Fig. 5.3. Variation of x-ray exposure, air kerma and absorbed dose with depth in the breast. For the 4.5 cm thick breast model of Figure 5.2c, normalized to 8.76 mGy air kerma (1 R) (free-in-air at the entrance skin surface) for a 0.37 mm Al HVL beam. The top curve of exposure (or air kerma) versus depth (light solid line) applies to both adipose and glandular tissue, but two separate absorbed dose curves result. The absorbed dose to glandular tissue (dark solid line, Dg) is consistently greater at a given depth than that for adjacent adipose tissue (dashed curve, Dad). The Dg curve is limited to the central region where glandular tissue is present.
good accuracy, so it has become the standard "mammographic dose" quantity. However, some comment is in order regarding three other widely used quantities: skin dose, midplane glandular dose, and average whole-breast dose. (The latter is approximated by the absorbed dose to a uniform breast phantom of the same thickness in Figure 5.2d).
The skin dose (Ds) has been widely quoted in the past, probably because it is most easily determined. However, it is a poor choice because skin is not the tissue at risk for radiation carcinogenesis, and the ratio Ds per Dg varies greatly with both beam quality and breast thickness. Midplane dose to the glandular tissue (Dmg) can be confused with Dg, and the latter may be substantially greater for screen-film techniques. The mean whole-breast absorbed dose (D) is reasonably close in value to Dg, for an "average breast" composition of 50 percent water, 50 percent fat by weight. There have been many publications on evaluation of D. However, D varies much more with changes in breast adiposity than does Dg (Stanton et al, 1984). The practical result is that estimates for D for patients with very dense or fatty breasts tend to involve much greater errors than those for Dg. Consequently, Section 5.3 describes evaluation of mean glandular dose (Dg) only.
5.3 Dose-Evaluation Principles
Direct evaluation of mean glandular dose (Dg) is complicated by the many variables on which x-ray exposure depends. Fortunately, Dg can be computed from the simple relationship:
Here DgN is the mean glandular dose (in millirad) resulting from an incident exposure (free-in-air) of 1 R, and Xa is the incident exposure (free-in-air) needed to produce a proper density image.8 To good accuracy, the value of DgN depends on only four quantities: the beam energy (HVL and operating potential), the x-ray tube target material (molybdenum, tungsten and, most recently, rhodium) and filter materials (molybdenum, rhodium, and sometimes aluminum), breast thickness, and breast composition. When breast composition is known, it is possible to construct simple working curves or tables to evaluate DgN (Dance, 1980; Stanton et al., 1984). Tables 5.2a through 5.2j give values of DgN for Mo target-Mo filter, Mo target-Rh filter, and Rh target-Rh filter for 50 percent glandu-lar-50 percent adipose tissue, 100 percent adipose tissue, and 100 percent glandular tissue (Wu et al., 1991; 1994). In addition, values of DgN for Mo target-Mo filter and 30 percent glandular-70 percent adipose are also provided by Wu.9 The effect on DgN of moderate departures from this composition is not great (NCRP, 1986). The exposure (free-in-air) required for proper image density (Xa) is
8In the SI system of quantities and units, Equation 5.2 would be expressed as: Dg = DgN Ka, where DgN is the mean glandular dose (in microgray) resulting from an incident air kerma (Ka) (free-in-air) of 1 mGy, and Ka is the incident air kerma (free-in-air) (in milligray) needed to produce a proper density image.
9Wu, X. (2000). Personal communication (University of Alabama Hospitals and Clinics, Birmingham, Alabama).
determined from x-ray output measurements (Sections 5.3.3 and 5.3.4). Parameterization of these data has been published by Sobol and Wu (1997).
5.3.2 Determination of Mean Glandular Dose (in millirad) per 1 R Entrance Skin Exposure (free-in-air)
Both experimental _and computational methods have been employed to evaluate DgN. An earlier method begins with measurements of exposure versus depth and backscatter factors, using a breast phantom. Refer to Figure 5.3, from which glandular tissue dose as a function of depth (z) [Dg(z)] can be determined as follows:
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