Vol.5 No.4 2013
Research paper : Dose standards for safe and secure breast cancer screening (T. Tanaka et al.)−230−Synthesiology - English edition Vol.5 No.4 (2013) of the glandular tissues (the absorbed dose per unit mass of glandular tissue). The International System (SI) of units for AGD is J/kg, and this unit is designated as grey (Gy). The AGD is used as a representative of the absorbed dose to glandular tissues, which decreases as X-rays penetrate deeper into the breast (direction from the X-ray source to the receiving device, see Fig. 2). However, the AGD may change according to the amount (percentage) and distribution of the glandular tissues within the breast or the compressed breast thickness. Therefore, in mammography quality control, the standard breast is a breast with a thickness of 45 mm (42 mm in some cases) with uniform distribution of adipose and glandular tissues by weight. The AGD is evaluated using this standard definition of a breast. To optimize (minimize) the AGD, reference levels (or minimum dose targets) have been set by related academic societies.The AGD for an individual human body is extremely difficult to evaluate, and it is evaluated using a phantom, which is a material that simulates the standard breast as recommended by academic societies. Figure 2 shows an evaluation of AGD using a mammography machine.The dosimeter is placed so that its reference plane matches the surface of the phantom, and the dose of X-rays irradiated onto the phantom surface (X-ray source side) is measured. Because the dose measured by the dosimeter in this case is expressed by the unit “air kerma”Term 4, it is necessary to convert from air kerma to AGD. This conversion factor can be determined using the Monte Carlo calculation, and the tables for corresponding X-ray qualities are given in quality control manuals and other references. Therefore, to evaluate the AGD, an evaluation of the radiation quality of X-rays from the mammography machine is necessary. However, the measurement of an energy spectrum in actual medical practice is not realistic considering the time and cost. Therefore, the mammography radiation quality is expressed as the thickness of the material (aluminum is used in mammography) required to reduce the amount of air kerma by half. This thickness is called the half-value layer. As described above, it is necessary for evaluation of the AGD to measure the dose (air kerma) and quality (half-value layer) using a dosimeter.2.4 Dosimeters for mammography X-raysThere are many types of dosimeters with different measurement principles. In the medical practice of mammography, 2 types of dosimeters are widely used: the ionization chamber type and the semiconductor type. The ionization chamber dosimeter measures the amount of ionization (ion-electron pair) generated by the interaction between X-rays and air. In mammography, X-rays with low energy (or those strongly attenuated by matter) are used. Therefore, the entrance window in an ionization chamber used for mammography X-rays is composed of a thin film (mainly metal vapor-deposited resin) that allows for high transmittance of X-rays. Because an ionization chamber can measure the X-ray dose close to the definition of air kerma, it is used as a reference dosimeter. However, because the volume of air changes according to environmental conditions such as temperature and pressure, correction for environmental conditions is necessary. Because the X-ray entrance window is composed of a thin film, care must be taken in handling the device. Additionally, because the thin film absorbs X-rays, the sensitivity of the ionization chamber is likely to change depending on the X-ray energy, which may create problems for the measurement of low-energy X-rays.Silicon is mainly used in semiconductor dosimeters, and the depletion layer formed by the P-N junction (application of reverse bias voltage) is used as the radiation sensitive layer. In the PIN–type semiconductor dosimeters, the intrinsic semiconductor layer (i layer) acts as the radiation sensitive layer. While the carrier of the current is the electron-ion pair in the ionization chamber dosimeter, the current is carried by the electron-hole pair in the semiconductor dosimeter. Therefore, the semiconductor dosimeter is referred to as a solid ionization chamber. Compared to the ionization chamber dosimeter, the semiconductor dosimeter is sturdy and convenient to use with no correction required for temperature and pressure, and it is therefore used frequently for dose evaluation in actual medical practice. However, because the surface SiO2 layer and the non-sensitive layer absorb a large amount of X-rays, the sensitivity is greatly dependent on X-ray energy in the low-energy region such as mammography X-rays.X-rayBreast support(Receiving element such as film is set underneath)Ionization chamber dosimeterCompression paddleApproximately 65 cm (distance between source and dosimeter)X-ray tube is set insideFig. 2 Example of the installation of a dosimeter for the dose evaluation of a mammography machineThe reference plane of the dosimeter is lifted from the breast support because of the thickness of the phantom. In the photograph, only the ionization chamber dosimeter is mounted.