The influence of backscatter radiation from the image detector on air kerma measurements in mammography

Calibration and routine QC testing in mammography present rather challenging procedures, given the geometry of the X-ray unit and the associated factors that affect the measurements. Some of these factors include the positioning of dosimeters at their respective reference points, backscatter from the image detector, and the presence of the compression paddle in the primary beam. Dosimeters used in calibration and QC procedures are ionization chambers (ICs) or semiconductor X-ray multimeters (XMMs). The main difference is that XMMs have lead shielding on the backside that prevents backscattered radiation from the image detector from reaching the active volume of the dosimeter, while the ICs might be sensitive to backscattered radiation since they can detect radiation from all directions. Mammography X-ray units have a vertical beam with an image detector in place, while calibration laboratories mostly use horizontal beam configurations free-in-air, which may make reproducing realistic clinical conditions difficult. The presence of the image detector cannot be avoided in clinical setups. In opposite, most calibration laboratories don’t use or have an actual image detector that could be used to mimic the realistic clinical conditions. Routine laboratory calibrations are done in terms of air kerma free-in-air which does not take backscattered radiation into consideration. In the scope of the 22NRM01 TraMeXI project, the transfer of calibration procedures from laboratories to clinics is being developed. For this purpose, an image detector has been introduced to the laboratory calibration setup. The measurements in terms of air kerma free-in air have been done with and without the presence of the image detector, to evaluate the influence on measured value. This effect was studied for different positions of dosimeters (3 ICs and 3 XMMs) relative to the image detector. The results can be used to better estimate the uncertainty related to the impact of backscatter