Vol.5 No.4 2013

Research paper : Dose standards for safe and secure breast cancer screening (T. Tanaka et al.)−229−Synthesiology - English edition Vol.5 No.4 (2013) Welfare (currently the Ministry of Health, Labour and Welfare)[3] recommended that patients aged 50 and older undergo screening (4th Elderly Health Care Project). The recommendation was revised in 2004,[4] and the age range was expanded to 40 years and older (5th Elderly Health Care Project). Enforcement of the Cancer Control Act began in 2007, and since March 2008, cancer screening was included in the health promotion project (based on the Health Promotion Act). With the expansion of the age range recommended for screening, the number of examinees has increased since the introduction of mammography in 2000, and this number surpassed 2.5 million in FY 2009.[5]2.2 Characteristics of mammography X-raysIn all diagnostic procedures involving X-rays, including mammography, an X-ray tube is used as the X-ray source. This tube is a device that generates X-rays by accelerating thermal electrons released from the filament to a metal plate (target) using high voltage (several kV to several hundred kV). The high voltage applied between the filament and target is called the tube voltage. These X-rays may include both characteristic and Bremsstrahlung X-rays depending on the combination of the target material and the tube voltage. The energy spectrum (radiation quality) is altered by passing the X-rays generated from the X-ray tube through a pure metal filter. The tube voltage and the material of the added filter and its thickness are adjusted to obtain X-rays with optimal quality for a given subject. Since the energy spectrum (radiation quality) is strongly depends on the target material and the filter, the expression of “target material /filter material” is commonly used. For example, if the target material is tungsten (W) and the filter is aluminum (Al), the energy spectrum (radiation quality) is expressed as W/Al.Compared to general radiography such as that used for the chest, the X-rays used in mammography have 2 features: (1) they have lower energy and (2) their energy spectra include characteristic X-rays.Regarding X-ray energy, while a tube voltage of approximately 80 kV is used in general X-ray radiography, 30 kV is used for mammography. Because the difference in the linear attenuation coefficientTerm 3 between glandular tissues and tumor tissue becomes larger as the X-ray energy decreases, low-energy X-rays are necessary to obtain high contrast images. However, X-ray absorption by the skin increases as the X-ray energy decreases. Therefore, in mammography, an X-ray tube voltage of approximately 30 kV is used to maintain both dose and image quality. At a tube voltage of approximately 30–80 kV, the absorption per unit length of matter, including air as well as the material of the dosimeter, increases as the X-ray energy decreases, which is a factor that prevents highly precise dosimetry.Another characteristic of mammography X-rays is their energy spectrum. Tungsten (W) is used as the target material of the X-ray tube in general X-ray radiography, whereas molybdenum (Mo) is mainly used in mammography. In mammography, the tube voltage settings between 25 and 35 kV are typically used, depending on breast tissue thickness and composition. This is because the tube voltage is above the ionizing energy of the Mo K-shell and characteristic X-rays are emitted in abundance. For mammography, a Mo target X-ray tube and a Mo additional filter are typically used. As a result, the energy spectrum contains an abundance of characteristic X-rays as shown in Figure 1. Using the Mo additional filter, all except the characteristic X-rays are filtered out, and the energy spectrum becomes monochromatic. This combination is used because X-rays with lower energy than the Mo characteristic X-rays lead to relatively strong radiation exposure of the skin, whereas high-energy X-rays decrease the image contrast.As described above, mammography is characterized by the use of low-energy X-rays that contain an abundance of characteristic X-rays.2.3 Dosimetry in mammographyAs an aid to the optimization of the radiation protection of the patient, dosimetric quantities need to be measured for radiography systems. In general X-ray radiography such as chest radiography, for example, the absorbed dose to the skin is used in dose evaluation.. However, in mammography, dose evaluation is done using a special dose called the average glandular dose (AGD) for the following reasons: only the breasts are exposed to X-rays, the glandular tissues are assumed to be most vulnerable to radiation in the breast, and the dose changes rapidly within the breast because the absorption per unit length is large, which is because of the low energy of the X-rays used.The AGD is calculated as the X-ray dose absorbed by all glandular tissues within the breast divided by the total mass Fig. 1 Examples of the X-ray spectra for mammography (Mo/Mo) and general radiography (W/Al)X-ray intensityX-ray energy (keV)General radiography(W/AI)Mammography(Mo/Mo)60504030201000.00.51.0


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