A Mammography Dose Measurement Protocol

Section 370 Section 370.APPENDIX A   Mammography Dose Measurement Protocol The technique factors used for performing a mammography examination shall not permit the mean glandular absorbed dose to exceed the limits specified in Section 370.110(e)(7) of this Part.  Radiation measurements shall be performed with an integrating radiation measuring device that is appropriate to the high beam intensity and mammographic kilovoltage peak (kVp) used, and sufficiently sensitive to determine compliance with the criteria specified in Section 370.110(i)(2) of this Part.  The instrument shall have been calibrated as specified in Section 370.110(i)(2) of this Part. The mammography exam dose limits are based on an average compressed breast value of  4.2 centimeters having an average density (i.e., 50 percent adipose and 50 percent glandular). Perform the following steps to determine the mean glandular dose to a nominal 4.2 centimeter compressed breast: a)         Measure and record the x-ray system's useful beam half-value layer (HVL). (See Section 370.110(e)(5) of this Part.)  Any compression device normally in the useful beam during mammography procedures shall be required to be placed between the x-ray tube target and measuring device when determining the HVL. The useful beam shall be collimated to a size encompassing the detector. AGENCY NOTE:  Filters used for the HVL evaluation should be placed as close to the target as practical.  The HVL for screen-film mammography should not exceed the minimum acceptable HVL by more than 0.1 millimeter of aluminum equivalent. b)         Determine the glandular dose to entrance exposure factor from the Mammography Dose Evaluation Table (see Table A of this Part) using the appropriate HVL, kVp and x-ray tube target-filter material. AGENCY NOTE:  The kVp of screen-film mammography systems with molybdenum target-filter combinations should be accurately measured to determine the appropriate glandular dose to entrance exposure factor from Table A of this Part. c)         If the equipment has the capability for variable source-image receptor distance, set the craniocaudal source-image receptor distance (SID) for the image receptor system used. d)         Position in the useful beam any compression apparatus normally used. AGENCY NOTE:  Some mammography systems have the capability of providing automatic adjustment of technique factors through feedback from the position of the compression device.  On such systems, the compression device should be lowered to a position 4.2 centimeters above the breast support assembly (BSA). The device should then be removed, inverted and replaced to allow placement of the phantom and measuring device on the BSA below the compression device.  If the compression device cannot be replaced in an inverted position, the device should be placed in the beam using auxiliary support. e)         Placement of the Radiation Measuring Device 1)         For systems equipped with automatic exposure control (AEC): A)        Place a properly loaded film cassette in the cassette holder. AGENCY NOTE:  The loaded cassette is placed in the cassette holder to simulate, as much as is possible, the conditions under which actual patient exposures are made.  Following radiation measurements, the film should be discarded and the cassette reloaded with unexposed film. B)        Place a mammography phantom (see the definition for "Phantom" in Section 370.20 of this Part) on the breast support assembly (BSA).  Align the phantom so that the edge of the phantom is aligned with the chest wall side of the BSA and the phantom is over the automatic exposure control device(s). C)        Place a radiation measuring device in the useful beam so the center axis of the device is parallel to the breast support assembly (BSA).  The geometric center of the measuring device shall be positioned 4.5 centimeters above the BSA, 2.5 centimeters from the chest wall edge of the BSA and immediately adjacent to either side of the mammography phantom. 2)         For systems not equipped with AEC, place a radiation measuring device in the useful beam so that the center axis of the device is parallel to the breast support assembly (BSA).  The geometric center of the measuring device shall be positioned so that it is centered 4.5 centimeters above the BSA, 2.5 centimeters from the chest wall edge of the BSA and at the center line of the BSA.  No part of the device's detector area shall be outside of the useful beam. f)         Collimate the x-ray field to the size normally used and assure that the area covered by the useful beam includes the detector area of the radiation measuring device and the mammography phantom if the equipment is equipped with automatic exposure. g)         Set the appropriate technique factors or automatic exposure controls normally used for a nominal 4.2 centimeter compressed breast. h)         Measure and record the exposure in air with the radiation measuring device. i)          Calculate the mean glandular dose for a 4.2 centimeter compressed breast by multiplying the measured exposure in millicoulombs per kilogram or in roentgens by the glandular dose to entrance exposure factor, which was determined using the procedure described in subsection (b) of this Section. EXAMPLE:  A mammography system is provided with a molybdenum target-filter combination, and the HVL and kVp are determined to be 0.3 and 30, respectively. Therefore, for a 4.2 centimeter compressed breast, the glandular dose to entrance exposure factor from the Mammography Dose Evaluation Table (Table A of this Part) would be 159 mrad.  The measured roentgen output determined in subsection (h) of this Appendix is determined to be 1.8 R. Therefore, the mean glandular dose would be 1.8 R multiplied by 159 mrad/R. This results in a mean glandular dose measurement of 286 mrad.  As such, the system would be in compliance with Section 370.110(e)(7) of this Part.