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Fig. 5.1. Effect of firm compression on breast contour. Breast is essentially spread out laterally (right) and made more uniform in thickness, so that x rays traverse less thickness (t) (centimeters). Consequently, a shorter exposure is required with corresponding dose reduction. Scatter to the image receptor R is also reduced, significantly improving image contrast. The resulting image improvement is indispensable in screen-film mammography.

Fig. 5.2. By making the sagittal (a) and transverse (b) cross sections of the breast more nearly rectangular with compression, computational model (c) can be utilized for estimating the mean glandular dose (Dg) (Hammerstein et al., 1979). The computational model for mean whole breast dose is (d). Outer hatched area in a, b and c represents skin and outer adipose layer thickness of 0.5 cm (Stanton et al., 1984).

Fig. 5.2. By making the sagittal (a) and transverse (b) cross sections of the breast more nearly rectangular with compression, computational model (c) can be utilized for estimating the mean glandular dose (Dg) (Hammerstein et al., 1979). The computational model for mean whole breast dose is (d). Outer hatched area in a, b and c represents skin and outer adipose layer thickness of 0.5 cm (Stanton et al., 1984).

A reference breast composition must be used when comparing doses from different techniques. By common usage, 50 percent water, 50 percent fat by weight has been an unofficial standard since 1976 for "average breast." The synthetic mix BR-12® closely matches the radiological properties of this composition (Hammerstein et al., 1979; ICRU, 1989) and is used extensively in dosimetry phantoms. Geise and Palchevsky (1996) have suggested that 30 percent glandular, 70 percent adipose might be a better match to the average patient. Although this suggestion has not been widely adopted in the radiological community, this Report contains data to calculate dose to a breast of this composition for the Mo target-Mo filter combination. (Table 5.2d). Other publications that indicate there is a lower glandular tissue content (e.g., less than 50 percent grandular tissue) in the average breast are those of Heggie (1996), Klein et al. (1997), and Kruger and Schueler (2001).

5.2.3 Why "Mean Glandular Dose?"

The mammography literature has frequently used the term "dose" when x-ray exposure was actually measured, usually free-in-air (i.e., no backscatter) or at the skin (with backscatter). Moreover, absorbed doses have often referred to the breast mid-plane value or to the average for the entire breast. To explain why mean glandular dose (Dg) is the preferred quantity, a brief conceptual discussion of x-ray exposure and absorbed dose is given below; this is followed by a comparison of the dosimetric quantities of most interest for mammography.

5.2.3.1 Absorbed Dose, X-Ray Exposure, and Air Kerma. Radiation effects result from the deposition of energy in tissue. Absorbed dose (D) is the energy imparted by ionizing radiation to matter in a volume element, divided by the mass of matter in that element. The International System (SI) special name for the quantity absorbed dose is the gray (Gy), where 1 Gy is an energy absorption of one joule per kilogram.

Although absorbed dose is the quantity most directly related to biological effects, it cannot be directly measured in the breast. However, x-ray exposure can be measured in suitable phantoms for given spectra and the result used to estimate absorbed dose (Hammerstein et al., 1979). X-ray exposure (X), is related to the ion concentration (number of ion pairs per gram) produced in a tiny volume of air at the location of interest, under specified conditions. More complete discussions of radiation quantities and their measurements are available in various radiological physics texts.

In practice, the distribution of x-ray exposure in suitable homogeneous phantoms is measured by thermoluminescent dosimeters or ionization chambers. The measurements can then be used to estimate the exposure distribution in a simplified model of the human breast, such as that of Figure 5.2c (Hammerstein et al., 1979). From that result, the desired absorbed dose (D) (in rad) from exposure (X) (in roentgen) at any location of interest can be directly computed using the appropriate exposure to absorbed dose conversion factor (fm) in a given material (m) (in rad per roentgen):

In the SI system of quantities and units, Equation 5.1a would be:

where_D is absorbed dose in milligray, Ka is air kerma in milligray, and (fm) is an air kerma to absorbed dose conversion factor in milligray per milligray air kerma for the material (m) of concern (adipose tissue, glandular tissue, etc.).

Published (fm) values for 10 to 40 keV x rays range from 0.58 to 0.65 and 0.90 to 0.92 mGy per milligray air kerma (0.51 to 0.57 and 0.79 to 0.81 rad per roentgen), respectively, for adipose (ad) and breast glandular tissue (g), respectively (Hammerstein et al, 1979). Because of the small change in (fm) over this energy range, single values of 0.62 mGy per milligray air kerma (0.54 rad per roentgen) (fad) _and 0.90 mGy per milligray air kerma (0.79 rad per roentgen) (fg) have been used for mammographic dose calculations (Stanton et al., 1984).

5.2.3.2 Depth Dose Distributions. Figure 5.3 illustrates how x-ray exposure, air kerma, and absorbed dose values change with depth in the simplified breast model of Figure 5.2c, when the latter receives a nominal x-ray exposure (free-in-air) at the entrance surface of the breast of 1 R (8.76 mGy air kerma), and the beam HVL is 0.37 mm Al. Such a beam would be representative of a screen-film technique with grid. With the breast in place, some of the incident x-ray energy is scattered back to the entrance location increasing the surface skin exposure (or air kerma) by about 10 percent. The light solid curve (top curve) in Figure 5.3 shows the relative depth-dose distribution for exposure (or air kerma) from the skin entrance to the exit surface.

The adipose tissue absorbed dose (dashed line,_bottom curve in Figure 5.3) is given by Equation 5.1, using the fad value of 0.62 mGy per milligray air kerma (0.54 rad per roentgen exposure). In the central volume, glandular and adipose tissue elements lie adjacent to each other. They can, therefore, receive the same exposure (or air kerma) at a given depth, but quite different absorbed doses. For example, the midplane exposure (or air kerma) at 2.25 cm depth is the same for both tissues but the absorbed dose to glandular tissue is about 30 percent higher than that for the adjacent adipose tissue. The single quantity most relevant to radiation risk is the mean glandular dose (Dg), which is obtained as described in Section 5.3.1 and Equation 5.2.

5.2.3.3 Mean Glandular Dose and Other Dose Terms Compared. The mean absorbed dose to the glandular tissue [mean glandular dose (Dg) ] is the preferred measure of potential carcinogenic risk from mammography. This quantity can be readily estimated with il) (/i o Q

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