Fluke Biomedical 6000-529 - Manual (Page 11)

Theory of Operation
Sample Applications
2
2-3
Measuring Breast Entrance Exposure and Mean Glandular Dose
Breast entrance exposure is the radiation exposure that would be measured at the point of radiation entry
to the breast, free-in-air with the breast removed from the beam. Mean glandular dose is the dose to
glandular tissue in a typical 50-50 adipose-glandular composition breast, compressed to a thickness of
4.5 cm, and averaged over the extent of the glandular tissue.
To measure entrance exposure, follow these recommended steps:
1. Set up the x-ray machine for a typical mammographic technique. Place a loaded cassette in the
cassette holder, of the size and type consistent with the examination being simulated. Set the
machine in the AEC mode and set the density control to the position most commonly used for the
examination.
2. Place a mammographic phantom on the cassette holder assembly at the position normally
occupied by the breast. Be sure the phantom completely covers the AEC sensor. Now, place the
Model 6000-529 ionization chamber just to the side of the phantom. The entrance window of the ion
chamber should be flush with the top of the phantom, and the chamber should be placed as close
as possible to the chest wall edge of the x-ray field being sure that the entire chamber is in the field
and that it does not shadow the AEC sensor. Now, lower the compression paddle until it contacts
the phantom and chamber. Take care not to put any mechanical stress on the chamber.
3. Connect the chamber cable to an electrometer, NERO, 4000M+ or other charge-measuring
instrument. Follow the instructions accompanying the instrument for details of instrument operation.
4. Make an exposure. Record the reading from the electrometer or other instrument. Apply whatever
corrections are necessary to yield an accurate exposure reading. If you are using the Model 6000-
531 preamplifier, and have set it up according to the instructions, the only correction that may be
necessary would be air density correction (see section page 2-1). If you are using a charge-reading
electrometer, you should multiply the electrometer reading (in nanocoulombs) by the calibration
factor (in Roentgens per nanocoulombs) listed on the calibration report that accompanied the
chamber. Record the result.
5. Repeat step 4 three more times. Average all four results. The final result is the breast entrance
exposure. You should now repeat the procedure for all other clinically used techniques.
6. To compute mean glandular dose, refer to Table 2-1. In the column labeled "HVL", locate the value
closest to the half-value-layer you measured in step 4 for the kVp setting used in making the
entrance exposure measurement. Now, for Mo/Mo tubes, find the column labeled by the same kVp.
Alternatively for W/AI tubes, locate the column labeled “W/AI." The number located at the
intersection of the column thus located and the row designated by the proper HVL should be noted.
This number, when multiplied by the entrance exposure in Roentgens will give mean glandular
dose in millirads.