Info

aTo convert values of DgN from millirad per roentgen to the SI system of quantities and units (in microgray mean glandular dose per milligray incident air kerma), multiply table entry by 1.14. bAdapted from Wu et al. (1994).

aTo convert values of DgN from millirad per roentgen to the SI system of quantities and units (in microgray mean glandular dose per milligray incident air kerma), multiply table entry by 1.14. bAdapted from Wu et al. (1994).

Wu et al. (1991; 1994) and X. Wu,10 which include the effects of operating potential, as well as HVL and values for different targets and filters.

5.3.3 Needed Measurements

Evaluation of the mean glandular dose ( Dg ) for a given mammographie view requires knowledge of the x-ray exposure (free-in-air) (Xa), the x-ray beam HVL, operating potential, and the compressed breast thickness. Both ionization chambers and thermoluminescent dosimeters have been used for this purpose. The ionization chamber response should be constant to ±10 percent for beams of 0.3 to 1.5 mm Al HVL, and have a National Institute of Standards and Technology traceable calibration. In addition, the ioniza-tion chamber-instrument system must provide accurate, reproducible readings, with at least 99 percent of saturation ioniza-tion chamber current for the highest measured exposure rate levels, negligible chamber leakage current, and electrometer zero drift. Additional information is given in ICRU (1973), Johns and Cunningham (1983), NCRP (1981), and Stanton et al. (1984). Ion-ization chambers designed for mammography are normally calibrated for free-in-air exposure or air-kerma measurements.

Accurate determination of the HVL for mammography x-ray beams requires great care (Wagner et al., 1990). Generally, the same ionization chamber used for exposure measurements may also be used for HVL measurements, which must be performed using proper geometry (Johns and Cunningham, 1983). The added aluminum filters must be high-purity aluminum and the thickness verified by micrometer. Type 1145 aluminum, which is 99.99 percent pure, is now commercially available in 0.1 mm thick sheets. For very low-energy measurements, uniformity of absorber material thickness should be checked radiographically and determination of thickness by precision weighing is recommended.

Lithium fluoride thermoluminescent dosimeter extruded ribbons ("chips") have been used successfully for measurement of x-ray exposure (free-in-air), at the entrance surface (with backscat-ter), and at depth in a phantom. Achievement of accurate results requires careful initial selection, handling and annealing of thermoluminescent dosimeter ribbons; also, multiple ribbons must be used for each measurement and corrections made for energy

10Wu, X. (2000). Personal communication (University of Alabama Hospitals and Clinics, Birmingham, Alabama).

dependence to insure accuracy (Hammerstein et al., 1979). Other corrections may also be required for residual signal after readout and for short-time fading. More extensive technical information on thermoluminescent dosimetry is available in specialized references (Robertson, 1974).

5.3.4 Application to Patient Dosimetry

The discussion in Sections 5.3.1 and 5.3.2 has dealt with the dose to a breast of reference composition (radiologically equivalent to 50 percent water, 50 percent fat by weight). The results are directly applicable to the comparison of dose levels from different techniques. A second important need is to monitor the patient dose. This Section explains in more detail how each of these important tasks may be accomplished.

5.3.4.1 Comparing Techniques. Values of DgN from Tables 5.2a through 5.2j may be used for comparing doses from different techniques. The value of Xa is determined using a BR-12® breast phantom, consisting of a stack of 1 cm thick BR-12® slabs of total thickness appropriate to the degree of compression used and the phantom surface location. Radiographs of the phantom are then made, varying the exposure time. The desired Xa value is the product of the measured exposure rate and the exposure time in seconds that yields the desired image optical density. Density of film images may be checked by densitometer. The mean glandular dose (Dg) can then be computed by Equation 5.2.

5.3.4.2 Monitoring Patient Dose. When screening programs are being established, the primary concern is the potential carcinogenic risk to a large group of women examined, rather than to specific individuals (Section 7). The average value of the mean glandular dose Dg for the group is hence most important, and the average breast thickness and composition most relevant. In a reasonably large population of women 40 y and older, this average composition differs only moderately from the composition of a reference phantom.

When there is concern by an individual woman about the dose received from a given mammography examination, dose calculations should be modified to account for the actual breast tissue composition of that patient, when possible.

5.4 Published Dose Recommendations and Surveys

5.4.1 Recommendations

Recommendations for acceptable mean glandular dose ( Dg ) delivered for a single view to a standard thickness (4.5 cm) compressed breast of average composition have been issued by various national organizations, as well as national and state regulatory agencies. These groups now all agree that for a single_view of a 4.5 cm compressed breast of average composition, the Dg should not exceed 3 mGy.

5.4.2 National Surveys

Both ACR, through MAP, and the FDA Center for Devices and Radiological Health have gathered data on Dg delivered to a standard thickness acrylic phantom which simulates a compressed breast of average thickness and composition. A summary of these data is presented in Table 5.3.

Table 5.3—Typical values of Dg from nationwide surveys.'

ACR-MAP (1992)

Image Receptor

0 0

Post a comment