 0.5 %) for scattering radiation, and the rest of the correction factors are fairly small (less than 0.1 %). The correction factors that are difficult to experimentally evaluate, such as the correction for scattering radiation, are evaluated using the Monte Carlo simulation.Figure 6 shows a photograph of the present mammography dose standard equipment. To develop the national standard for mammography, the correction factors of the free air ionization chamber (the primary standard instrument for soft X-ray dose) were evaluated for the radiation quality of mammography X-rays without developing a new primary standard instrument (free air ionization chamber) specific for mammography X-rays. This significantly shortened the time required for the development of a standard and allowed for a quick response to the social demand for dissemination of the standard. As shown in Figure 6, the free air ionization chamber is installed on the XY stage and can be shared with the dose standard for soft X-rays (W/A1 radiation quality).Fig. 6 Apparatus used for developing the mammography X-ray dose standardFig. 5 Schematic diagram of the primary standard (free air ionization chamber)High voltage (+2500 V)9.0 cm2.0 cmAir pass length; 8.0 cmSignalDiaphragmX-rayShielding wallHigh voltage electrodeCollecting electrodeGuardelectrodesThe reference plane of the diaphragmCollecting volumeNational primarystandardXY stageXY stageSoft X-ray irradiation systemMammography X-ray irradiation system


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